Global aspects of orographic precipitation

Orographic precipitation around the world is particularly important for water resources as it determines the discharge in the most of world's rivers. River discharge in turn controls hydropower, irrigated agriculture and urban water resources. Over the last three decades, several field projects have studied orographic precipitation in mid-latitudes (ALPEX, MAP, IMPROVE, COPS, PACJET, etc.) and in the subtropics (TAMEX and Hawaiian Rainband) . Recently, a tropical field project has been completed (i.e. DOMEX in Dominica). Based on all these results, we are now in a position to compare mountain ranges in different climate zones on the basis of the controlling environmental parameters: water vapor source, water vapor flux statistics, blocking and deflection, diurnal forcing, conditional instability, and melting level. Using these factors, we have studied 68 representative mountain ranges around the world; categorizing them into classes with similar physical characteristics. We also compared conventional and stable isotope measures of mountain drying ratio.

One important environmental characteristic is the horizontal flux of water vapor (WVF) towards a mountain range. To a first approximation, this WVF will control the orographic precipitation. The mountain ranges with the most WVF in mid-latitudes are the Canadian Rockies (SWly flow) in the northern hemisphere and the southern Andes (NWly flow) in the southern hemisphere. More than 10 percent of the time, the WVFs there exceed 400 kg/m/s. The isotope-derived drying ratio exceeds 40% for these ranges. The Japanese Alps experience similarly high values of WVF in SWly flow.

In the tropics, two categories of WVF are found. In the eastern Caribbean, nearly steady easterly tradewinds carry a WVF between 200 and 500 kg/m/s throughout the year. These fluxes impinge on Dominica and other Caribbean and Central American mountains. Although the annual precipitation on Dominica exceeds 6000mm, the drying ratio is less than 1%. In southeast Asia, the monsoon months of June, July and August bring strong westerlies against the Western Ghats in India and the Arakan Range in Myanmar. These WV fluxes often exceed 800 kg/m/s. These sites have the highest monthly averaged WVF found anywhere in the world, about 600 kg/m/s. While these Caribbean and Monsoon examples have opposite wind direction, they both have reversed flow aloft, modifying the airflow and convection response to forced orographic ascent.

Another important environmental characteristic of orographic precipitation is the moist stability. As expected, the tropical regions are conditionally unstable. In these regions, upon sudden orographic lifting, initially small neutrally-buoyant temperature and moisture anomalies trigger convection. Convective orographic precipitation is also common in mid and high latitudes but such conditions are highly transient. As frontal cyclones pass over terrain, they bring periods of stable and unstable flow, producing both stratus and convective orographic precipitation. Contrary to previous views, the convection does not always increase the amount of precipitation.