Land Surface Impacts on Weather Forecasts in Coastal Regions

The impact of the land surface on weather forecasts in coastal regions remains a significant challenge to understand and represent in models. In this presentation, two coastal regions of the United States with different land surface characteristics are considered: a moist area in the Mid-Atlantic, and an arid area in the southwestern United States. The sensitivity of weather forecasts to soil moisture (SM) perturbations during the warm season is examined for a selected offshore low-level flow event in each coastal region by using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). For each event, a set of seven 120-hour, cloud-resolving simulations are implemented: a control simulation with initial SM set to an intermediate value, and six experimental simulations in which initial SM is uniformly increased or decreased relative to the control by equal amounts. Differences in atmospheric quantities, including lower-tropospheric temperature and moisture, between the control and each experimental simulation are examined across land and adjacent offshore areas of each coastal region. The simulations indicate that SM perturbations can have notable impacts on low-level temperature and moisture over both land and offshore areas for both coastal regions. In the East Coast region, dry SM perturbations tend to have larger impacts on low-level temperature and moisture compared to moist SM perturbations. In the West Coast region, dry SM perturbations tend to have smaller or similar impacts on low-level temperature and moisture compared to moist SM perturbations. For both coastal regions, the largest impacts of SM perturbations on temperature and moisture over the lower troposphere tend to occur within the planetary boundary layer (PBL) over land areas and above the PBL over offshore areas.