Analysis and Application of Sheppard’s Airflow Model to Predict Mechanical Orographic Lifting and the Occurrence of Mountain Clouds

Abstract

Mechanically driven orographic lifting is important for air pollution dispersion and weather prediction, but the small dimensions of mountain peaks often prevent numerical weather models from producing detailed forecasts. Mechanical lifting in stratified flow over mountains and associated thermodynamic processes were quantified and evaluated using Sheppard?s model to estimate the dividing-streamline height zt. The model was based on numerical weather model profile data and was evaluated using ground-based measurements on a tall, axisymmetric mountaintop for which the nondimensional mountain height hND = hN/U∞ is frequently between 1 and 10 (here h is mountain height, N is Brunt?Väisälä frequency, and U∞ is upstream horizontal wind speed). Sheppard?s formula was successful in predicting water vapor saturation at the mountaintop, with a false-prediction rate of 14.5%. Wind speed was found to be strongly related to the likelihood of forecast errors, and wind direction, season, and stratification did not play significant roles. The potential temperature (water vapor mixing ratio) at zt in the sounding was found to be slightly smaller (larger) than at the mountaintop, on average, indicating less lifting than predicted and/or turbulent mixing with higher-altitude air during parcel ascent. Detailed analysis revealed that this difference is a result of less lifting than predicted for small U∞/(Nh), whereas Sheppard?s model predicts the relative increase in uplift with increasing U∞/(Nh) correctly for U∞/(Nh) > 0.2.