Differences in the mean wind and its diurnal variation between wet and dry spells of the monsoon over southeast India

The differences in the vertical structure of mean wind and its diurnal variation from the surface to upper troposphere between wet and dry spells are studied using a unique data set consisting of surface and remote sensing (SOund Detection and Ranging and wind profilers) measurements made at Gadanki (13.5 degrees N 79.2 degrees E). Special emphasis was given to study the variation of low-level jet (LLJ) and tropical easterly jet (TEJ), two most conspicuous features of the Indian summer monsoon. Largest and significant wind differences between the spells are observed in the lower troposphere (< 4 km) with stronger winds during the dry spell than during wet spell. The spatial variation of LLJ and TEJ depicts intriguing differences between the spells. The LLJ splits into two branches over the Arabian Sea during the dry spell, while only one branch is apparent during the wet spell. On the 100 hPa level, the longitudinal extent of the TEJ is larger during the dry spell than during wet spell. The vertical variation (in particular, in the lower troposphere) of the amplitude and phase of diurnal cycle is significant and, to some extent, different during the dry and wet spells. The plausible mechanisms responsible for the change in the phase of the diurnal cycle with altitude are discussed. The amplitude of the diurnal cycle increases with altitude up to 2 km and then decreases. Largest amplitudes in the entire troposphere and lower stratosphere are found in the height region of 1-2km. Given that LLJ peak magnitude and height exhibit a strong diurnal variation, one should be careful in obtaining characteristics of the LLJ and nocturnal LLJ from observations made at a fixed time. The diurnal variation of LLJ and TEJ exhibits distinct spatial variability in both spells. The results are compared and contrasted with earlier reports on intraseasonal variability of the LLJ and TEJ.