Observed diurnal and intraseasonal variations in boundary layer winds over Ganges valley

Abstract

The diurnal and intraseasonal variability in the surface winds are examined using high resolution measurements during the Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) Phase III Integrated Ground based Experiment, conducted in the Ganges valley during 2014–15. The yearlong observations of lower boundary layer wind from the Ganges valley are discussed in this study. Wind speed and directions at the surface levels obtained from SOund Detection And Ranging (SODAR) and tower are compared and found, mostly SODAR measurements are consistent with tower during high wind conditions (greater than 4 ms−1) except under weak circumstances. Our analysis revealed that, the mean wind speed found to be a maximum (7.86 ms−1) during the monsoon season in the air layer between 10 m and 200 m level and shows a sharp increase near to the surface (at 20 m level). Analysis also depicts seasonal variations in wind speed are predominant in the lower boundary layer and it is closely linked to the large-scale flow patterns/disturbances. The turbulent kinetic energy (TKE) was found to be maximum at 165 m (0.62 m2s-2) during the pre-monsoon due to enhanced surface heating. The 93% and 75% of the wind speed profiles exhibited unstable conditions in the monsoon and post monsoon conditions. Moreover, the unstable layers are characterized by high wind speed and temperature structure function (CT2)in day time. On large scale, the wind speed gets strengthened (reduced) in the wet (dry) spell in the lower levels with distinct diurnal cycle in the Indo Gangetic Plain. The advection of moisture in the nocturnal periods in connection with the wet spells in the near surface level is observed and the wind speeds in the daytime (above 150 m) are also found to be stronger in the range 3.5–5 ms−1 in the dry spells due to the enhanced mixing. The diurnal cycle is more evident in the dry spells in the lower levels. Rainfall peak found in July 2014 is connected with the quasi-biweekly mode periodicity of 2–16 days and further, power spectrum analysis reveals that the presence of high frequency oscillations of periodicity less than 10 days and between 10 and 20 days in the surface layer parameters.