Characteristics of Thermally-Induced Near-Surface Flows over An Enclosed Crater: Observations of the Meteor Crater Experiment(METCRAX)

Meteorological measurements of the Meteor Crater Experiment(METCRAX) were used to analyze the characteristics of the thermally-induced flows over an enclosed crater(Arizona, USA). Despite its relatively small size, this crater acts as an effective wind shelter. Daytime upslope winds and nocturnal downslope flows are regularly observed on its west/east sidewalls. The times of diurnal wind-direction shift(i.e., around sunrise and sunset) were slightly mismatched between the sites based on the east and west sidewalls, owing to the crater's shadowing effects. Under conditions of relatively weak synoptic forcing, the nocturnal downslope flows prevail throughout the night, despite quite a low wind velocity near the surface. ?

Numerical Simulation of the Critical Scale of Oasis Maintenance and Development in the Arid Regions of Northwest China

Oasis is a special geographic landscape among the vast desert/Gobi in Northwest China (NWC). The surface sensitive heat flux and latent heat flux at Zhangye Oasis during 1 to 11 August 1991 are simulated using the NCAR nonhydrostatic mesoscale model MM5 Version 3. The horizontal grid resolution is set as 1km. By comparing the simulation results with HEIFE observations, it is proved that the model can be used to simulate the surface energy and water mass exchange of arid and semiarid regions in NWC. Based on the above results, the influence of different oasis scales on the local atmospheric field near the ground surface, and the critical scale of oasis maintenance, in NWC are studied dynamically. The following conclusion is obtained: the local thermal circulation between the oasis and the desert/Gobi is formed in the oasis downstream if the oasis scale is larger than 4 km. This local thermal circulation between the oasis and the desert adjacent to the oasis helps to conserve water vapor over the oasis. At the same time, it transfers the abundant water vapor from the oasis into the desert/Gobi near to the oasis to supply relatively plentiful water vapor for desert crops to grow on the fringe of the oasis. So, it is advantageous for oasis extension. However, if the scale of the oasis is smaller than 4 km, it is not easy for the local thermal circulation between the oasis and the desert/Gobi to take shape. This study provides a new standpoint for oasis maintenance and development.

EFFECT OF THE THERMAL FORCING ON THE SECONDARY CIRCULATION OF TYPHOONS

A non-dimensional secondary circulation equation for typhoons has been derived and then 11-yr com- positing typhoon data were used to estimate the thermally forced secondary circulation.The main results have been obtained as follows: (1)The diabatic heating and Cu vertical heat mixing are major thermal forcing factors.They have the same magnitude of order.(2)The effects of eddy flux and Cu horizontal mixing of heat are of minor im- portance.(3)Ekman pumping and Cu vertical heat mixing cooperatively work.This feedback process is favorable for the enhancement of the secondary circulation of typhoons.