图书馆知识库

This study evaluates the monthly, seasonal, and annual characteristics of near-surface air temperature as a function of both elevation and geographical coordinates over Pakistan, using a climatic data set from 53 observation stations ranging between 5 and 2317 m asl and stepwise multiple regression. Forcing processes for temperature gradient (TG) variation were determined by analyzing its relationship with moisture flux (qv) and gradients of rainfall and saturation vapor pressure (e(s)). The bi-model pattern of TG(E) (temperature gradient with topographic elevation), which earlier studies (TAAC 113(3):671-682, TAAC 132:1129-1141) from the southern slopes of the Himalayas have observed, is also consistent in this study region. However, the forcing strengths, mechanisms, and processes for the monthly variations in TGs magnitude and their diurnal range are somewhat different. The annual cycle of TG(E) in this study is inversely (positively) associated with the rainfall (e(s)) gradient. Monsoon and orographic controls for TG(E) are stronger in summer, especially in the day, while the influences of inversions and mountainous topoclimate are higher during the non-monsoon period, in particular in winter and at night. Mountain barrier effects are higher in summer, which considerably hamper the relationship between temperature and geographical coordinates and rainfall and elevations. Steeper values of mean TG(E)s in the non-monsoon period are the result of strong dry convectional cooling at higher elevations due to high thermal forcing effects at lower elevations. Sensible heating reduction due to westerly-driven precipitation and cloud cover in daytime in the northern mountainous region (or higher latitudes) further strengthens the TG(E) value in this period. This effect is weaker in the surrounding regions; therefore, the TG(E) magnitude in this study is expected to be higher, especially in the pre-monsoon season. Gradients are more shallow in winter (excluding latitude), but values are greater than those in summer, due to distinct variations in moisture and cloud amount, i.e., lower in winter and higher in summer, particularly at lower elevations. Observed distinct differences in TG diurnal range between elevation (high) and latitude (low) in this study are due to a wide variation in temperature, topography, and elevations within the same latitudinal belts. In this study, the lowest gradient values for elevation are observed in August and the highest in May, 1 month later than observed in adjacent mountain regions due to the late arrival of monsoon moisture in summer and intense thermal forcing effects following the wet early pre-monsoon months. Distinct variability of gradient magnitudes between the northern mountainous region and the southern flat terrain is due to differences in topoclimates, moisture amounts, elevations, and geographical coordinates.