Characteristics of solar radiation and its distribution above the canopy of tropical seasonal rain forest in Xishuangbanna, Southwest China

Based on the observation information of solar radiation in a tropical seasonal rain forest in west China's Xishuangbanna through 2003,annual,seasonal and daily variation of energy balance components,and values of components relative to global radiation,long-wave radiation,albedo and transmissivity were analyzed in order to discuss solar radiation characteristics in tropical seasonal rain forests.The results show that yearly average of total daily global radiation is 13.75MJ/m²,with 59.98%,28.68%,11.34% and 7.13% of global radiation partitioned into net radiation,effective radiation,reflective radiation and transmission radiation separately.Because of the seasonal differences of weather conditions,velocity of above-canopy solar radiation is smaller in foggy-cool seasons while greater in rainy seasons.As for the dry-hot season,daily variation of solar radiation components is different to a certain degree due to leaves shedding concentratedly in this season. In addition,transmission radiation should not been neglected in a tropical seasonal rain forest,especially when it is greater in the dry-hot season.During a day,transmission radiation is greater around midday;especially in the dry-hot season,there is the phenomenon of "sudden emergence",with a transmissivity as high as 49.9% at 12:00.Through a day,upward long-wave radiation present an one-peaked pattern,while downward longwave radiation shows a pattern of two peaks and two troughs,which results in positive effective radiation above the canopy.So,as far as annual average values are concerned,tropical seasonal rain forest canopies always act as heat sources to heat up the surrounding air.Though values of upward long-wave radiation change monthly and seasonally,daily variations are all similar.However,influenced by weather conditions,not only the values of downward long-wave radiation change with month and season,but also daily variations of downward long-wave radiation differ in different seasons.Finally,effective radiation above the canopy have different values in different months,and there exist greater daily variation differences among seasons.Besides the general fact that daily albedo is higher in the early morning and dusk,the albedo has the tendency of higher values in the forenoon and lower in the afternoon during the daytime.Also,daily transmissivity shows a tendency of maximum value at the noon and smaller in the forenoon and afternoon.In the tropical seasonal rain forest in Xishuangbanna,above-canopy solar radiation components and values of components relative to the global radiation,and annual,seasonal and daily variations of long-wave radiation,albedo and transmissivity are all closely related with local climate conditions and forest community appearance variations.