This study investigates the physical mechanisms of orographic enhancement of precipitation in a representative variety of storms, under different regimes of stability, type of storm, and topography. The study will focus on understanding the dynamical and microphysical processes contributing to growth and fallout of orographic precipitation, and hence to a physical basis for understanding and parameterizing the time scales of precipitation growth and fallout. This study will utilize data from the Mesoscale Alpine Programme (MAP) and Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) field studies of midlatitude cyclones moving over mountains by fully exploiting those datasets and by extending the analysis to warmer and more unstable orographic precipitation regimes.
In the midlatitude cyclone regime of MAP and IMPROVE, the Principal Investigator will contrast the orographic precipitation processes in the stable to nearly neutral conditions during the frontal phase of the storm to the moderately unstable conditions in the postfrontal phase of the storm. The latter is an important precipitation producing stage over the western U.S. mountains but has been previously neglected in the literature. Studies of MAP and IMPROVE to date have focused only on the frontal phase of these storms. The PI will extend these studies to include the postfrontal convective stage. MAP and IMPROVE have shown the importance of coalescence and riming to shorten the time scales of growth and fallout of precipitation, the importance of moist nearly neutral unblocked flow in promoting these processes by favoring rapid ascent over the terrain, and the role embedded vertical-motion cells (owing to buoyancy and/or shear) in enhancing the microphysical processes. In the postfrontal phase of the storm the key growth mechanisms of coalescence and riming will be examined in relation to how they are affected by the vertical motions in postfrontal convection. The study will evaluate how well models represent the small-scale dynamical and microphysical processes in relation to aircraft and radar data collected in MAP and IMPROVE in both the frontal and post frontal phases of storms.
The analysis of orographic precipitation mechanisms will be extended to two important warmer orographic regimes. First, the Principal Investigator will examine the interaction of the balanced circulation of a tropical cyclone with a mountain range. Tropical cyclones passing over rugged terrain produce some of the most devastating flooding on earth when the warm organized cyclonic circulation encounters a topographic barrier. As a representative of this regime, the Principal Investigator will examine a model simulation of the passage of Typhoon Nari (2001), which represents a devastating cyclone over Taiwan. The simulated storm encounters a large 2D mountain range rising abruptly out of the sea. Preliminary investigation suggests that when the eyewall vortex circulation extends across the mountain barrier, a mountain wave circulation forms and interrupts the eyewall vertical motion. The mountain wave appears to temporarily lock the strong upward motion and precipitation to the terrain. Both coalescence at low levels and graupel formation aloft appear to shorten the time scales of the precipitation growth and concentrate the rain on the windward slope. The proposed study will explore this hypothesis of tropical storm/mountain wave interaction in the model results and perform experiments with the typhoon approaching the barrier from different directions.
The second warm regime to be examined will be a very highly unstable flow interacting with a large mountain barrier. This type of regime is notable for producing major floods such as the Big Thompson and Black Hills floods in the U.S. and the recent 2004 floods in India. The Principal Investigator will address the very warm, highly unstable orographic regime by examining convection upstream of and over the Himalayas. This region is ideal because a persistent moist flow produces many realizations of deep, intense convection. The vertical structure of the convection in this region will be examined in Tropical Rainfall Measuring Mission (TRMM) satellite data and compared to model simulations over the Himalayan region to determine how low-altitude coalescence and high-altitude riming, both of which shorten the time scales of the growth and fallout of precipitation in this highly convective environment. Preliminary analysis of the TRMM data suggests that the riming is so robust that graupel particles produce high reflectivity at altitudes up to 17 km.
Broader impacts resulting from the proposed activity: This study will contribute to the societal goal of improving predictions of heavy precipitation and flooding over mountainous regions.
