Orography strongly affects precipitation when baroclinic precipitation systems pass over mountain ranges and when deep convection occurs in regions of terrain modified flow. This research investigates both of these types of orographic precipitation by focusing on: (1) Physical mechanisms by which the widespread precipitation of a baroclinic storm system is enhanced on the windward slope of a midlatitude mountain barrier; (2) Factors affecting the location, intensity, and structure of heavily precipitating convective systems in the vicinity of tropical mountain ranges.
For midlatitude baroclinic systems, the Principal Investigator (PI) will test hypotheses, derived from the Mesoscale Alpine Programme (MAP, based in the Alps) and Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE, based in Oregon), that windward slope enhancement can occur either: 1) when strong low level flow of near neutral or slight static instability rises easily over orography such that cells form over the first sharp rise of terrain, or 2) when weak low-level flow is bounded above by a layer of shear and overturning cells. In either case, the cellularity leads to enhancement of precipitation over the lower slope of the terrain. The PI will further examine the MAP and IMPROVE data to better understand these processes, and expand the investigation to include a dataset from the Hydrometeorological Testbed (HMT) in northern California, which will help determine the broader applicability of the processes identified in MAP and IMPROVE II to west coast U.S. storms in general.
For tropical convective systems, the PI will continue to explore convection in the Himalayan region, which has a robust combination of high mountains, well-defined upstream moisture sources, and different types of land surface properties. In preliminary studies of a satellite database that have been compiled and high-resolution model output, the PI has found that extremely deep convection in the northwestern subcontinent is controlled by flow down from high plateaus overriding low-level moist flow. The low-level flow is channeled in long converging trajectories over dry, hot land surfaces by the shape of the mountain barrier, and convection is released at the first sharp peak of terrain, so that the heavily precipitating convective systems occur near the base of the barrier. When large maritime convective systems come ashore over the wet river delta of the northeastern subcontinent, it appears that the stratiform components of these systems are enhanced by orographic uplift on and ahead of the windward slopes of the terrain. The research will broaden the satellite database to determine how these processes account for the climatological precipitation pattern near the Himalayas and how they vary diurnally and seasonally. In the latter half of the project the PI may develop a satellite data base for the Andes to test the generality of concepts derived from the Himalayas.
In both the midlatitude and tropical phases of this research the PI has established collaborations with groups running high-resolution forecast models. The above described research will use output from these models to test and extend the physical understanding gained from the field project and satellite datasets. The PI also will be evaluating the models' accuracy in representing the orographic precipitation processes in both midlatitudes and tropics.
Intellectual merit: This research builds on the PI's previously established expertise in the dynamics and physics of precipitating clouds. It will help fill a knowledge gap in atmospheric sciences regarding orographic effects on precipitating systems.
Broader impacts: This study will contribute to the societal goal of improving predictions of heavy precipitation and flooding in mountainous regions.
The purpose of this project was to develop a better understanding of how mountains influence precipitation at all locations over the globe. To accomplish this purpose we picked precipitation regimes in both low and high latitudes, so that we could determine the physics and dynamics leading to precipitating cloud systems in both thermodynamically stable (i.e. high-latitude) and unstable (i.e. low-latitude) regimes. We also chose venues near the world's two highest mountain ranges, the Himalayas and Andes. The study grew out of our earlier work, which was based largely on frontal systems in higher latitudes, specifically on storms passing over the Alps and Cascades. The current study continued the midlatitude work by analyzing the passage of a frontal system over the southern Andes. It was similar to frontal systems passing over the western U.S., and this study was able to reveal how the precipitation process enhancement by the topography in this type of storm begins upstream over the ocean, for both dynamical reasons associated with blocking and microphysical reasons for enhancing particle growth in clouds forming upstream of the mountains. Apart from this frontal study, we focused on lower latitude cloud systems in unstable environments and have gained insight into how the mountains influence the formation of intense convection through capping and triggering. The capping is due to midlevel flow over the high terrain descending and preventing convection from erupting premature. In the vicinity of the Andes and Himalayas, the triggering occurs when the low-level capped flow encounters foothills or other small mountains upstream of the primary range. Another effect we found in the Himalayan region was that when mesoscale convective systems produce large regions of stratiform precipitation, the latter is enhanced strongly as they ascend a large mountain barrier. This type of enhancement is common over northeastern India, Bangladesh, and Burma, but is rare in northwestern India or Pakistan. One significant finding of this study was that when mesoscale systems with large stratiform regions form anomalously in northwestern India and Pakistan, where they are normally rare, devastating floods can occur. To arrive at this conclusion, we performed case studies of the devastating floods of 2010 in Pakistan and India. This research is intellectually meritorious in that it has broadened understanding of orographic effects on precipitation. It is has potentially broader impacts in understanding how precipitation near mountainous regions relates to large scale conditions that might change in a warming world, and it has implications for possible improvement of forecasts of flooding in South Asia.
