No 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.

Approved for public release; distribution is unlimited.ii REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

To investigate synoptic interactions with the San Andres Mountains in southern New Mexico, the Weather Research and Forecasting (WRF) model was used to simulate several days in the period 2018–2020. The study domain was centered on the U.S. Department of Agriculture (USDA) Agricultural Research Service’s Jornada Experimental Range (JER) and the emphasis was on synoptic conditions that favor strong to moderate winds aloft from the southwest, boundary layer shear, a lack of moisture (cloud coverage), and modest warming of the surface. The WRF simulations on these synoptic days revealed two distinct regimes: lee waves aloft and SW-to-NE oriented Longitudinal Roll Structures (LRS) that have typical length scales of the width of the mountain basin in the horizontal and the height of the boundary layer (BL) in the vertical. Analysis of the transitional periods indicate that the shift from the lee wave to LRS regime occurs when the surface heating and upwind flow characteristics reach a critical threshold. The existence of LRS is confirmed by satellite observations and the longitudinal streak patterns in the soil of the JER that indicate this is a climatologically present BL phenomenon.