Introduction

Namias (1958) was among the first in the meteorological community to address the influence of surface soil moisture anomalies on the atmospheric circulations at the seasonal scales. However, the advanced development of the soil moisture effect to climatic change was from middle 1970s. Charney (1975) pointed out that soil moisture can greatly affect surface albedo and evaporation during his research on biophysical feedback between land and atmosphere in the Sahel. Numerical simulations disclosed that the influence of soil moisture on surface fluxes is more important than those of surface albedo, soil structure and types (Mccumber, 1981). Sensitivity tests by Zhang et al. (1982) showed that the daytime planetary boundary layer structure us most sensitive to moisture availability, roughness length, albedo and thermal capacity, in that order. Several studies also showed that soil moisture had effect on not only local circulation in which anomaly exists, but also regions nearby (Rind, 1982; Rown-tree et al., 1983; Yeh et al., 1984). Researches (Ookouchi et al., 1984; Avissar et al., 1989; Mahfouf et al., 1987; Pinty et al., 1989; Avissar et al., 1994) further pointed out that horizontal uniformity of soil moisture can induce local circulation and may contribute to the formation of strong synoptic system (Lanicci et al., 1987; Chang et al., 1991).

Comparisons between dry and wet soil affection on regional climate with numerical model by Shukla et al. (1982) showed that the precipitation in July in major continent from dry soil is 40-50% weaker than that from wet soil, which means the contribution of land evaporation to precipitation is unelectable, and more sensible heat flux from dry soil can lead to stronger convection precipitation with abundant vapor transport in the region. In main monsoon regions such as India subcontinent where monsoon circulation is dominant, the precipitation from dry soil is stronger than that from the wet soil, which may be explained as the monsoon precipitation is controlled by water vapor transport from monsoon circulation rather than inland evaporation. Meanwhile, surface temperature at dry surface is 2-3°C

higher than that of the wet one; however, the pressure at dry surface is lower than that if the wet one. Sensitive test by Walker et al. (1978) also showed that dry soil can lead to the rise of temperature with stronger precipitation and wet soil is helpful to the duration of the precipitation. Yeh et al. (1984) investigated the effect of irrigated soil to the climate with GFDL GCM and pointed out that the increased soil moisture can lead to the greater precipitation and lower temperature. A study by a coupled vegetation-climate model to perform comparisons of different surface conditions in Sahel by Xue (1990) showed that surface condition changes have strong feedback on the precipitation and cloud amount but also the temperature can be affected therefore regional climate change.

Wang (1991) studied the influence from soil moisture anomaly with IAP-GCM (Institute of Atmospheric Physics) and also found that positive soil moisture anomaly leads to increased precipitation and humidity, lower temperature and higher pressure. However, the effect is weak 20 days later. Similar to other studies, his study shows the impact of soil moisture anomaly can propagate outside the region. One mechanism that soil moisture affecting climate change is through changing surface heat and hydrology process, therefore the feedback on flux transport between surface and the atmosphere.

It was well expressed the importance of soil moisture to the climatic change, especially in the areas of continental climate. However, due to the lack of in situ soil moisture observation data, numerical simulation is the mostly used and convenient tool in present studies. With current observation dataset few diagnosis and analysis on the relationship between soil moisture and climatic change can be performed, excepting for some field observation experiment, which are not long enough to detect long term climatic variation at large scale. Further comprehension on the mechanism between soil moisture and long term climatic change interaction at large scale is necessary though difficult for climatic change research at present.

Global distributions of soil moisture gauge stations are shown by Fig. 7.1. Comparing to observation at global scale, there is advancement in regional scales. For instance, in situ soil moisture data from former Soviet union and China agricultural meteorological gauge stations are available from 1980s (Robock et al., 2000). Originally serving agricultural meteorology, in situ soil moisture measurement, such as soil moisture, is of great importance in studies of climate change and land surface process interaction.

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