Diurnal variation in vertical air motion over a tropical station, Gadanki (13.5°N, 79.2°E), and its effect on the estimation of mean vertical air motion

Abstract

The long‐term measurements of vertical air velocity (w) made with the Indian mesosphere‐stratosphere‐troposphere (MST) radar are used to investigate the diurnal variation of w in different seasons. The effect of the diurnal variation of w and gravity wave variance on the estimation of mean w is also studied. Significant diurnal variation is observed in the vertical air motion at Gadanki with a variation larger than 6 cm s−1. This variation is, in fact, larger than the mean vertical air motion. Downdrafts are observed in the evening hours in the lower troposphere in all the seasons. A large part of this subsidence is due to the bias caused by upward propagating gravity waves, as evidenced by large hourly variance values in evening hours. In the middle and upper troposphere, the diurnal variation is dominated primarily by 12‐h oscillation, which is conspicuously present in all the seasons, albeit with different amplitudes. The 24‐h variance caused primarily by tidal and relatively long period oscillations (>2 h) and mesoscale weather disturbances is almost comparable to the variance due to short‐period gravity waves (<1 h). The variance for periods <1 h shows significant diurnal variation with peak values during afternoon‐evening. It implies that the bias in w should also have such a diurnal cycle, because the bias (downward) is directly proportional to the variance in w. Given that w and the bias sources vary considerably during the course of the day, any estimate of mean w (taken over a season/month) from data pertaining to a particular hour (hours) of the day will represent the mean w of that particular hour, but may not truly represent the seasonal/monthly mean w.