Occurrence of low-level jets and multi-layer clouds over various urban agglomerations located in Indo-Gangetic Plain

The continuous heat release over cities/urban areas during night owing to urban heat island effect causes turbulent mixing and sustained supply of moisture and cloud condensation nuclei to the boundary layer. This sustained supply of moisture helps in persistence of low-level clouds over these urban areas as compared to rural/suburban areas during night. The various dynamical process including cloud formation, transport and dispersion of moisture and pollutant at boundary level at night time is highly influenced by the nocturnal low-level jets .These nocturnal low-level jets are found to be stronger over urban . These low-level jets are associated with high vertical wind shear production which enhances the turbulent mixing below boundary layer and plays a critical role in formation of nocturnal stratus clouds. In this study we have identified the vertical location of low-level jets using radiosonde data.  We get two peak for the frequency of occurrence of low-level jets, first at around 500-1000 m (boundary-level jets) and another at 1500 m altitude for winter, pre-monsoon, monsoon and post-monsoon seasons. We have also shown the diurnal (morning and evening) variation in the low-level jet frequency for these four seasons. The measurements from radiosonde in this study are taken at 05:30 am Local time (00:00 UTC) and 5:30 pm (12:00 UTC). We have also identified the types of clouds classified on the basis of number of layers over the study areas and associated it with the occurrence of low-level jets.