Chapter Menu

• 7.0 Overview
  • Learning Objectives
• 7.1 Synoptic Weather Systems
  • 7.1.1 Tropical Easterly Waves
    • 7.1.1.1 Background and Climatology
    • 7.1.1.2 Structure
    • 7.1.1.3 Formation of AEWs
    • 7.1.1.4 Lifecycle over Africa
    • Box 7-1 The Easterly Wave as a Convectively Coupled Wave
    • 7.1.1.5 Monitoring and Tracking
    • 7.1.1.6 Downstream Transformation (into tropical cyclones)
    • 7.1.1.7 Easterly Waves over the Atlantic, Caribbean, and East Pacific
    • 7.1.1.8 Intraseasonal Variability
  • 7.1.2 Equatorial Inertio-gravity Waves
    • 7.1.2.1 Basic Description and Coupling with Convection
    • 7.1.2.2 Formation Mechanisms and Theoretical Solutions
  • 7.1.3 Tropical Upper Tropospheric Troughs and Upper Cold Lows
    • 7.1.3.1 Structure, Weather, and Climatology
    • 7.1.3.2 Impact on Tropical Cyclones
  • 7.1.4 Subtropical Cyclones
    • 7.1.4.1 Structure, Lifecycle, and Impacts
    • 7.1.4.2 Formation and Transformation
  • 7.1.5 Monsoon Depressions
    • 7.1.5.1 Structure and Characteristic Weather
    • 7.1.5.2 Formation and Lifecycle
    • 7.1.5.3 Climatology
  • 7.1.6 Arabian Sea Mid-tropospheric Lows
    • 7.1.6.1 Structure and Formation
    • 7.1.6.2 Climatology
  • 7.1.7 Wind Surges
    • 7.1.7.1 Trade Wind Surges
    • 7.1.7.2 Westerly Wind Bursts
    • 7.1.7.3 Cold Wind Surges
  • 7.1.8 Tropical-Extratropical Interactions
    • 7.1.8.1 Cyclones, Fronts, and Cold surges
    • 7.1.8.2 Rossby Wave Energy Propagation
    • 7.1.8.3 The Subtropical Jetstream, MJO, and Atmospheric Rivers
    • 7.1.8.4 Extratropical Interactions with Easterly Waves
    • 7.1.8.5 Tropical Temperate Troughs
    • 7.1.8.6. Tropical Intrusion over South America
• 7.2 Mesoscale Weather Systems
  • 7.2.1 Mesoscale Definition and Classification
    • 7.2.1.1 Defining Mesoscale
    • 7.2.1.2 Dynamical Concepts of Mesoscale Circulations
  • 7.2.2 Mesoscale Convective Systems
    • 7.2.2.1 Thunderstorms and Lightning
    • 7.2.2.2 Structure and Lifecycle of Tropical MCSs
    • 7.2.2.3 Environments of Tropical MCSs
    • 7.2.2.4 MCS Propagation
    • 7.2.2.5 MCS Interaction with Large-scale Tropical Circulations
    • 7.2.2.6 Global Distribution and Impacts
    • 7.2.2.7 Heat and Moisture Transport
    • 7.2.2.8 Mesoscale Convective Vortex (MCV)
    • 7.2.2.9 Atmospheric Electrical Effects
    • 7.2.2.10 Chemical Transport by MCSs
  • 7.2.3 Mesoscale and Local Circulations
    • 7.2.3.1 Sea-land Breeze
    • 7.2.3.2 Mountain Effects
    • 7.2.3.3 Island Circulations in the Trades and Monsoons
  • 7.2.4 Tropical Severe Weather
    • 7.2.4.1 Ingredients for Severe Thunderstorms
    • 7.2.4.2 Tornadoes
    • 7.2.4.3 Waterspouts
    • 7.2.4.4 Dust Devils
• Summary
• Questions for Review
• Appendix 7A
• Brief Biographies
• References

7.2 Mesoscale Weather Systems »
7.2.2 Mesoscale Convective Systems »
7.2.2.3 Environments of Tropical MCSs

Vertical Wind Shear

While the main ingredient for cumulus convection is moist air that is warmer than its environment (Section 5.2.2.2Section 5.2.2.2), the vertical wind shear is critical for determining the organizational mode and longevity of MCSs.^234,254,255 The CAPE is key to their updraft strength and depth.²⁵⁶

Tropical versus Midlatitude Shear

In general, tropical MCSs develop in environments with weaker cold pools and weaker horizontal wind shear than the midlatitudes. Both tropical and midlatitude MCSs have weak or straight-line mid-level shear profile with most of the vertical wind shear found in the low-levels (often generated by a low-level jet). The 950 and 650 hPa shear in non-squall tropical clusters (the least intense MCSs) is less than 5 m s^-1 while tropical squall lines with bow echoes (which have the highest likelihood of strong, straight-line surface winds) have mean shear of about 13 m s^-1. By comparison, 10 m s^-1 is the upper threshold for weak shear environments in midlatitude MCSs.

Figure 7.96 shows how shear organizes tropical convective structure and orientation, with lines generally perpendicular or parallel to the shear in the low and mid-troposphere.^257,258 With weakly sheared environment, convection developed along outflow boundaries (Panel 1), With strong low-level shear and weak midlevel shear, the main convective band is perpendicular to the low-level shear (Panel 2 in Fig. 7.96). Without strong low-level shear, lines form parallel to the low-level shear (Panel 3), while more complex structures occur with strong low and mid-level shear (Panel 4).

Updrafts in MCSs

In MCSs with a trailing stratiform region, updraft speeds range from 5-17 m s^-1 in the convective region and 35-45 cm s^-1 in the stratiform region. Mesoscale downdrafts, in the stratiform region, of the typical tropical MCS are in the 5-25 cm s^-1 range. How do these MCS features form and evolve?

1) The traditional view is of updrafts created because of static instability, also known as buoyant or gravitational instability. Parcels of warm, moist (with high equivalent potential temperature θe) air from the boundary layer, accelerate upward after being forced above the local level of free convectionlevel of free convection, entrain varying amounts of environmental air via in-cloud turbulence as they rise, then decelerate and eventually stop and spread out laterally at or near the equilibrium level, where they are neutrally buoyant (Section 5.2.2.2Section 5.2.2.2). However, the idea undiluted updrafts in “hot towersâ€� over the oceanic equatorial trough has been challenged by recent field and satellite observations.²⁵⁹ Such undiluted updrafts are relatively rare over tropical oceans; rather, freezing is an important source of additional buoyancy.²⁵

2) The upward air motion that maintains a mature MCS occurs in the form of a deep slantwise ascending layer of air.²⁶⁰ In this view, θ_e is conserved along streamlines.^261,262 If the large-scale environment is unstably stratified and sheared, air must flow through the storm along a unique set of streamlines. For a typical environment of strong low level shear, the updraft would then be of a layer ascending on a slantwise path through the storm.

3) Another theory is that of gravity wave responses to the heating by the MCS.^263,264 Simulations of MCSs found that once convective cells become clustered into an organized mesoscale group, this structure serves as a quasi-steady heat source. A layer of 3- to -6 km inflow is then drawn up through the system as a gravity wave response to the heating. The layer of inflow air enters the convective region, rising and exiting as the middle to upper level front-to-rear flow.

4) Moist absolutely unstable layer (MAUL) layer lifting operates when the layer is potentially unstable²⁶⁵ (Fig. 7.97). Radar observations of MCSs during TOGA-COARE showed sloping updrafts with finite layers of air, 0.5-4.5 km depth, rising over a downdraft cold pool.²⁶⁶

Thermodynamic evolution

The pre-squall environment is marked by a moist boundary layer, moderate-high CAPE, low level of free convection (Fig. 7.98). As the squall line passes, the following features are observed: a pre-squall mesolow (due to subsidence warming in the mid-to upper troposphere), a mesohigh (due to heavy precipitation and convective-scale downdrafts), and a wake low (due to subsidence warming and as a surface expression of the descending rear inflow jet) (Fig. 5.19). Post squall environments are characterized by subsidence, adiabatic compression, which warms and dries the lower troposphere, as indicated by an onion-shaped sounding²³¹ (Fig. 7.98). A squall line example is shown but the process is similar for other MCSs.

Squall Line MCS environments

Tropical squall line development is aided by strong low-level wind shear (for GATE squall lines, the critical threshold was 13 m s^-1 in the 950-650 hPa layer, mostly perpendicular to the leading line). Squall lines, in turn, transport momentum upgradient, increasing the low-level vertical wind shear.

The organizational mode of the squall line, e.g., its slope and size of the stratiform region, is dependent on the relative strengths of the downdraft-generated cold pool and the vertical wind shear.^254,255,267 The strength of the cold pool influences its propagation speed. The convectively-generated cold pool acts as a gravity current because it is denser than its environment. New convection is formed by uplift along the boundary of the cold pool. Its speed is given by:

where h is the height of the spreading cold pool, g is the gravitational constant, is the environmental virtual potential temperature and is the perturbation virtual potential temperature of the cold pool. Gust fronts can precede storm cells of a few tens of km (20 to 40 km). Normally, the density current head has a rotor circulation. The typical depth of the cold pool is ~ 1 km. It is often thinner over the ocean (200-300 m) than over continents (up to 3 km). With passage of the cold pool, winds intensify and rotate, temperature drops 2 to 10° C, and pressure jumps 1 to 2 hPa. The moisture content also falls.

When the shear is stronger than the cold pool, the updraft is tilted away from its source of warm, moist air and the tops of the cells are “shorn offâ€�. When vertical shear is weak, the cold pool moves out from the original updraft, cutting off the supply of warm, moist air (the high θ_e air). When the shear vorticity is balanced by the cold pool vorticity, the updraft remains upright, the cold pool triggers lifting, and the convective system is long-lived^254,255 (Fig. 7.99).

How the stratiform region evolves reflects the orientation of the vertical shear vector relative to the squall line motion (Fig. 7.100). The commonly observed trailing stratiform type develops where dominant shear is perpendicular to the line. The parallel stratiform region has more line parallel shear in the upper-levels and the leading stratiform has weaker low-level shear than the other two types (Fig. 7.100). It is not clear how the archetypes can be applied generally in the tropics given the generally weaker shear compared with the midlatitudes.

At the large-scale, the dominant forcing of squall clusters in the western Pacific and tropical Atlantic is convergence in the ITCZ. Systems observed during the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) formed several hours after large-scale convergence was established and moisture flux increased in the low-middle troposphere.

Squall clusters also occur with easterly waves, which are noted for having low-level convergence, divergence above 200 hPa, and non-divergence in between. They normally form ahead of the easterly wave trough, move at about twice the speed of the wave, and tend to die just behind the ridge. Amazon squall lines have added mesoscale forcing from the sea breeze front.^48,268

Bow echo environments

Bow echoes most commonly occur in an environment with strong deep layer shear in combination with high CAPE, steep midlevel lapse rates, and a strong cold pool. Bow echoes sometimes develop within pre-frontal squall lines and tropical cyclone rainbands. For example, squall lines and bow echoes in the developing Tropical Storm Nicole (2010) produced tornadoes, damaging strong surface winds, and heavy rainfall (COMET module, “Severe Weather and Flooding in Jamaica�, http://www.meted.ucar.edu/radar/tropical_cases/navmenu.php?tab=2&page=1.0.0).

MCC environments

Most MCCs initiate within prominent baroclinic zones (such as the monsoon trough, stationary fronts, and the Mei-Yu/Baiu front), which have locally large values of lower-tropospheric vertical wind shear and CAPE. Development is favored when a convectively-generated surface cold pool combined with substantial low-level vertical wind shear leads to a deep layer of slantwise ascent and a large stratiform cloud region. Most of the wind shear in MCC environments is found in the low-levels (often generated by a low-level jet).

MCC formation can happen with or without large-scale forcing (Fig. 7.101). If the sign of cold-pool-induced circulation is always the same as that associated with the ambient shear then the convection will be long-lived. This secondary mechanism (Fig. 7.101b) is likely in Northern Australia MCCs that have weak baroclinic forcing. In that region, MCCs are associated with the monsoon trough and are often precursors to tropical cyclones.

Large-scale environments conducive to MCCs usually have: a low-level jet of air with low static stability, high equivalent potential temperature (θ_e), oriented nearly perpendicular to the baroclinic zone, flowing towards the genesis region, and is forced to rise over a relatively shallow, surface-based layer of relatively cool air.^235,239 The surface cool layer is overlaid by pronounced warm advection accompanied by strong lower-tropospheric veering. A local maximum in absolute humidity and a local minimum in static stability favored MCC initiation. Low-level convergence, upper-level divergence, and a mid-level vorticity maximum, and weak mid-level shear are also characteristic of the mean genesis environment. West of Borneo in the Maritime Continent is a common location for MCCs.^269,270

There, favorable strong low-level convergence and low-level wind shear are produced from the interaction of northeast monsoon winds and the sea breeze²⁶⁹ (Fig. 7.102).²⁷¹ MCCs also form along the shores of northwest South America, where initial thunderstorms develop along the mountain slopes in response to diurnal heating of elevated terrain. Then it is suggested that thermally-forced gravity waves propagate offshore and initiate MCCs during the nighttime.²⁷¹

Non-squall cluster environment

A distinctive feature of non-squall cluster environments is the absence of strong shear between 700 and 250 hPa. They have deep layer moisture and maximum low-level convergence during the growing stage of the system lifecycle. Maximum vertical motion is close to 300 hPa and occurs during the mature phase.

During GATE, clusters formed where the mean 950-650 hPa environmental shear was about 6 m s^-1 and mostly parallel to the lines. Non-squall clusters are similar to squall clusters in terms of conditional instability and formation relative to the African easterly wave trough. However, large, longer-lived non-squall clusters tend to move at less than the speed of the easterly wave and dissipate once they fall behind the trough axis. Similar winter monsoon clusters near Borneo are initiated from offshore breezes and a quasi-stationary vortex that develops cross-equatorial monsoon flow (Section 3.5.3Section 3.5.3).

A

Absolute angular momentum

For the atmosphere, the absolute angular momentum, per unit mass of air, is the sum of the angular momentum relative to the earth and the angular momentum due to the rotation of the earth.

Absolute vorticity

See Vorticity.

Absorber

Anything that retains incident electromagnetic radiation due its physical composition.

Absorption

The process by which incident radiant energy is retained by a material due to the material's physical composition.

Absorption band

A portion of the electromagnetic spectrum where radiation is absorbed and emitted by atmospheric gases such as water vapor, carbon dioxide, and ozone.

African easterly wave

A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.

Aggregation

The clumping together of ice crystals after they collide.

Anomaly

The deviation of a quantity over a specified period from the normal value for the same region. For example, El Niño is identified by sea surface temperature anomalies.

Atlantic Multidecadal Oscillation (AMO)

A natural oscillation of the North Atlantic SST between warm and cool phases. The SST difference between these warm and cool phases is about 0.5°C and the period of the oscillation is roughly 20-40 years (the period is variable, but is a few decades long). Evidence suggests that the AMO has been active for at least the last 1,000 years.

Attenuation

Any process in which the intensity of radiation decreases due to scattering or absorption.

Atmospheric Window

A portion of the electromagnetic spectrum where radiation passes through the atmosphere without absorption by atmospheric gases such as water vapor, carbon dioxide, and ozone.

Available potential energy (APE)

The portion of the total potential energy available for adiabatic conversion to kinetic energy. The total potential energy is a combination of the APE and the potential energy representing the mass distribution needed to balance the mean atmospheric motions.

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B

Backscatter

That portion of radiation scattered back toward the source.

Baroclinic

Dependence on the horizontal temperature contrast between warm and cold air masses., In a baroclinic atmosphere, the geostrophic wind varies with height in direction as well as speed and its shear is a function of the horizontal temperature gradient (the thermal wind equation).

Barotropic

The atmosphere has the same horizontal structure at all levels in the vertical. This is equivalent to the absence of horizontal temperature gradients.

Barotropic-Baroclinic Instability

Barotropic and baroclinic instability analyses are used to explain the growth of a small perturbation to the flow. A perturbation growing due to baroclinic instability draws its energy from the available potential energy (APE). A perturbation growing due to barotropic instability draws its energy from the kinetic energy of the background flow. A perturbation growing through both APE and mean kinetic energy conversion to kinetic energy of the growing system (intensifying the system) is developing through combined barotropic baroclinic instability.

Best track

As defined by the National Hurricane Center, it is a subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center positions fixed during operations.

Beta (β) effect

Denotes how fluid motion is affected by spatial changes of the Coriolis parameter, for example, due to the earth's curvature. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude. The asymmetric flows resulting from the interaction of the vortex with the changing Coriolis parameter is known as the β-gyres.

Beta (β) plane

An approximation of the Coriolis parameter in which f = f₀ + βy, where β is a constant. The Coriolis parameter is assumed to vary linearly in the north-south direction. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude.

Blackbody

An object that absorbs all incident radiation and emits the maximum amount of energy at all wavelengths.

Blended precipitation estimate

An estimate that is derived by combining low earth-orbiting microwave measurements, which have high resolution but low frequency, with the more frequently available geostationary IR.

Bow echo

An organized mesoscale convective system, so named because of its characteristic bow shape on radar reflectivity displays. Bow echoes are typically 20–200 km long and last for 3–6 hours. They are associated with severe weather, especially high, straight-line surface winds, which are the result of a strong rear-inflow jet descending to the surface.

Brightness temperature

The Planck temperature associated with the radiance for a given wavelength.

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C

Center

Location of the vertical axis of a tropical cyclone, usually defined by the location of minimum wind or minimum pressure. The cyclone center position can vary with altitude.

Cloud track winds

Winds derived from tracking movement of cloud elements using IR and water vapor images from geostationary satellites.

Conditional Instability of the Second Kind (CISK)

A theory for tropical cyclone development that relates boundary layer moisture convergence (driven by Ekman pumping) to the potential for tropical cyclone intensification. As the storm intensifies, the moisture convergence must increase, providing a feedback to the system. As with WISHE, CISK relies on the presence of an incipient disturbance.

Coordinated Universal Time (UTC)

Same as Zulu (Z) and Greenwich Mean Time (GMT).

Coriolis parameter, f

A measure that is twice the local vertical component of the angular velocity of a spherical planet, 2Ω sinφ, where Ω is the angular speed of the planet and φ is the latitude.

Cyclogenesis

The formation of a cyclone.

Cyclone

An closed circulation of low pressure, rotating counter-clockwise in the Northern Hemisphere and clockwise in the SH.

Cyclone Phase Space (CPS)

A concise, three-parameter summary of the structure of a storm. It can be used to describe the structure of any synoptic or meso-synoptic cyclone.

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D

Deposition

The process by which molecules are changed from the vapor phase directly to the solid phase, such as from water vapor to ice.

Doppler Effect

The apparent shift in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave.

Doppler radar

Radar that uses the Doppler effect to detect radial velocity of targets based on the phase shift between the transmitted pulse and the received backscatter.

Dvorak Technique

a classification scheme for estimating the intensity of TCs from enhanced IR and visible satellite imagery. It is the primary method of estimating intensity everywhere, except the North Atlantic and North Pacific where aircraft reconnaissance is routine.

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E

Eddy angular momentum flux (EAMF)

Flux (net transport) of angular momentum into a circle centered on the storm. If EAMF is positive, the flow inside the circle will become more cyclonic; negative EAMF render the system less cyclonic (more anticyclonic). See Box 8-6 for a definition and discussion of angular momentum in tropical cyclones.

Ekman layer

Thin horizontal layer of water at top of the ocean that is affected by wind.  That layer has a force balance between pressure gradient force, Coriolis force and frictional drag.

Ekman layer instability

A shearing instability in a boundary layer where the mean flow results from a balance between the forces of friction, pressure gradient, and Coriolis. The instability appears as vortex rolls in the form of a spiral band.

Ekman pumping

The force balance determining the vector wind is modified by friction at the Earth's surface. The addition of friction changes the force balance to slow the winds and change their direction: winds now flow into a low and out of a high pressure system. Winds flowing into a low because of friction are forced upwards and out of the boundary layer. This process is known as Ekman pumping.

El Niño-Southern Oscillation (ENSO)

An oscillation of the ocean-atmosphere system in the tropical Pacific which affects global  weather and climate. El Niño, the warm phase of ENSO, is a quasi-periodic (2-7 years) warming of ocean surface waters in the equatorial and eastern tropical Pacific and an eastward shift in convection from the western Pacific climatological maximum. Changes occur in the tropical trade easterlies, vertical wind shear,  and ocean height. Cool ocean temperature anomalies are observed in the tropical western Pacific extending eastward into the subtropics of both hemispheres. "La Niña" refers to the less intense, anomalous  cool phase of ENSO. The Southern Oscillation refers to the atmospheric pressure difference between Darwin and Tahiti that is correlated with El Niño.

Electromagnetic (EM)

Energy carried by electric and magnetic waves.

Emission

The process by which a material generates electromagnetic radiation due to its temperature and composition.

Emissivity

The emitting efficiency of an object compared to an ideal emitter (or blackbody). A blackbody has an emissivity of one.

Emitter

Anything that radiates measurable electromagnetic radiation.

Empirical Orthogonal Function (EOF)

See Principal Component Analysis.

Energy

The capacity to do work or transfer heat. Measured in SI units as Joules.

Entrainment

The integration of unsaturated environmental air into the turbulent cloud-scale circulation. The antonym of entrainment is detrainment.

Explosive Deepening

A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 hPa hr^-1 for at least 12 hours or 5 hPa hr^-1 for at least six hours.

Extratropical

A term used to indicate that a cyclone has lost its “tropical” characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone’s primary energy source from the release of latent heat of condensation to baroclinic processes.

It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force. Given that these dangerous winds can persist after the cyclone is classified as extratropical, the Canadian Hurricane Centre (for example) follows them as “Former hurricane XXX.”

Extratropical Transition (ET)

The evolution of a poleward-moving initially tropical cyclone resulting in an extratropical cyclone. In the process of this evolution the energy source of the storm shifts from latent heat release to baroclinic development.

Eye (of tropical cyclone)

The approximately circular area of light winds at the center of a tropical cyclone. It is surrounded entirely or partially by clouds in the eyewall.

Eyewall / Wall Cloud

The full or partial ring of thunderstorms that surround the eye of a tropical cyclone. The strongest sustained winds in a tropical cyclone occur in the eyewall.

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F

Field of View (FOV)

Generally associated with the ground resolution from the detector standard viewing location, field of view is the solid angle through which a detector observes radiation.

Fraction of Photosynthetically Active Radiation (FPAR)

An index that measures how much sunlight the leaves are absorbing.

Frequency

The number of recurrences of a periodic phenomenon per unit time. The frequency, v, of electromagnetic energy is usually specified in Hertz (Hz), which represents one cycle per second.

Fujiwhara Effect

The mutual advection of two or more nearby tropical cyclones about each other. This results in cyclonic rotation of the storms about each other.

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G

Gale Force Wind

A sustained surface wind in the range 17 m s^-1 (39 mph, 63 km hr‑1 or 34 knot) to 24 m s^-1 (54 mph, 87 km hr‑1 or 47 knot) inclusive, and not directly associated with a tropical cyclone.

Geostationary or Geosynchronous orbit

An orbit whose rotation period equals that of the Earth. The altitude of a geostationary orbit is approximately 35,800 km. Its orbit keeps it above a single point on the equator.

GOES

Geostationary Operational Environmental Satellite (operated by NOAA).

GOES Precipitation Index

An estimate of precipitation that uses 235K as the IR temperature with the best correlation to average precipitation for areas spanning 50-250 km over 3-24 hours.

GPS

Global Positioning System, a network of defense satellites established in 1993. Each satellite broadcasts a digital radio signal that includes its own position and the time, accurate to one billionth of a second. GPS receivers use the signals to calculate their position to with a few hundred feet.

GPS radio occultation

The technique by which satellite receivers intercept signals from GPS and infer the deviations in the signal's path caused by temperature and moisture gradients.

Gravity waves

Oscillations usually of high frequency and short horizontal scale, relative to synoptic- scale motions, which arise in a stably stratified fluid when parcels are displaced vertically. Gravity is the restoring force.

Greenwich Mean Time (GMT)

Mean solar time of the meridian at Greenwich, England, used as the basis for standard time throughout most of the world. Also referred to as Zulu (Z) and Coordinated Universal Time (UTC).

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H

Hadley Cells

Circulation cells in which air rises in the ITCZ, sinks into the subtropical highs, and returns to the equatorial low along the trade winds. George Hadley proposed a model (1735) of the global atmospheric circulation with rising motion at the equator, where there is surplus heating, and sinking motion at the poles, where there is net cooling. Hadley's model did not account for the Coriolis effect, which leads to average westerly motion in the mid-latitudes. The Hadley model does explain the circulation within 30 degrees of the equator.

Horizontal Convective Rolls

Lines of overturning motion with axes parallel to the local surface. These rolls result from a convective instability (high density over low density – often corresponding to cool air over warm) and can mix strong winds from above down towards the surface.

Hurricane

A tropical cyclone in which the maximum sustained surface wind (using the local time averaging convention) is at least 33 m s^-1 (74 mph, 119 km hr^-1 or 64 knot). The term "hurricane" is used for in the Northern Atlantic and Northeast Pacific; "tropical cyclone" east of the International Dateline to the Greenwich Meridian; and "typhoon" in the Pacific north of the Equator and west of the International Dateline.

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I

Inertial period

The time taken to complete one rotation. In the tropical cyclone this is calculated by dividing the circumference at the radius of interest (commonly, the radius of maximum winds) by the wind speed at that radius.

Infrared (IR)

Electromagnetic energy within the wavelength interval generally defined from 0.7 to 100 microns.

Irradiance

The energy per unit time incident upon a unit area of a given surface, measured in SI units as Wattsm^-2.

Insolation

The incoming solar radiation that reaches the earth and its atmosphere.

Intensity

The peak sustained surface wind in the region immediately surrounding the storm center, or the minimum central pressure measured in the eye.

Intertropical Convergence Zone (ITCZ)

The zone where the northeast and southeast trade winds converge. It is marked by low pressure, rising motion, and thunderstorms, which occur with strong surface heating. Its latitudinal position shifts in response to the solar maximum and heating response of the surface. It is recognized in satellite images as a band of thunderstorms across the tropics. It is often, but not always, co-located with the zone of low pressure known as the "Equatorial Trough".

Intraseasonal

Varying on time scales shorter than one season.

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J

Joule

SI unit of energy equal to 0.2389 calories.

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K

Kelvin waves

At the equator, eastward propagating waves with negligible meridional velocity component and Gaussian latitudinal structure in zonal velocity, geopotential, and temperature, symmetric about the equator.

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L

Landfall

The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for the strongest winds to be experienced over land even if landfall does not occur.

Leaf Area Index (LAI)

The ratio of green leaf area to the total surface area occupied by vegetation.

Longwave (LW)

Electromagnetic energy lying in the wavelength interval generally defined from 4.0 microns to an indefinite upper limit.

Low earth orbit (LEO)

An orbit that is located at an altitude generally between 200 and 1000 km.

Low earth orbit satellite

A satellite that has a low earth orbit. Most have paths crossing the poles and can provide synchronous observations (e.g., the NOAA series or Defense Meteorological Satellite Program systems). The TRMM is an LEO satellite that orbits between ±35º latitude.

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M

Madden-Julian Oscillation (MJO)

Tropical rainfall exhibits strong variability on time scales shorter than the seasonal. These fluctuations in tropical rainfall often undergo a 30-60 day cycle that is referred to as the Madden-Julian Oscillation or intraseasonal oscillation. The MJO is a naturally occurring component of the Earth's coupled ocean-atmosphere system that significantly affects the atmospheric circulation throughout the global tropics and subtropics.

Maritime Continent

The region of Southeast Asia that comprises many islands, peninsulas, and shallow seas (including countries such as Indonesia, Malaysia, Papua New Guinea, and the Phillipines and covers approximately 12°S to 8°N, 95°E to 150°E).

Meridional

North-south, crossing latitudes; by convention the meridional wind from the south is positive.

Mesoscale

Spatial scale of 100-1000 km and temporal scale of hours to a day; between synoptic and convective scale. Tropical clouds are most often organized into mesoscale systems.

Mesoscale convective complex (MCC)

A large, quasi-circular mesoscale convective system that produces heavy rainfall and severe weather. In some MCCs, a mid-tropospheric vortex forms and remains after the deep convection has dissipated.

Mixed Rossby-Gravity (MRG) Wave

A divergent Rossby wave, resulting from conservation of potential vorticity and buoyancy forcing. These waves propagated westward along the equator. Meridional velocity is symmetric about the equator. Zonal wind, temperature, and geopotential area antisymmetric about the equator.

Monochromatic

Of or pertaining to a single wavelength, or in practice, perhaps a very narrow spectral interval.

Monsoon

A term whose roots are from the Arabic for "season", it is a seasonal wind reversal. The monsoon has inflow to a surface heat low and an offshore flow from high pressure during the winter when the land cools relative to the ocean. The Indian monsoon is the most prominent but it has been recognized that that monsoon region extends from Southeast Asia to West Africa. The summer monsoon is a vital source of moisture; its arrival, duration, and amount of precipitation modulates the economies of these regions.

Monsoon Gyre

A closed, symmetric circulation at 850 hPa with horizontal extent of 25° latitude that persists for at least two weeks. The circulation is accompanied by abundant convective precipitation around the south-southeast rim of the gyre.

Monsoon Region

Refers to the combination of features including a monsoon trough, confluence zone, and the ITCZ.

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N

Nadir

The satellite viewing angle directly downward (viewing zenith angle = 0 degrees). Also used to refer to the sub-satellite point location.

Navier Stokes equations

The set of three equations describing the time evolution of the atmospheric motions. They are often referred to as "the equations of motion" or "the momentum equations".

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Ocean conveyor belt

The name given to summarize the pattern of global ocean currents. The surface ocean currents generally transport warm salty water polewards, out of the tropics. The water cools as it moves polewards, becoming increasingly dense (remember that salty water is more dense than fresh water). This water sinks in the North Atlantic and also in the Southern Ocean near Antarctica. The deep water currents transport the water around the globe until it rises to the surface again, once more part of the surface ocean currents.

Opaque

A physical description of a material which attenuates electromagnetic radiation.

Optical depth

A measure of the cumulative attenuation of a beam of radiation as a result of its travel through the atmosphere.

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P

Pacific Decadal Oscillation (PDO)

The PDO is a basin-scale pattern of Pacific climate variability; PDO climate anomalies are most visible in the North Pacific and North American regions, with secondary features in the tropics. The phases of the PDO persist for 20-to-30 years. Causes for the PDO have not yet been explained.

Planck's Law

An expression for the variation of monochromatic radiance as a function of wavelength for a blackbody at a given temperature.

Planetary Boundary Layer (PBL)

The layer of the atmosphere that extends upward from the surface to heights of 100 to 3000 m. The boundary layer is directly influenced by surface forcing such as friction, heating, and evapotranspiration.

Polar orbit

An orbit whose path crosses the polar regions. This type of orbit is located at an altitude generally between 200 and 1000 km, and can provide sun-synchronous observations.

Polar Orbiting Environmental Satellite (POES)

A satellite which has a polar orbit, such as the NOAA series or Defense Meteorological Satellite Program systems.

Potential evapotranspiration

A measure of the maximum possible water loss from an area under a specified set of weather conditions.

Potential Intensity (PI)

The largest possible intensity (maximum wind, minimum pressure) expected to be possible for a particular tropical cyclone.

Potential vorticity

A scalar measure of the balance between the vorticity and the thermal structure of the atmosphere.

Principal component analysis

A mathematical technique for identifying patterns in data by reducing multidimensional data to a smaller number of dimensions. A number of variables that are (possibly) correlated are transformed into a new coordinate system. The transformation identifies the components that account for variability in the data. The first principal component often accounts for the most of variability in the data. Also known as Empirical Orthogonal Function (EOF) analysis.

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Q

Quasi-Biennial Oscillation (QBO)

An oscillation in the lower stratospheric zonal winds averaged around the equator. It is typically diagnosed from the zonal winds between 30-70 hPa (although it is evident as high as 10 hPa). The QBO has a varying from about 24 to 30 months. The zonal winds change by about 40 m s^-1 between the maximum easterly and maximum westerly phase.

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R

Radar (Radio Detection And Range)

An instrument that detects objects remotely by transmitting high-frequency pulses to the atmosphere and measuring the "backscatter" or echoed pulses from that object. Weather radar transmits microwave (mm-cm) pulses; the returned signal is interpreted to determine where it is precipitating.

Radiance

A measure of radiant intensity produced by a material in a given direction and per unit wavelength interval, measured in Watts/m 2 /steradian/micron. Monochromatic radiance is the most fundamental unit measured by satellite instruments.

Radiation

Energy transferred by electromagnetic waves.

Radius of Maximum Winds

The distance from the center of a tropical cyclone to the location of the cyclone's maximum winds. In well-developed systems, the radius of maximum winds is generally found at the inner edge of the eyewall.

Rapid Deepening

A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 hPa hr^-1 or 42 hPa for 24 hours.

Recurvature

The poleward motion of a tropical cyclone taking it from the mean tropical easterlies to the midlatitudes westerlies. This change in the advection of the storm results in curvature in the storm track.

Reflection

The process by which incident radiation is scattered in the backward direction (backscattered).

Reflectivity

The fraction of incident radiation reflected by a material.

Relative vorticity

See Vorticity.

Remnant Low

Used for systems no longer having the sufficient convective organization required of a tropical cyclone (e.g., the swirls of stratocumulus in the eastern North Pacific).

Retrieval

The process or end result of a process where physical quantities such as water vapor, temperature, and/or pressure are extracted from measurements of total upwelling radiance to space; here involving the GOES sounder.

Reyleigh Instability

Instability of a horizontal fluid layer heated from below or cooled from above. The onset of convection due to this thermal instability was first documented by Lord Rayleigh and Henri Bérnard (although the latter discovery was an instability due to surface tension differences that occur when a fluid is unevenly heated). Mesoscale cellular convection in the atmospheric boundary layer appears similar to Rayleigh-Bérnard convection.

Riming

The formation of ice by the rapid freezing of supercooled water drops as they impinge upon an object such as an ice crystal or aeroplane wing.

Rossby Radius of Deformation

The Rossby radius is the critical scale at which rotation becomes as important as buoyancy, which allows an initial disturbance to be sustained. It is a function of the absolute vorticity, stability, and depth of the disturbance. When a disturbance is wider than L_R, it will persist; systems that are smaller than L_R will dissipate.

Rossby Wave

A planetary wave, resulting from conservation of potential vorticity. Gradients of potential vorticity provide a restoring mechanism to allow propagation of the waves. This text focuses on Rossby waves centered on the equator equatorial (n=1) Rossby waves.

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S

Saffir-Simpson scale

A scale that links the observed damage and the effects of wind, pressure and storm surge that could lead to such damage. Initial wind damage scale was defined by Herbert Saffir and later expanded by Robert Simpson to include storm surge.

Scattering

The process by which a material interacts with and redirects incident radiation (in any given direction).

Scatterometer

A radar that infers near-surface wind velocity by sending pulses of microwave energy to the ocean surface and measuring the backscatter from small-scale waves. Scatterometry wind retrievals can be ambiguous during rain, since rain creates additional backscatter and attenuates the radar beam.

Shortwave (SW)

Electromagnetic radiation generally defined as having a wavelength shorter than 4.0 microns.

Size

The mean radius of a tropical cyclone enclose by winds of at least 17 m s^-1. Size may also be defined as the outer closed isobar of the surface pressure.

Solar declination angle

The angle between the rays of the Sun and the equatorial plane of the Earth. It is zero during an equinox and 23.5° during a solstice.

Southern Oscillation Index (SOI)

The normalized difference in sea level pressure between Darwin, Australia and Tahiti, French Polynesia.

Specific humidity

The mass of water vapor per unit mass of air (including water vapor), usually denoted by q and measured in units of grams per kilograms.

Spectral

A descriptor for radiometric quantities or measurements which have a limited wavelength range.

Split window

A pair of regions of the electromagnetic spectrum which are closely located in wavelength, but have slightly different attenuation characteristics. Used to denote the 11- and 12-micron regions in which greater water vapor attenuation at 12 microns causes slightly different brightness temperatures.

Stefan-Boltzmann Law

The energy emitted per unit area (from all wavelengths and represented by the area under the blackbody curve) is proportional to the 4 th power of the absolute temperature

Steradian

The unit of measure of solid angles, equal to the angle subtended at the center of a sphere.

Storm Surge

An abnormal rise in sea level accompanying a tropical cyclone or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

Storm Tide

The actual level of sea water resulting from the astronomic tide combined with the storm surge.

Subtropical Cyclone

A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.

The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.

A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing (dying frontal) zone of horizontal wind shear, with radius of maximum sustained winds generally less than about 50 km (30 miles). The entire circulation may initially have a diameter less than 160 km (100 miles). These generally short-lived systems may be either cold core or warm core.

Subtropical Depression

A subtropical cyclone in which the maximum sustained surface wind speed does not exceed 17 m s^-1 (39 mph, 63 km hr‑1 or 34 knot).

Subtropical Storm

A subtropical cyclone in which the maximum sustained surface wind speed is at least 17 m s^-1 (39 mph, 63 km hr‑1 or 34 knot).

Synthetic Aperture Radar (SAR)

Works like other radars except that it has very fine resolution in the azimuthal direction. It synthesizes the fine resolution normally achieved with a large antenna by combining signals from an object along a radar flight track and processing the signals as if obtained simultaneously from a single large antenna. The distance over which the signals are collected is known as the synthetic aperture.

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T

Trade Winds

Prevailing easterly winds flowing from the subtropical highs that affect equatorial and subtropical regions. Trade winds are mostly east to northeasterly in the Northern Hemisphere and east to southeasterly in the Southern Hemisphere. During the monsoon, easterly trades are replaced by mostly westerly winds.

Transmission

The process by which incident radiation propagates forward through a material.

Transpiration

The process by which water vapor enters the atmosphere through the stomata in the leaves of plants.

Thermocline

The inversion layer separating the near-surface warm waters from the colder, deeper layers of oceans and lakes.  It is about 1km deep and is thermally stratified. In the ocean, it also separates the fresher waters near the surface from the saltier waters below.

Total Precipitable Water (TPW)

The depth of water in a vertical column of unit cross-sectional area if the total atmospheric water vapor contained in that column, extending between the surface and the "top" of the atmosphere, completely condensed.

Tropical Cyclone

A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperatures and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects). Also see Hurricane.

Tropical Cyclone Season

The portion of the year having a relatively high incidence of tropical cyclones. Also known as "Hurricane Season" or "Typhoon Season".

Tropical Depression

A tropical cyclone in which the maximum sustained surface wind speed is not more than 17 ms^-1 (39 mph, 63 km hr‑1 or 34 knot).

Tropical Disturbance

A discrete tropical weather system of apparently organized convection – generally 185 to 550 km (100-300 n mi) in diameter – originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.

Tropical Storm

A tropical cyclone in which the maximum sustained surface wind speed ranges from 17 ms^-1 (39 mph, 63 km hr‑1 or 34 knot) to 33 ms^-1 (74 mph, 119 km hr^-1, 64 knot).

Typhoon

See Tropical Cyclone and Hurricane.

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U

Ultraviolet (UV)

Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays (approximately 0.03 to 0.4 microns)

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V

Virtual Potential Temperature

In a system of moist air the potential temperature of dry air having the same density and pressure as the moist air.

Visible

The region of the electromagnetic spectrum which is detectable to the human eye (approximately 0.4 to 0.7 microns).

Vorticity

The local rotation of the flow, calculated as the the curl (cross product) of the vector wind. Vorticity has units of inverse seconds (s^-1).

“Relative vorticity” is the vorticity calculated for the observed winds. It is called “relative” since the winds are the flow relative to the Earth’s rotation.
The vertical component of the vorticity vector is most often used since it is much larger than the other vorticity components. This is because the horizontal winds in tropical cyclones are much greater than the vertical wind component.

“Absolute vorticity” is the vorticity calculated for the total motion of the atmosphere the combination of the observed winds and the Earth’s rotation.

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W

Walker Circulation

The east-west circulation cells that form along the equator in response to differential surface heating.

Warning

A warning that sustained winds exceeding the threshold for either tropical storm or tropical cyclone and associated with such a storm are expected in a specified coastal area in 24 hours or less.

Watch

An announcement for specific coastal areas that either tropical storm or tropical cyclone conditions are possible within 36 hours.

Wavelength

The distance a wave will travel in the time required to generate 1 cycle, denoted by λ. A length measured from the midpoint of a crest (or trough) to the midpoint of the next crest (or trough).

Wavenumber

The reciprocal of the wavelength, denoted by κ.

Water Vapor Channel (or water vapor IR channel)

A spectral band in which the radiance is attenuated by water vapor. This usually refers to the 6.7 micron channel in this module.

Weighting function

A mathematical expression representing the relative radiance contribution provided from a given level of the atmosphere (usually a function of atmospheric pressure).

Wind-Induced Surface Heat Exchange (WISHE)

A tropical cyclone development theory based on a conceptual model of a tropical cyclone as an atmospheric Carnot engine. Consistent with its Carnot engine roots, WISHE relates (i) fluxes of heat and moisture from the ocean surface and (ii) the temperature of the tropical cyclone outflow layer to the potential for continued storm development. The fluxes increase with surface wind speed providing a feedback to the system. As with CISK, WISHE relies on the presence of an incipient disturbance.

Wind profiler

Vertically pointing radar which operates on the same principle as horizontally-scanning Doppler radar; provides best measurements of vertical air motion inside convective storms

Wien's Displacement Law

The wavelength of maximum blackbody emission is inversely proportional to its absolute temperature.

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X

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Y

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Z

Zonal

East-west, crossing longitudes; by convention, the zonal wind from the west is positive.

Zulu (Z)

Used to represent the same clocktime at GMT and UTC. See Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC)

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