Sensitivity of Weather Forecasts to Soil Moisture Perturbations in Coastal Regions

Land-atmosphere interactions and their impacts on offshore weather in coastal regions remain significant challenges to understand and represent in models. In this presentation, we consider two coastal regions of the United States with different land surface conditions: 1) a moist and vegetated area in the Mid-Atlantic, and 2) an arid and mountainous area in the southwestern United States. The terrestrial coupling index (TCI), for which larger positive values indicate greater sensitivity of surface latent heat flux to soil moisture variability, is calculated for each region and month during 2000–2020 using the 0.1° ERA5-Land dataset. There are larger positive values of TCI throughout most of the year in the West Coast region compared to the East Coast region, suggesting that there is stronger land-atmosphere coupling in the West Coast region compared to the East Coast region. The primary focus of this presentation is to examine the sensitivity of weather forecasts to soil moisture perturbations in the West Coast region for an offshore low-level flow event during a month with large positive values of TCI. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used to run pairs of simulations out to 120 h at convection permitting horizontal resolutions, in which one simulation includes soil moisture perturbations at forecast initialization and the other simulation does not. Differences in atmospheric quantities, including lower-tropospheric temperature and humidity, between the simulations in each pair are examined across the West Coast region and adjacent offshore areas. In addition, the simulations for the West Coast region are briefly compared to simulations that were previously performed for an offshore low-level flow event in the East Coast region during August 2020 to examine regional variability of the sensitivity of weather forecasts to soil moisture perturbations. The simulations for the East Coast region indicate that soil moisture perturbations can have notable and persistent impacts on the lower troposphere over land and over offshore areas throughout the 120-h forecast. For example, dry soil moisture perturbations can result in notable increases in potential temperature over offshore areas primarily above the boundary layer, between approximately 500 and 2000 m above ground level, with the largest increases tending to occur during the evening and overnight hours.