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 z t . 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 h ND ϭ hN/U ϱ is frequently between 1 and 10 (here h is mountain height, N is Brunt-Väisälä frequency, and U ϱ 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 z t 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 ϱ /(Nh), whereas Sheppard's model predicts the relative increase in uplift with increasing U ϱ /(Nh) correctly for U ϱ /(Nh) Ͼ 0.2.

The Aethalometer calibration and determination of iron concentration in dust aerosols

“…1). This approach is in agreement with the estimates obtained by using the Sheppard's airflow model (Kleissl & Honrath, 2005), which take in consideration orographic lifting and the occurrence of mountain clouds. That model, applied by Kleissl (2005) to the period under study uses as input the FNL data.…”

Occurrence of upslope flows at the Pico mountaintop observatory: A case study of orographic flows on a small, volcanic island

“…Nevertheless, the model has been shown to provide a reasonable description of the degree of uplift. On the basis of numerical simulations of inviscid flow, Ding et al [2003] concluded that the model is successful because “the energy provided by pressure field roughly offsets the energy loss due to friction/turbulence for axisymmetric hills.” We recently analyzed the accuracy of z t estimates from Sheppard's formula, using observations of clouds, specific humidity, and temperature at the PICO‐NARE station that is the subject of this paper [ Kleissl et al , 2006]. The results confirmed the findings of other investigators [ Hunt and Snyder , 1980; Vosper et al , 1999; Ding et al , 2003] that z t predictions from Sheppard's model are generally accurate, with a tendency to slightly overestimate the degree of uplift.…”

“…We have evaluated the accuracy of these z t calculations by using them to predict the occurrence of orographic clouds at Pico summit. In this analysis, presented elsewhere [ Kleissl et al , 2006], temperature and humidity at z t were used to determine whether condensation would occur if air was lifted from z t to the summit. The predictions were evaluated using relative humidity measurements at the PICO‐NARE station, with the presence of clouds indicated by RH values above 98%.…”

Interannual variability of ozone and carbon monoxide at the Whistler high elevation site: 2002–2006

“…Bachelor site with that from nearby radiosondes. Kleissl et al (2006Kleissl et al ( , 2007 provided a detailed examination of the effects of buoyant upslope flow or mechanical lifting on the Azores Pico site through the use of observed vertical soundings supplemented by those from a European numerical weather model. A simplified approach is used here to estimate the timing of the boundary layer influence on the mountain peak CO and O 3 mixing ratios.…”