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Tips for Post-frontal Stratus Forecasting over the Upper Midwest

(last updated 8/3/2005)

From Philip Schumacher, NWSFO FSD (Notes from the post-frontal stratocumulus forecast discussion, personal notes and communication)

Elements for stratocumulus formation and maintenance:

• Broad areas of low-level moisture (below 800 hPa)
• Inversion with moisture located below
• Cyclonic flow
• Low-level lift
• Low level instability (formation only)
• Warm air advection above inversion and/or cold air advection below inversion
• Subsidence above inversion

Elements for dissipation of stratocumulus:

• Advection of cold air aloft or warm air at the surface
• Dry air advection into lower levels
• Solar heating (mainly in warm season) so surface hits convective temperature
• Subsidence forcing the inversion to the ground
• Mechanical mixing of moisture

What to examine for possible initiation:

1. Observed soundings:

• Compare postfrontal sounding to "near front" sounding
• Examine changes in inversion depth temporally and spatially
  • Inversion persistence is favorable for post-frontal Sc development
• Compare observed sounding to model soundings
  • Is the inversion as strong in the model as observed?
  • Does the model have the sub-inversion layer as saturated as the observed soundings?

2. Satellite imagery:

• Compare location of cloud edge to low level RH in models
• Compare evolution of cloud edge movement to changes in RH field in the models

3. Model tendencies to note:

• Tendency to dry out low-level RH too fast
  • Compare 24-hr forecast differences in RH rather than 12-hr changes
  • Look for gradients in RH rather than focusing on a specific value (i.e., 70 or 90%)
• Tendency to lower or weaken the inversion too fast
  • Check if surface or lower boundary layer flow goes to southwest. If not, then inversion is likely to persist.
  • Compare low-level (near or just above inversion) to surface (below inversion) temperature advection. Warm air advection above and /or cold air advection below will help maintain or strengthen the inversion.
• Do not handle evaporation from local water bodies well
  • Look for local moisture sources, especially when the air temperature is much colder than the water temperature (mainly in autumn)
  • Lakes that could have large influence are Lake Winnipeg, Lake Manitoba, Lake of the Woods, Upper and Lower Red Lakes, and Devils Lake

The tendencies in models to both weaken the inversion and dry out the low levels too fast are the result of the model trying to create more continuous gradients in temperature, wind, and moisture. To do this, the model has a diffusion routine to prevent what it sees as unrealistically large gradients. In addition, the models do not do a great job of handling surface sources of cooling (like snow cover) and typically allow the surface to heat more readily and affect the lowest layers. Finally, model terrain does not have sufficient resolution to accurately simulate topographical features such as the Red River Valley and, therefore, have difficulty in accounting for the deeper inversions that occur over this region.

4. Model parameters to examine for initiation:

Low-level cyclonic vorticity and average 900-800 hPa layer RH:

• Assumptions
  • Ekman layer develops, which leads to cyclonic flow with low-level lift
  • 900-800 hPa layer is representative of low-level moisture
  • Atmosphere will become unstable enough for Ekman pumping to become efficient lifting mechanism
• Strengths
• Accounts for a dynamic mechanism for lift that does not require convective instability
• Models are generally better at predicting low-level vorticity than lift
• Maximum cyclonic vorticity tends to pinpoint areas where stratocumulus will last for the longest time
• By overlaying the 900-800 hPa layer RH field, you can see where the atmosphere may be too dry to support stratocumulus, even when ample cyclonic vorticity exists
• Weaknesses
  • Once stratocumulus is in place and when the sun angle is low, Ekman pumping is no longer necessary to maintain the cloud deck
  • In the cold season, 900-800 hPa layer may be too high to be representative of low-level moisture
  • If no inversion exists or is weak, using this method may over-predict stratocumulus by not accounting for mixing at the top of the inversion, thereby eroding the clouds from above

The Minneapolis cumulus rules (difference in the surface boundary layer dewpoint and the 850 hPa temperature):

MSP Cu rule thresholds:

T850 - Tsfc	Skies
0 to -2	SCT cumulus
-3 to -5	BKN cumulus
< -6	OVC skies with SHRA

• Assumptions
  • Atmosphere mixes to 850 hPa
  • Atmosphere will become convectively unstable
• Strengths
  • In summer can be a good indication of when cold air aloft will result in Cu formation
  • Provides indication of amount of sky expected to be covered with Cu
  • Does not rely on model correctly forecasting low-level RH
• Weaknesses
  • If inversion is below 850 hPa, will result in indicating too little Cu
  • Once Sc is in place, will not forecast erosion of the Cu field very well, especially in the cold season when sun angle is to low to allow for efficient mixing

Boundary layer convergence and relative humidity:

• Assumptions
  • Divergence = subsidence and convergence = lift
  • Lowest 50 hPa Rh is representative of low-level moisture
• Strengths
  • 950 to 900 hPa layer is probably below the inversion throughout the year
  • Convergence (even weak) can help pinpoint locations of thicker Sc, especially when moisture is present
  • By looking at lowest layers for convergence, it confines lift to below the inversion
• Weaknesses
• Once Sc is in place and when the sun angle is low, lifting mechanism is not necessary to maintain Sc
• If inversion is not in place, it will over-predict cumulus
• In Northern plains, 950 hPa may be too close to the surface (at times). Therefore, changes in RH may be due to diurnal temperature changes rather than advection or mixing of drier air.

Parameters to examine for maintenance or dissipation:

• Overlay the boundary layer RH with the surface winds. As long as the RH is high and the surface winds remain southerly or southeasterly, stratus probably will not dissipate
• Overlay the two lowest levels of RH, along with the surface wind, and the lapse rate between two layers centered on the inversion level. With little change in the RH and lapse rate, you can see that the inversion is remaining in place. Even if model decreases the RH a little but lapse rate remains nearly stable, then stratus is unlikely to dissipate.
• For the same layers, overlay the 6-hour change in RH and lapse rate. By plotting the changes in both, you can examine if there are significant changes in either parameter that would allow for increased mixing or significant drying below the inversion
• Overlay the differential temperature advection centered on the inversion level. This will help to evaluate whether the large-scale processes are acting to affect the inversion. In many cases, the models may try to warm the surface in order to decrease the lapse rate or use mixing to decrease the lapse rate. In a lot of cases, especially in winter or with very strong inversions, neither mechanism will directly affect the inversion. Instead, only temperature advection can readily weaken the inversion.

Other model techniques:

• Use time sections to look for strong mid-level subsidence. If there is strong subsidence, it will eventually erode the clouds. However, if there is weak subsidence, it will not be strong enough to erode the clouds but will begin to lower ceilings.
  
• Use forecast soundings to see if and how fast the inversion is lowering. If realistic (compare to observed soundings), you can see if the inversion could begin thinning the stratus from above so that even the inefficient mixing could begin to poke holes in the cloud deck.