Xuejia Wang

Influences of Two Land-Surface Schemes on RegCM4 Precipitation Simulations over the Tibetan Plateau

The effects of different RegCM4 land-surface schemes on Tibetan Plateau (TP) precipitation simulations were investigated. Two groups of ten-year (1992–2001) simulation experiments (hereafter referred to as BATS and CLM) were performed based on two land-surface schemes (BATS and CLM3.5, resp.) and were compared with observed data using the same domain, initial, and lateral boundary conditions, cumulus convective scheme, and spatial resolution. The results showed that the CLM monthly precipitation more closely matched the observed data compared with BATS. BATS and CLM both overestimated summer precipitation in the northern TP but underestimated summer precipitation in the southern TP. However, CLM, because of its detailed land-surface process descriptions, reduced the overestimated precipitation areas and magnitudes of BATS. Compared to CN05, the regional average summer precipitation in BATS and CLM was overestimated by 34.7% and underestimated by 24.7%, respectively. Higher soil moisture, evapotranspiration, and heating effects in the BATS experiment triggered changes in atmospheric circulation patterns over the TP. Moreover, BATS simulated the lower atmosphere as warmer and more humid and the upper atmosphere (~150 hPa) as colder than the CLM simulations; these characteristics likely increased the instability for moist convection and produced more summer precipitation.

Effects of modified soil water‐heat physics on RegCM4 simulations of climate over the Tibetan Plateau

To optimize the description of land surface processes and improve climate simulations over the Tibetan Plateau (TP), a modified soil water-heat parameterization scheme (SWHPS) is implemented into the Community Land Model 3.5 (CLM3.5), which is coupled to the regional climate model 4 (RegCM4). This scheme includes Johansens soil thermal conductivity scheme together with Nius groundwater module. Two groups of climate simulations are then performed using the original RegCM4 and revised RegCM4 to analyze the effects of the revised SWHPS on regional climate simulations. The effect of the revised RegCM4 on simulated air temperature is relatively small (with mean biases changing by less than 0.1°C over the TP). There are overall improvements in the simulation of winter and summer air temperature but increased errors in the eastern TP. It has a significant effect on simulated precipitation. There is also a clear improvement in simulated annual and winter precipitation, particularly over the northern TP, including the Qilian Mountains and the source region of the Yellow River. There are, however, increased errors in precipitation simulation in parts of the southern TP. The precipitation difference between the two models is caused mainly by their convective precipitation difference, particularly in summer. Overall, the implementation of the new SWHPS into the RegCM4 has a significant effect not only on land surface variables but also on the overlying atmosphere through various physical interactions.

Variations in soil temperature at BJ site on the central Tibetan Plateau

The temporal and spatial variation in soil temperature play a significant role in energy and water cycle between land surface and atmosphere on the Tibetan Plateau. Based on the observed soil temperature data (hourly data from 1 January 2001 to 31 December 2005) obtained by GAME-Tibet, the diurnal, seasonal and interannual variations in soil temperature at BJ site (31.37° N, 91.90° E; 4509 a.s.l.) near Naqu in the central Tibetan Plateau were analyzed. Results showed that the average diurnal variation in soil temperature at 4 and 20 cm depth can be described as sinusoidal curve, which is consistent with the variation of solar radiation. However, the average diurnal variation in soil temperature under 60 cm was very weak. The average diurnal amplitude in soil temperature decreased by the exponential decay function with the increase of soil depth (R2=0.92, p<0.01). It is demonstrated that the average diurnal maximum soil temperature decreased by the exponential decay function with the increase of soil depth (R2 =0.78, p<0.01). In contrast, the average diurnal minimum soil temperature increased by the exponential grow function with increasing of soil depth (R2=0.86, p<0.01). There were a linear negative correlation between the average annual maximum Ts and soil depth (R2=0.96, p<0.01), a logarithmic function relationship between the average annual minimum soil temperature and soil depth (R2=0.92, p<0.01). The average seasonal amplitude in soil temperature followed the exponential decay function with the increase of soil depth (R2=0.98, p<0.01). The mean annual soil temperature in each layer indicated a warming trend prominently. During the study period, the mean annual soil temperature at 4, 20, 40, 60, 80, 100, 130, 160, 200 and 250 cm depth increased by 0.034, 0.041, 0.061, 0.056, 0.062, 0.050, 0.057, 0.051, 0.047 and 0.042 °C /a, respectively.