Yongkang Xue


Department: Geography
UCLA Dept. of Geography
1165 Bunche Hall
Los Angeles, CA 90095-1524
Phone: (310) 825-1122
Fax: (310) 825-1071
Email: yxue@geog.ucla.edu

Keywords: Geography, China, Asia

Prof. Xue has a number of reseach projects underway. Among these is one focusing on interactions of land-surface processes and the East Asian monsoon. Prof. Xue's team of researchers are investigating the influence of thermal heating gradients and changes in Bowen ratio associated with vegetation and snow distributions, land-use and land-cover (LULC) changes, and other land surface processes on seasonal and interannual variability of the East Asian (EA) summer monsoon and on prediction of major components in the EA water cycle. The EA summer monsoon is a major part of the global monsoon and exerts great influence on regional and global climate and water balances. Observational evidence shows that precipitation and river discharge in East Asia have had significant variations at different temporal and spatial scales. However, the causes of the variability and changes in EA monsoon, especially the role of land surface processes, are poorly understood. This uncertainty is partially due to the complex topography in East Asia and difficulty in properly simulating the very dynamic EA monsoon processes. A coupled National Center for Environmental Prediction (NCEP) general circulation model/Simplified Simple Biosphere Model (SSiB) with an imbedded high-resolution regional Eta model will be used for the proposed study. These state-of-the-art models have been tested in EA simulations and have produced reasonable results.

Observational data sets will be applied to validate, calibrate, and constrain model simulations of the observed monsoon variability and changes in East Asia at different temporal scales. These data sets include field measurements taken from the Tibetan Plateau, station data for precipitation and other surface variables, the profiles of atmospheric variables from atmospheric sounding over China, and river runoff station data. New land data from remote sensing will also be applied to specify land-surface boundary conditions and to help understand the causes of monsoon variability due to land conditions.

Five 20-year GCM runs from 1982 through 2001 will be performed and analyzed. These simulations include: land surface parameterizations with or without explicit vegetation processes, vegetation data sets with only seasonal variations and vegetation data sets with seasonal as well as interannual variations based on satellite observations, LULC conditions with both natural variations and anthropogenic effects and LULC conditions with mainly natural variations, and real sea surface temperatures (SSTs) and climatological SSTs. The results from these experiments will be compared with the observed data and with each other. Using the simulated data of precipitation, snow cover, surface air temperature, soil moisture and atmospheric circulation, as well as specified vegetation parameters, we will examine the relationship between seasonal and interannual variations of the monsoon and land surface conditions over East Asia. The feedback mechanisms will be investigated. The role of surface characteristics in western and eastern parts of the Tibetan Plateau, including snow cover, in EA monsoon on-set and evolution is one of the foci. Simulations for selected years with climate anomalies will be further tested using the Eta model with the GCM output and the GAME/4DDA reanalysis data as lateral boundary conditions. The regional model results will provide insight into the impact of large-scale circulation anomalies and land surface conditions on the intensity, structure, and variability of the EA monsoon and potential predictability of the EA monsoon variability.