WRF simulations of mesoscale convective systems at convection-allowing resolutions

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Duda, Jeffrey
Major Professor
William A. Gallus, Jr
Committee Member
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Geological and Atmospheric Sciences

Mesoscale convective systems (MCS) were studied using both idealized and real data

WRF simulations using grid spacings in the range from 0.5 km to 12 km with an emphasis on

3 km to determine the necessity of a convective parameterization scheme. The Kain-Fritsch

(KF) convective parameterization scheme was used as it is considered to give the best

forecasts of precipitation in mesoscale models.

The idealized simulations were used to conduct three sets of sensitivity tests. One set

tested the ability of the model to adequately resolve typical two-dimensional squall line

structure by varying the vertical grid resolution. It was determined that using 81 vertical grid

levels was sufficient to model squall lines. A second set tested the sensitivity of the partition

of precipitation into microphysics and convective scheme components to horizontal grid

spacing. A zone of grid spacing values from about 1 km to about 6 km was identified over

which the partition shifts from approximately 10% of precipitation from the KF scheme to

anywhere from 60% - 100%. This zone was found to be insensitive to microphysics scheme

and somewhat sensitive to initial conditions. The amount of precipitation produced per

activation of the deep convective part of the KF scheme was also found to shift significantly

across this range of horizontal grid spacing values. The third set tested the sensitivity of

precipitation forecasts to five treatments of the scheme. While two of the treatments

included the lack of a convective scheme and the unmodified KF scheme, the other three

treatments involved modifications to the scheme. These modifications included removing

the linear dependence of grid-resolvable vertical velocity on grid spacing, coarsening the

vertical motion, temperature, and water vapor mixing ratio fields before the KF scheme ran,

and coarsening the heat and moisture tendencies as well as convective scheme precipitation

after the KF scheme ran. When applied to a set of three-dimensional real data cases, it was

found that the use of no convective scheme and the unmodified KF scheme generally

performed the best. However, due to a small sample size, the spread of the data was large

and more tests are needed.

The conclusions from the sensitivity tests were that the KF scheme becomes less

active as grid spacing decreases below horizontal grid spacings of 6 - 8 km. Below 1 km

grid spacing, the KF scheme certainly should not be used. However, from 1 km to 3 km, it

likely is not necessary to use the KF scheme. Using it at 3 km does not hurt the forecast,

however. As other research has shown, there is some use for the KF scheme above 3 km.

Three-dimensional real data WRF simulations were conducted at 3 km horizontal

resolution for a set of 39 cases involving MCSs across the United States. The KF scheme

was not used. Convective initiation was found to err by approximately 150 km in the westsouthwesterly

direction with a nearly zero mean timing error. Large scatter was found

between the strength of large-scale forcing and the model skill at forecasting initiation, but

traditional skill measures (ETS and bias) showed that stronger-forced cases were better

forecast in the upscale evolution of the MCSs. Case studies were performed for a few cases

to illustrate the ways in which the WRF can succeed or fail to accurately predict convective


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Sat Jan 01 00:00:00 UTC 2011