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

Abstract

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°N 79.2°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 1–2 km. 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.

A Versatile 205 MHz Stratosphere–Troposphere Radar at Cochin – Scientific Applications

“…The associated upper air circulation extends to the upper troposphere and even to the lower stratosphere. The altitude of jet-core, strength of wind speed, and the north-south oscillations of LLJ and tropical easterly jet stream show strong association with monsoon activity 18 .…”

Climatological characteristics of nocturnal eastward‐propagating diurnal precipitation peak over South India during summer monsoon: Role of monsoon low‐level circulation and gravity waves

“…The WGs in South India block these prevailing winds and reduce the strength of the southwesterly winds on the leeward side of the mountains. Under these conditions, it was observed that lower atmospheric winds exhibited systematic diurnal variability (Mohan & Rao, 2016;Ruchith et al, 2014) over the leeward side of the WGs (Figure 5; see Figure S3). During the afternoon and evening (1430-1730 LST), the anomalous southwesterly in southeast India showed signs of weakening.…”

Nocturnal Southerly Moist Surge Parallel to the Coastline Over the Western Bay of Bengal

“…Land surface effects of the Indian subcontinent, including both their dynamic and their thermodynamic roles, can be major influencing factors regarding the nocturnal moist surge toward the Gangetic Plain over the western BoB. This is because the Indian subcontinent, which is located further windward, can have substantial impact on the low‐level monsoon westerlies over and around the Indian subcontinent (Mohan & Rao, 2016; Ruchith et al., 2014). In the daytime, the mixed layer develops from the surface to 850 hPa across the Indian subcontinent (Figure 4a).…”