Mitigating Excessive Drying from the Use of Observations in Mesoscale Modeling

Abstract

Observations can be used to enhance mesoscale model forecasts by both improving the initial conditions and adjusting the model state toward the observations during its forward integration. Since observations are not available at the same density as the model grid points, spatial and temporal error correlations must be specified in order to effectively assimilate the observations into the model. Overdrying is found while applying the Weather Research and Forecasting Model to five cases centered over Southern California. This overdrying is caused by incorporation of observations in the initial conditions both at a discrete time and over a preforecast data assimilation period. Nonphysical dryness results from the assumption that model moisture error at the observation location is correlated with model moisture error at other locations in the model without regard to the relative magnitude of the moisture at those two locations. Modifying the model and its preprocessor to remove this assumption greatly reduces the occurrence of excessive model dryness without degrading the overall model performance in predicting moisture.

Assimilation of Aircraft Observations in High-Resolution Mesoscale Modeling

“…The Mellor-Yamada-Janjić (MYJ) scheme [23] is applied to parameterize the atmospheric boundary layer, modified as in Reen et al [24]. Microphysics are represented using the Thompson microphysics scheme [25] and on the coarser two domains (27 and 9 km) the Kain-Fritsch cumulus parameterization [26] is invoked.…”

“…The Noah land surface model [29] represents land surface processes. This combination of physics parameterizations is very similar to that used in Reen et al [24] which applied a 9 km WRF domain centered over southern California; the only difference is the use of the Thompson microphysics scheme here. All of the physics parameterizations chosen for this study have been widely used in other studies, with the exception of the modifications to the MYJ scheme which have however been applied previously over this region [24] and in other studies [30,31].…”

“…Five case days were simulated over the southwestern United States; simulations started at 12 UTC on each of the five days (7 February,9 February,16 February, 1 March, and 5 March 2012) and ended at 12 UTC on the following day. Since these are the same case days used in Reen et al [24], the case description from that study is used in this section with minor modifications. Within the five case days, there are days with and without strong synoptic forcing.…”

“…Nudging. TAMDAR observations are assimilated during a 6 h preforecast (12-18 UTC) using observation nudging [18,24,33]. For each TAMDAR observation of temperature, moisture, and wind, the difference between the WRF-ARW value and the TAMDAR value (the innovation) is calculated throughout the time period when the model is sufficiently close to the observation time.…”

“…An overall observation nudging weight of 8 × 10 −4 s −1 is applied here. A modification to mitigate overdrying by observation nudging developed in earlier research [24] was also applied.…”