Diurnal variability of convection over northwest Indian subcontinent observed by the Doppler weather radar data

摘要

The diurnal cycle of convection over a sub-tropical semi-arid inland station-Delhi-has been analyzed in this study based on three different rainfall episodes. Two of these cases represent convection in association with low precipitable water content ( 40 mm) and moderate vertical wind shear (between 4 and 10 m/s) while the third case represents convection under high precipitable water content (> 60 mm) and low vertical wind shear ( 2 m/s). It has been noted that for all the three cases, convection was initiated during the morning hours in the form of single cells, which evolved into multi-cellular convection zones later on those days. The most common mesoscale organization of the clouds in all the three cases had been in the form of convective lines which moved along the mean steering flow in the lower troposphere. However, for case 1, squall line formation and movement were observed during a period of 6 h, which was aided by the high unidirectional shear in the lower-to-middle troposphere, that was absent in other cases. These squall lines were associated with severe surface winds. The convection zones were found to be short lived with less stratiform outflow for case 1, more stratiform outflow for case 2 and longest lifetimes and most stratiform outflow for case 3. This study also indicates that the primary peak of convection and associated rainfall over the region, irrespective of the season, is in the afternoon hours between 1730 Indian Standard Time (IST) and 2030 IST, and lags the diurnal temperature maximum (around 1430 IST) by 3-5 h. When there is sufficient moisture in the atmosphere and convection persists throughout the entire diurnal cycle, a second peak in convection and associated rainfall appears over the region in the early morning hours (between 0230 and 0530 IST). This night time-early morning peak has a greater fraction of stratiform clouds at the beginning of a rainfall episode. As the moisture build up in the atmosphere on day 2 and later of a long-lived episode, new convection was initiated in the night time with increase in the night time rainfall intensity. This implies that pre-monsoon convection over Delhi in the presence of low moisture is primarily unimodal, characterized by short bursts of intense convection with narrow and short-lived cells. Monsoon convection on the other hand, is essentially bimodal, with the early morning peak, often pre-dominating over the afternoon peak and characterized by longer lived cells which are less intense than cells of the pre-monsoon weather systems.