In-depth look at a precip "bomb" in the Eta model

The 22-km operational Eta model began making precipitation bull's-eyes over the plains in mid-July 2001 and continued generating such episodes after the 24 July 2001 model upgrade. These precipitation bull's-eyes with amounts over four inches, mostly in a 6-hour period, are being generated primarily by the grid-scale scheme in high-CAPE environments with strong mesoscale forcing. The forecast events are spurious and unphysical but are occurring under conditions favoring strong convective systems with heavy rains. These grid-scale storms become the model's focal point for convergence and vertical motion, causing the forecast to miss the area where the MCS actually occurs.

Warning - Eta might not still make precip "bombs" This case occurred in summer 2001. The new grid-scale precipitation scheme introduced with the 12-km Eta on 27 Nov 2001 has a higher saturation threshold, allowing the convective parameterization to take care of its business more before grid-scale clouds are generated requires more cloud water before producing precipitation advects precipitation downwind, producing more diffuse patterns rather than grid-point bulls eyes. All of these factors should reduce grid-scale storms. Warm season tests on the 10-km central US nest showed less problem with precipitation bulls eyes. However, warm season testing covered too small a sample to definitively conclude spurious grid-scale storms won't ever occur. Additionally, this case study may still be useful because cases of grid-scale convection are still common in other models you will see in detail the effect of grid-scale convection on model forecast fields you will see model soundings showing that the convective parameterization is unable to stabilize the environment in these situations of strong mesoscale convergence of high-CAPE air - a condition that will still occur in the Eta model even if these precipitation bulls eyes do not occur you will learn how the BMJ convective parameterization combines with the grid-scale scheme to make an MCS.

The purpose of this case example is to make forecasters aware that the Eta model was exhibiting this behavior in the most recent summer season and to probe what is going on in the model as these events develop. Previously, the Eta using the BMJ convective parameterization had never generated such episodes of grid-scale convection. The problem is more widely known in the AVN and in the NSSL experimental version of the Eta running with the Kain-Fritsch convective parameterization.

The example examined here is from the 12 UTC 26 July 2001 22-km operational Eta model run. On the following pages you will see what happens inside the model before and during two episodes of grid-scale thunderstorms, one over South Dakota during the day-one forecast period and the other over Kansas during the day-two forecast period. Another case example will be developed to show the role of resolution on forecast impacts, comparing events in the AVN, the 12-km operational Eta with the new (late fall 2001) Eta grid-scale precipitation scheme, and perhaps others such as one of the 48-km short-range ensemble runs or a case from the experimental high-resolution nonhydrostatic Eta model.

An example of this kind of event occurring in a research model, with comparisons of simulations using 50-km, 18-km, 6-km, and 2-km grid spacing, appears in Belair and Mailhot (2001). Their 18-km results appear similar to forecasts from the 22-km Eta model. As regional operational models move toward finer resolution, parts of the convective system circulation will be explicitly resolved, so that more of the precipitation should be produced by the grid-scale scheme.

In going through this case, when you click the Display Graphic button , an image or loop will pop up in a separate window. If one is already displaying, it will be replaced by the one you just selected. Sometimes a loop may only load one picture — just click the button again and it should work. Loops automatically start looping. To freeze a frame, click the Stop button . To advance one frame, use the Plus 1 button . To resume looping, use the Forward button . Loop speed is increased by repeatedly clicking the Fast button and decreased by doing the same to the Slow button .

Credits

By Dr. Stephen Jascourt, UCAR/COMET

Thanks to:

• Keith Brill for GEMPAK help
• Bill Bua for some data and graphics
• Dave Reynolds for first calling attention to the model problem

References

Belair, S. and J. Mailhot, 2001: Impact of horizontal resolution on the numerical simulation of a midlatitude squall line: Implicit versus explicit condensation. Mon. Wea. Rev., 129, 2362-2376.

Bryan, G. H. and J. M. Fritsch, 2000: Moist absolute instability: The sixth static stability state. Bull. Amer. Meteor. Soc., 81, 1207-1230.