Spatial variations in the diurnal patterns of winter precipitation in India

The present study focuses on spatial patterns of diurnal variability in winter precipitation across the Indian subcontinent. Hourly precipitation data during the months of January and February from 1980 to 2002 were procured for 80 stations spread across the subcontinent. The methodology consisted of calculation of the first harmonics by harmonic analysis for three variables that included frequency, total amount, and intensity of precipitation events. There were substantial variations in the strength of the diurnal cycle prevailing over the subcontinent, with most of the peninsular region and eastern part of the northern Gangetic Plain showing relatively stronger diurnal cycles. The variance explained by the first harmonic was significant at the 90% confidence level for larger parts of the peninsular region and northern plains. The weakest patterns were observed in the interior of the continent over central India. The times of maximum for hourly frequency, total amount, and intensity of precipitation were predominantly during the latter half of the 24-hour period, during late evening hours to just before sunrise hours. This was due to the enhanced warm front lifting during the late night to early morning hours as a result of nighttime inversions in the warm sector. Further, along the foothills of the Himalayas, the times of maximum were observed during predawn hours just before sunrise, caused by the downslope movement of katabatic winds and their convergence with the cyclonic storms in the low-lying areas, leading to enhanced precipitation. Along the west coast, the times of maxima were also concentrated in the midnight to early-morning hours due to the convergence of inland moving seas breeze fronts with the northeast trades.