GFS T170 Grid-Scale Precipitation Bomb during Flood Event in IA

vs

Abstract

This case discusses one occurrence of a well-known problem with the GFS; grid-scale precipitation bombs at T170L42 resolution. Anecdotal evidence suggests that these precipitation bulls'-eyes have been more frequent this warm season, particularly in the central and northern Plains and Midwest. The time-scale, spatial scale, and effect of the GFS precipitation bombs on the forecast are examined. We also discuss whether GFS forecasts that produce such features can be useful to the operational meteorologist.

Learning objectives

In this case study, the main lessons are:

1. Heavy precipitation bulls-eyes result from the inability of the GFS convective scheme to remove instability in the presence of a deep, conditionally unstable moist layer. This typically occurs in regions of overrunning during the warm season, but can occur at all seasons in the proper environment.
2. The grid-scale scheme attempts to relieve the instability, resulting in a grid-scale thunderstorm.
   • Latent heating from condensation in deep conditionally unstable layer results in development of:
     • A strong updraft through the depth of the troposphere, with a velocity maximum at 400-500 hPa and maximum velocity approaching 0.5 ms^-1.
     • A convergence/divergence couplet vertically straddling the updraft (from mass conservation). The lower troposphere convergence feeds additional moisture into the system and maintains instability within the updraft for extended periods.
     • A vorticity maximum develops below the area of maximum vertical velocity produced by the grid-scale thunderstorm. in response to vertical stretching of the atmospheric column.
   • Large quantities of cloud ice are produced in the upper troposphere, and heavy grid-scale precipitation occurs in an area a few grid boxes in size.
3. If a grid-scale downdraft can be produced by the precipitation scheme (through evaporative cooling, for example), then the grid-scale thunderstorm is suppressed.
4. The effects of the grid-scale thunderstorm are to:
   • Overdevelop or even create spurious frontal waves
   • Spin up spurious vorticity maxima in the middle troposphere
   • Produce what may be spurious precipitation maxima (or precipitation maxima through the wrong processes, i.e. through the grid-scale rather than convective precipitation parameterization).

    

In going through this case, when you open a separate graphic, 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. 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. Bill Bua

Thanks to:

Bill Nichols, lead forecaster at Davenport, IA, for bringing attention to this case

Mike McClure, forecaster at Davenport, IA for reviewing this case

Stephen Jascourt (for discussions, and review)

START CASE