P13 Mitigating excessive drying caused by observation nudging

Reen, Brian P., Robert E. Dumais, Jr., Jeffrey E. Passner, United States Army Research Laboratory

WRF-ARW observation nudging provides the capability to assimilate observations of moisture, temperature, and wind in a computationally efficient manner. Recent WRF-ARW simulations using observation nudging resulted in regions where the model predicted water vapor mixing ratios of zero. Based on further investigation we made changes to resolve the issue. These changes will be presented.

In order to limit the ability of observation nudging to nudge moisture to zero, the observation nudging technique was altered. The magnitude of any negative water vapor mixing ratio innovation applied at a given point was limited to not exceed the model water vapor mixing ratio at that point. This change is generally effective in preventing observation nudging from creating excessively dry conditions, when this change is implemented in concert with other modifications.

Another modification found to decrease the prevalence of excessively dry conditions was altering the MYJ PBL scheme to decrease the background turbulent kinetic energy and change the PBL depth diagnosis technique. As expected, decreasing the nudging strength also decreased the occurrence of excessively dry areas. Additionally, we modified the Cressman analysis in OBSGRID that is used to enhance initial conditions with observations to avoid introducing very dry areas.