The Influence of Irrigated Soil Moisture on Modeled Land-Atmosphere Interactions and Simulated Flows in the San Joaquin Valley, California

California’s San Joaquin Valley (SJV) is subject to some of the most complex atmospheric dynamics within the United States, which present challenges to both effective weather and air quality forecasting and water resource and fire hazard management strategies. During the summer, the dynamics in the region are characterized by weak synoptic forcing, up/down valley flows, and entrapment of air between the Costal, Tehachapi, and Sierra Nevada mountain ranges. Moreover, smaller-scale land-surface characteristics, such as heterogeneous land use and variable irrigation, and localized katabatic/anabatic winds from the surrounding mountain slopes lead to atmospheric conditions that vary significantly in space and time. The importance of both regional and microscale dynamics and their relationship in the SJV present a unique challenge to modeling land-atmosphere interactions and the planetary boundary layer; any simulation requires a large enough domain to accurately portray regional flow patterns, but also demands sufficiently fine resolution to ensure that energy partitioning, land use, and planetary boundary layer parameterizations have appropriate boundary conditions to accurately portray the region. Of particular importance in the SJV is the accurate representation of irrigated soil moisture as a boundary condition in models due to its widespread use within the basin during the long, dry Mediterranean summer and its influence on the partitioning of surface energy. We analyze the performance of Weather Research and Forecasting (WRF) model simulations within the SJV during typical summertime conditions to investigate how irrigated soil moisture is represented in various land-surface models and subsequently, quantify how these depictions impact energy balance partitioning, surface and air temperatures, low-level winds and biases in planetary boundary layer heights. Specifically, we compare observations from the California Baseline Ozone Transport Study airplane flights, the NOAA Earth Systems Research Laboratory wind profiler network, NASA remote sensing products, and the California Irrigation Management Information System to results from different WRF model runs. These model runs test different combinations of land surface models (i.e. RUC, NOAH, and NOAH-MP) and planetary boundary layer schemes (i.e. MYNN 2.5, YSU, and ACM2), and are used to assess biases within each scheme combination to illustrate the sensitivity and importance of accurate representation of irrigated soil moisture within simulations. Initial findings suggest the underrepresentation of soil moisture results in larger planetary boundary layer heights, smaller than observed Bowen ratios, and larger than expected sensible heat fluxes, which could be attributed to erroneous partitioning of energy in the SJV within WRF simulations. In addition, results show that Bowen ratios over the SJV are smaller than observed, while sensible heat fluxes are too large. Future work includes testing high-resolution data assimilation techniques for use in land surface models to better constrain irrigated soil moisture and quantify the improvements on simulations within the SJV.