High-resolution real-time modeling of the marine atmospheric boundary layer in support of the AOSN-II field campaign

The Navy's COAMPS atmospheric model has been used to provide twice-daily, real-time forecasts of 72-h duration in support of the AOSN-II field campaign during August 2003. The model was configured with four grid meshes with a minimum horizontal grid increment of 3 km. A statistical analysis of the model forecasts using available buoy observations demonstrates the skill of the atmospheric model predictions with characteristic wind speed RMS and bias errors of 1–3 m s−1 and less than 0.5–1.0 m s−1, respectively, and temperature RMS errors of 1–2 °C and biases typically less than 1.0 °C. The highest-resolution mesh provides more accurate wind-speed variances during the upwelling periods in the nearshore region, where the wind-speed gradients and diurnal variations are the largest. Persistent and strong low-level winds from a northwesterly direction were associated with a mid-tropospheric trough and jet stream that was substantially stronger than the overall mean monthly conditions. During the relaxation periods, which were relatively infrequent during August 2003, the trough was weaker than the mean conditions with a closed low west of the coast that resulted in southerlies near the coast. During upwelling-favorable conditions, the mean marine inversion was substantially stronger than during the relaxation periods with the surface-stress field exhibiting maxima in the lee of prominent coastal capes and topography, particularly during the periods of strong larger-scale low-level flow. The persistent mean stress pattern that is topographically locked, yields maxima in the stress curl pattern that induce favorable regions of coastal upwelling near and downwind of the coastal promontories and headlands.