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  • Author or Editor: Changhai Liu x
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Xinyuan Feng
,
Changhai Liu
,
Roy Rasmussen
, and
Guangzhou Fan

Abstract

A plateau vortex refers to a shallow meso-α-scale cyclonic vortex that is usually confined to near-surface levels (500 hPa) over the Tibetan Plateau during warm seasons. It is the major precipitation-producing weather system over the plateau, but the knowledge of its climatology and understanding of generation mechanisms are limited because of the lack of adequate observations in this harsh mountainous region. In this study, the high-resolution NCEP Climate Forecast System Reanalysis data have been used to perform a statistical survey of these vortices over 10 warm seasons (April–October of 2000–09). The purpose is to document their climatological features, including genesis, size, life cycle, propagation, and diurnal variation.Results show that ~103 plateau vortices occur on average every year. Most are detected from May through August, with the maximum monthly count in July. The primary area of origin exhibits a west–east orientation in correspondence with a large-scale confluence zone, and the most concentrated source lies in the area of 33°–36°N, 84°–90°E in the high elevated central and western plateau. Significant diurnal variations are observed, characteristic of a preferential genesis during late afternoon to evening hours and a late night dissipation peak. The vortex events have an average life span of ~15 h and an average horizontal dimension (effective diameter) of ~280 km. In accordance with the steering environmental flow, an overwhelming majority travel eastward with a mean translation speed of ~10 m s−1. A small fraction of systems (approximately nine cases annually) move off the plateau, predominantly from the eastern edge.

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Xiaoqin Jing
,
Bart Geerts
,
Yonggang Wang
, and
Changhai Liu

Abstract

Two high-resolution (4 km) regional climate simulations over a 10-yr period are conducted to study the changes in wintertime precipitation distribution across mountain ranges in the interior western United States (IWUS) in a warming climate. One simulation represents the current climate, and another represents an ~2050 climate using a pseudo–global warming approach. The climate perturbations are derived from the ensemble mean of 15 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). These simulations provide an estimate of average changes in wintertime orographic precipitation enhancement and finescale distribution across mountain ranges. The variability in these changes among CMIP5 models is quantified using statistical downscaling relations between orographic precipitation distribution and upstream conditions, developed in Part I. The CMIP5 guidance indicates a robust warming signal (~2 K) over the IWUS by ~2050 but minor changes in relative humidity and cloud-base height. The IWUS simulations reveal a widespread increase in precipitation on account of higher precipitation rates during winter storms in this warmer climate. This precipitation increase is most significant over the mountains rather than on the surrounding plains. The increase in precipitation rate is largely due to an increase in low-level cross-mountain moisture transport. The application of the statistical relations indicates that individual CMIP5 models disagree about the magnitude and distribution of orographic precipitation change in the IWUS, although most agree with the ensemble-mean-predicted orographic precipitation increase.

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Xiaoqin Jing
,
Bart Geerts
,
Yonggang Wang
, and
Changhai Liu

Abstract

This study analyzes the control of upstream conditions on the distribution of wintertime precipitation across mountain ranges in the interior western United States using 10 winters of high-resolution regional climate model data. Three mountain ranges, the Wind River Range, the Park Range, and the Teton Range, are selected to explore the statistical relations between the precipitation distribution and upstream wind, stability, and cloud conditions. A 4-km-resolution simulation is used for the former two ranges, and a 1.33-km-resolution simulation driven by the 4-km-resolution simulation is used for the Teton Range, which is smaller and steeper. Across all three mountain ranges, the dominant factor controlling precipitation is the mountain-normal low-level wind speed. Statistically, stronger wind results in heavier precipitation and a lower upwind precipitation fraction. The low-level wind generally veers with height during precipitation events, but the amount of veering does not unambiguously affect the precipitation distribution or intensity. The more the terrain blocks the upstream flow, the more the precipitation shifts toward the upstream side of the mountain and the weaker the overall precipitation rate is. A higher cloud-base temperature and a lower cloud-base height typically are associated with heavier precipitation. Deeper clouds tend to produce heavier precipitation and a slightly lower windward/leeward contrast. Convective precipitation proportionally falls more on the lee slopes than stratiform precipitation. The upstream and macroscale cloud conditions identified herein predict both the mean precipitation rate and the upwind precipitation fraction very well for the three ranges studied here.

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