3.2 Use of Satellite Imagery

3.2.2 Polar-Orbiting Satellites

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Polar-orbiting satellites provide excellent spatial coverage of the Earth’s polar regions, and are equipped with an increasing and impressive array of multispectral imaging and all-weather sounding capabilities. In addition to the standard visible and infrared imagery, active and passive microwave instruments are providing information on atmospheric temperature and moisture, integrated water vapor, cloud water and ice, precipitation, and sea surface winds.

3.2.3 Standard Visible and Infrared Imagery

Both geostationary and polar-orbiting infrared and visible satellite imagery can reveal critical information on the presence or formation of low-level cloud features commonly associated with polar low precursors.

An outbreak of cold polar air flowing from land or ice out over a relatively warm ocean surface is one of the key precursors to polar low formation. The low-level instability associated with cold air outbreaks over relatively warm water leads to the formation of open-cell cumuliform cloud patterns often seen as cloud streets in visible and infrared imagery. Closed-cell cloud formations, on the other hand, are associated with more stable air masses or air masses that have been significantly modified by the ocean surface.

This image illustrates both stable and relatively unstable air masses separated by a secondary low-level baroclinic zone. The cloud band marks the baroclinic zone where polar low formation would be favored.

The presence of upper-level forcing, or some other triggering event, is key to the formation of a polar low and can often be detected with visible and infrared satellite imagery. For example, the "comma" cloud signature, is associated with vorticity advection in advance of the short wave and is one of the more recognizable features linked to the forcing and subsequent formation of polar lows.

The examples shown here illustrate key cloud features associated with polar low formation and development.

Vortex Formation

Polar low vortex formation can typically be seen as an organization of convective clouds into a more circular pattern with a dry slot appearing near the center of circulation.

Spiraliform and Comma

Spiraliform and comma signatures are the most common organizing cloud patterns observed with developing polar lows. The spiraliform signature is characterized by bands of convective cloud elements curving inward towards the polar low’s center of circulation and marking development of the low beyond the cyclogenesis phase. The comma signature also indicates development beyond the cyclogenesis phase, but is typically smaller in dimension than the upper-level comma and may take on a more blurry appearance. Active deep convection within the comma-head region generates cirrus outflow that forms an anticyclonically curved cloud shield.

Cloud-free Eye

A cloud-free or nearly cloud-free eye is a characteristic trait of mature polar low, indicating the presence of a low-level warm core within the low and potentially strong surface wind conditions.

Merry-go-round Signature

Example of a merry-go-round signature with multiple polar lows or mesoscale cyclones. Arrows indicate position of the cyclones.

The merry-go-round signature occurs when multiple polar lows or mesoscale cyclones form within the circulation of a cold upper-level cyclone. The mesoscale cyclones are thought to form in response to upper-level shortwave troughs embedded within the larger scale flow of cyclone. These cold core systems may be part of the circumpolar vortex or represent the latter stages of an occluded synoptic cyclone.

Infrared imagery can also provide an indirect means of establishing the depth of the polar low circulation by assigning cloud heights to the deeper convective cloud elements. This can be accomplished by matching cloud-top temperatures with radiosonde observations, numerical model temperature profiles, or a representative climatological sounding.

3.2.4 Shortwave Infrared Channels

Daytime and nighttime 3.7- and 11-micrometer imagery used in combination can improve the identification of polar low cloud types and characteristics. Developing and mature polar low systems are highly convective in nature and contain significant quantities of liquid water and ice. A precursor upper-level comma cloud, potentially mistaken for a polar low, would contain more ice and could be distinguished by utilizing the 3.7-micrometer channel's sensitivity to water and ice.

High resolution shortwave infrared imagery is available onboard NOAA and NASA-EOS Terra and Aqua satellites. Lower spatial resolution (4km GOES, 3km MSG) shortwave imagery is also available from geostationary satellites including GOES and MSG.

11 µm IR    11-3.8 µm

3.2.5 Passive and Active Microwave Observation

Microwave imagery available from the NOAA-AMSU and DMSP-SSM/I sensors can assist with the detection of developing polar low circulation centers. This is similar to applications developed for locating tropical cyclone centers. The higher microwave frequencies (85-GHz channel with SSM/I; 89- and 150-GHz channels with AMSU) can penetrate high thin cirrus, revealing the structure of cloud formations, precipitation, and surface winds organizing around the center of circulation. Cloud water appears relatively bright/warm at these frequencies (SSM/I 85.5 GHz; AMSU 89- and 150-GHz) whereas cloud ice scatters microwave radiation and diminishes/cools the observed brightness temperature values seen by the instrument. These differences provide another tool for assessing the intensity of developing polar low events.

Recent research work has demonstrated the efficacy of the AMSU 53-GHz channel. As it is sensitive to low-level atmospheric temperature, it can reveal the warm-core structure of moderate-to-strong polar low events (Moore and VonderHaar, 2003).

3.2.6 Microwave Observation for Sea Surface Winds

Active microwave scatterometry and passive radiometry provide indirect measurements of winds at the sea surface from polar-orbiting satellites and can be powerful tools to assist in both the detection and intensity of polar low circulations.

Quikscat, ERS-2, SeaWinds, and the future ASCAT instrument to be flown on Metop, the next generation European polar-orbiting satellite, incorporate scatterometers to transmit and receive microwave radiation and provide both wind speed and direction. Wind vectors for most of these systems are capable of measuring wind speeds between 0 and 20 m/s with a 2 m/s accuracy, and have a directional accuracy of about 20 degrees.

 

Current passive microwave radiometers such as the SSM/I are only capable of measuring emitted microwave energy. As a result, only wind speed and not direction can be measured. Derived SSM/I wind speeds can range from 0 to 40 m/s with a 2 m/s accuracy and are valid at 10 meters above sea level.

SSM/I derived surface wind speeds over open sea for a polar low near 65 deg N, 2 deg E. The black arrow indicates storm center as estimated from IR imagery.

A new passive microwave observing technology is being developed for NPOESS, the next generation U.S. polar-orbiting series of satellites. It will provide both wind speed and direction.

References

Bader, M.J., G.S. Forbes, J.R. Grant, R.B.E. Lilley, and A.J. Waters, 1995: Images in weather forecasting. Cambridge University Press.

COMET Module: Satellite Meteorology: GOES Channel Selection https://www.meted.ucar.edu/training_module.php?id=784

COMET Module: Polar Satellite Products for the Operational Forecaster, Module 1: POES Introduction and Background http://www.meted.ucar.edu/ist/poes/index.htm

Heinemann, Günther, 2001: Newsletter of the European Polar Low Working Group, September 2001.

Heinemann, Günther, 2003: Proceedings of the European Geophysical Society (EGS) Polar Low Workshop, Cambridge, UK, 11-13 June 2003.

Hewson, T.D., G.C. Craig, and C. Claud, 2000: Evolution and mesoscale structure of a polar low outbreak, Q. J.R. Meteorol. Soc., 126, 1031-1063.

Moore, Richard W. and T.H. Vonder Haar, 2003: Diagnosis of a polar low warm core utilizing the Advanced Microwave Sounding Unit. Weather and Forecasting, 18, 700-711.

Parker, M.N. and Edward Hudson, 1991: Polar Low Handbook for Canadian Meteorologists. Environment Canada, Atmospheric Environment Service.

QuikScat data and images are produced by Remote Sensing Systems and sponsored by the NASA Ocean Vector Winds Science Team.

Rasmussen, Erik A. and John Turner, 2003: Polar Lows: Mesoscale Weather Systems in the Polar Regions, Cambridge University Press.

SSM/I data and images are produced by Remote Sensing Systems and sponsored by the NASA Pathfinder Program for early Earth Observing System (EOS) products.