The Characteristics And Parameterization Of Land Surface Processes And Its Relationship With Climate Over Northern China

Semi‐ and arid areas in Northern China accounts for around 40% of the land area of our country.The former workers had focused on its land surface processes (LSP). But researches about LSP characteristics on regional scale and their responses to climate are still tiny, and land surface processes under different weathers and different underlying surfaces also lack summary. Therefore, by dividing Northern China into three parts including the northwest China, Loess Plateau and northeast China according to the climate types and geographic location, using the data provided by " Experimental Co‐observation and Integral Research in the Semi‐arid and Arid Regions over North northern China " (observation period is July‐September, 2008), this paper analyzed the characteristics of LSP on regional scale and under different weather and difference underlying surface, summarized the responses of LSP to climates of different aridity degrees, and conducted preliminary analysis on LSP parameterization, hoping to provide some meaningful reference for the parameterization of LSP on regional scale in semi‐ and arid areas over Northern China. The main conclusions are as follows:(1) The Spatial Characteristics of LSP: Both global radiation and upward shortwave radiation increase gradually from the southeast to the northwest due to the influence of summer monsoon. The trends of downward and upward longwave radiation with latitude do not agree in the east and west of northern China, which is probably because the factors controlling longwave radiation are different in the east and west. Net radiation minimum center appeared in Loess Plateau while maximum centers occur in both the northwest and the northeast China, and in each of the three regions net radiation is degressive from south to north. Both Albedo and Bowen ratio increase along latitude, representing the hydrologic characteristics of regional climate. Sensible heat and latent heat flux have opposite tendencies with increasing latitude, and soil heat flux also appears extremum centers in each region.(2) The Regional Characteristics of LSP: Land surface radiation and energy processes show regional characteristics, which is attributed to difference in the available water content in the atmosphere and land. Global radiation is mainly influenced by latitude; but other radiation processes and energy process are sensitive to hydrologic property of climate and the thermal, hydrologic, optical properties of underlying surface. Upward shortwave radiation is smallest in northeast China and highest in the northwest China. Downward longwave radiation in northeast is the most obvious and is weakest in Loess Plateau, while upward longwave radiation is strongest in the northwest and is weakest in northeast. The regional differences of land surface energy processes are more apparent. In the northwest most of available energy is used to heat the atmosphere. In Loess Plateau the energy for heating atmosphere accounts for 30% and 50% is consumed by evaporation. Nearly half of energy in the northeast is used for evaporation and large part of the other half is lost through heating atmosphere. Loess Plateau and northeast China belong to semiarid areas and their energy distribution has certain similarity, but differences still exist due to available water surface differences.(3)Characteristics of LSP under different Weathers: Obvious discrepancy is found between clear‐sky and non‐clear‐sky LSP, but the difference in LSP between cloudy and rainy sky is not that distinct. The diurnal variation of LSP is evident under clear‐sky, and the diurnal variations of LSP are seen slightly fluctuating over time under cloudy sky. Rainy sky LSP vary in a more stable way relative to cloudy sky but is not as obvious as that of clear sky. In addition, the characteristics of LSP under different weathers showed marked regional differences.(4) Characteristics of LSP under different underlying surfaces: No evident differences in Global radiation and downward longwave radiation under different underlying surfaces are observed. Upward longwave radiations of different underlying surfaces show similarity to that of land surface temperature characteristics but are also greatly influenced by the canopy. The upward shortwave radiation and albedo are sensitive to the hydrologic, thermal and optical properties of underlying surface: alpine forest is lowest in reflectivity, the second is farmland and grassland, and desert underlying surface is highest. Net radiation of desert underlying surface, grassland, farmland, and forest increases in turn. The proportion of sensible heat to net radiation is highest under desert underlying surface, followed by grassland, farmland, and lowest under alpine forest. The proportion of latent heat is ranked oppositely to that of sensible heat. In addition, the proportion of sensible and latent heat flux under certain underlying surface depends on available water content of underlying surface and shows regional characteristic. Land surface energy closure rate is highest under desert underlying surface, followed by grassland, farmland, and the rate of alpine forest is lowest. The closure rate is related to measurement error in latent heat flux as well as the non‐uniform degree of underlying surface.(5) The reponse of LSP to Climate: LSP is considerably affected by climate. The response of radiation and energy processes variables as well as soil and atmospheric hydrologic, thermal variables is evident and the response strength pass significance test except variables such as global radiation, downward longwave radiation, albedo and the soil heat flux.(6) The paraterization of LSP: In various underlying surface, dynamic roughness length under the grass is smallest, secondly farmland, orchards and forest are highest. Dynamic roughness length decreases with increasing wind speed in the case of tall vegetation but not for short vegetation. Thermal and latent heat roughness length are basically in a 10‐3 order, smaller than land surface model reference, which may lead to latent and sense heat bias in model simulation. Momentum transfer coefficients C_D of desert and grass underlying surfaces are nearly the same and are smaller than other underlying surface. C_D of the farmland ranges between 1×10^‐2 and 3×10^‐2 either in stable or unstable condition. The C_D of alpine forest is highest, about 5 10^‐2. The heat transfer coefficient C_H for various underlying surfaces is 1×10‐3²Ã—10^‐2. The parameterization of albedo on regional scale is carried out based on the assumation that albedo can be separated into two parts, the short‐term part and the long‐term part. The heat conductivity of forest and grass underlying surfaces is basically less than 1, and thermal diffusitivity is in 10^‐7 order. The soil thermal conductivity of farmland is between 0.5 and 2 changes, and thermal diffusitivity is in 10^‐6 order. In the orchard underlying surface, thermal conductivity reaches 3.2, and thermal diffusitivity is in 10^‐6 order.