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PhD Defense

Department of Atmospheric Science

Wed., July 18, 9:00 am, EN6085

Regional Climate Study of Orographic Precipitation in the Interior Western United States

Xiaoqin Jing

University of Wyoming

Abstract

The Interior Western United States (IWUS) is mostly arid, but also home to the headwaters of several major river systems, however, quantitative precipitation estimation QPE is especially challenging in the complex terrain, and general circulations models (GCMs) have large uncertainties in modelling the orographic precipitation in the IWUS. This work aims to study the orographic precipitation distribution in the IWUS using high resolution convection-permitting Weather Research and Forecasting Model (WRF) simulations, and understand its future changes in a warmer climate.

The seasonal precipitation distribution in the IWUS is evaluated using gauge network, a ground-based radar dataset, four gauge-driven gridded datasets, and a 4-km resolution WRF simulation. The WRF simulation captures wintertime orographic precipitation distribution well, and biases over specific mountain ranges are identical to those in an independent WRF simulation, suggesting that these biases are at least partly due to errors in the snowfall measurements, or the gridding of these measurements. Due to the good performance of the simulation, we analyze the control of upstream conditions on the wintertime precipitation distribution across mountain ranges using the high-resolution WRF data. The results show the dominant factor controlling precipitation is the mountain-normal low-level wind speed. Other factors, such as stability, cloud base height and temperature, cloud depth, and cloud type, also affect the precipitation distribution. The upstream and cloud conditions identified herein predict both the mean precipitation rate and the upwind precipitation fraction (UPF) very well for the three ranges using the statistical relations developed here.

In order to explore how future changes in ambient conditions in a changing global climate influences the orographic precipitation distribution, another WRF simulation, which represents an anno ~2050 climate, is conducted using a pseudo global warming (PGW) approach. The climate perturbations assumed here are from the ensemble mean of 15 GCMs. The results show the ensemble mean climate change yields an increase in wintertime precipitation over IWUS mountain ranges, with little change in UPF. Some individual CMIP5 members disagree on this, although most agree with the precipitation increase. Light (heavy) precipitation events become less (more) common across IWUS mountains, which is mainly due to enhanced low-level atmospheric moisture in a warmer climate. Weakening cross-mountain winds and rising cloud base heights tend to offset this change, but their impact is minor.


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