The relationship between the direction of the diurnal rainfall migration and the ambient wind over the Southern Sumatra Island

The present study investigates the climatological relationship between the direction of the diurnal migration of rainfall and the ambient wind and its mechanism over the Sumatra Island, making use of long-term data (1998-2012) from TRMM satellite product and re-analysis. Migration systems over the Southern region are especially focusedtargeted on, where diurnal cycle of precipitation shows strong seasonality based on FFT analysis.

We make Tthe time series of zonal wind averaged for lower troposphere (1000 hPa to 500 hPa) is first constructed over the focused area. Using the mean (U ̅) and standard deviation (σ) of the time-series, we identify 4 regimes and calculate a composite of diurnal rainfall in each regime; (1) Strong Westerly regime (SW; U≥U ̅+σ), (2) Westerly regime (W; 0When the ambient wind in the lower troposphere is westerly (SW and W regime), rainfall systems characterized by convective rain migrate eastward. Under the same wind condition, cloud systems dominated by stratiform rainfall simultaneously propagate westward. In SW regime, however the migrating signal of diurnal precipitation cycle is considerably weaker. In the easterly wind regime (SE and E regime), the convective and stratiform rainfall migrates only to the west. The migrating system of rainfall also develops or re-develops over the sea at the midnight to morning each regime.

The eastward migration speed of convective rain system is approximately 5 m s-1 and 8 m s-1 in WS and S regime, respectively. These propagation speeds almost agree with the ambient wind speed in the lower troposphere. The westward propagation speed of cloud system characterized by stratiform precipitation is about 5 ms-1 in W regime. This migration speed is also associated with the speed of easterly background wind in the mid-upper troposphere. On the other hand, the westward propagation of cloud system composed of convective and stratiform rain is about 8 ms-1 and 8-12 ms-1 in E and SE regime, respectively. The relatively slower speed somewhat corresponds is loosely tied to the ambient wind in the mid-upper troposphere. The relatively faster speed, however, is not clearly explained bydo not agreed with the wind speed.

The direction of the rainfall system migration appears to be strongly controlled by ambient zonal wind. These results suggest that the mechanism of the eastward (westward) migration systems characterized by convective (stratiform) type rain are advection by the background wind in the lower (mid-upper) troposphere. On the other hand, the mechanism of westward propagation systems dominated by convective rainfall remain unclear in our study.