Modeling of orographic precipitation with multilevel coupling of land-atmosphere interactions.

摘要

A three-dimensional model that simulates regional distributions of orographic precipitation was developed. The model consists of two parts: (1) an independent circulation module designed to interpolate synoptic wind fields from the macroscale (100's of kms) to the mesoscale (10's of kms); (2) a Lagrangian transport module to track adiabatically the evolution of moist air masses and the generation of precipitation across the study domain. Total water and moist static energy are treated as conservative tracers but for the scavenging effect of precipitation. The removal of precipitable water from the atmosphere is governed by a spatially varying precipitation efficiency parameter that requires calibration. A new evaporative cooling scheme to account for phase and mass exchanges during the downfall of hydrometeors was also developed. The model was applied to the Olympic and the Sierra Nevada Mountains.; In addition, a new methodology to facilitate adaptive multilevel coupling for the simulation of land-atmosphere interactions is illustrated through three distinct modeling experiments. A first experiment consisted of coupling two simple circulation models solving each separately for the potential and rotational components of atmospheric wind fields. The resulting wind fields improved greatly the ability of the precipitation model to reproduce leewind storms in the Olympic Mountains. In the second experiment, three versions of the precipitation model operating at resolutions of 40, 60 and 80 km were intermittently coupled to establish the path of independent storms across the Sierra Nevada region. Detailed spatial distributions of precipitation at low computational costs were obtained by solving advective transport at the coarser spatial resolution and restricting precipitation processes to the finest resolution. Finally, a third experiment consisted of linking the 40 km implementation of the precipitation model for the Sierra Nevada to a 1D surface energy model to simulate directly runoff, soil moisture and snow cover distributions, and the recycling of moisture between the atmosphere and the land-surface. The two models were coupled by a 2D finite-element mesh with a 10 km grid-spacing. This case-study provides insight into the intra-annual dynamics of the hydrological cycle in mountainous regions, which is impossible to observe directly.; The dissertation concludes by discussing and recommending potential directions for further research in the context of mountain hydrology. The necessity for studies addressing issues related to climate, landscape evolution and water resources management is stressed.