Notes for Convective Weather Refresher

Notes for Convective Weather Refresher - Notes for...

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Notes for Convective Weather Refresher Usage recommendation: Print these notes, then view the presentation in "Slide Show" mode using the notes as a guide. The notes here are identical to those attached to each slide in the Powerpoint file. Slide, topic Explanation (self-explanatory pages and self-evident main points omitted) 4 Background Here is an example of what happens when the convective parameterization fails to relieve instability fast enough. On the far left is a model sounding showing a saturated layer with a lapse rate steeper than moist-adiabatic. This layer is produced by strong mesoscale convergence and lift in the resolved grid-scale motions and is a characteristic signature of such lift in a moist environment. The plot next to it shows vertical velocity at the same time in green, an hour later in blue, and two hours later in magenta. The saturated unstable layer is at the top of the region of strong ascent. The model continues to lift this region, forming a grid- scale cumulonimbus rising over several hours. The extreme vertical velocities clearly violate the model’s hydrostatic assumption, making the pressure and velocity forecast evolution physically unrealistic. The strong convergence and vertical motion bulls-eye results in a locally-intense vorticity maximum. On the US map, the pre-storm ambient vorticity is shown, with typical values of 3 to 12 x 10 -5 s -1 . The upper zoomed-in area shows 3-hour precip contours and shaded vorticity (same color scale) at the time of the sounding shown in the left-most plot, which was for a grid point to the southeast of the precipitation bulls-eye at that time. The vorticity has now increased to over 20 x 10 -5 s -1 . The lower zoomed-in area shows the same for 3 hours later, with the vorticity maximum exceeding 39 x 10 -5 s -1 co-located with the precipitation bulls-eye near the location of that sounding. Precipitation at that sounding location is listed in the table. The convective parameterization did trigger, but it could not relieve instability fast enough to prevent the model from making a thunderstorm updraft core on the grid scale. Most of the precipitation coming from the grid-scale scheme in a convective situation is a classic characteristic of a model grid-scale blow-up. A grid-scale thunderstorm is desirable in a very high resolution nonhydrostatic model (the storm would grow more rapidly and have a narrower updraft), but is exactly what must be avoided in operational NWP models that are not at high enough resolution to resolve a convective updraft. The purpose of the convective parameterization is to avoid this scenario. 5 Background Note how the sharpness of the trough as well as the wind speeds and CAPE are different in these two model runs. The models were the same except for the convective parameterization. The model convection affected not only the precipitation but also the model mass, horizontal wind, and vertical motion fields.
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6 Background The skill of model precipitation forecasts in convective situations is poor, even for
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This note was uploaded on 04/01/2008 for the course MEA 214 taught by Professor Staff during the Spring '08 term at N.C. State.

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Notes for Convective Weather Refresher - Notes for...

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