Seasonal trend in the occurrence of nocturnal drainage flow on a forested slope under a tropical monsoon climate

Using data for one year, we examined the vertical wind speed profiles on a mountain slope covered with forest in northern Thailand under a tropical monsoon climate. We defined two profile patterns: higher wind speeds at greater heights ( Pattern 1) and lower wind speeds at greater heights ( Pattern 2). We classified 9.4% of the data as Pattern 2; this pattern tended to occur during the night, at low wind speeds, and with high outgoing longwave radiation. In addition, stable stratification and decoupling between the canopy surface air and the overlying layers were observed when Pattern 2 occurred frequently. These facts suggested that Pattern 2 was caused by a nocturnal drainage flow. The occurrence of Pattern 2 showed a clear seasonal trend, indicating that there is a seasonal trend in the occurrence of nocturnal drainage flows. Pattern 2 was observed more frequently between August and February and less frequently between March and July. This corresponded to the seasonal trend in wind speed, but did not correspond to the seasonal trend in the outgoing longwave radiation.