Features of nocturnal low level jet (NLLJ) observed over a tropical Indian station using high resolution Doppler wind lidar

Abstract

High resolution Doppler wind lidar measurements made during the period 01 April 2012 to 31 March 2014 over Pune (18°32′N, 73°51′E, 559m Above Mean Sea Level), India have been used to study Nocturnal Low Level Jet (NLLJ) occurrence and its characteristics. Vertical profiles of horizontal wind in the altitude range from 100m to 3000m (at every 50m interval) and averaged over 5min have been used to study time–height variations during local nighttime. On several occasions during nighttime the wind profiles showed a narrow region of strong wind speed below 1000m altitude from surface, suggesting the presence of the low level jet. Analysis of the data indicates that NLLJ occurs more frequently (~66%) during pre-monsoon season (March–May) and on only 14% of the nocturnal period during SW monsoon season (June-September). Mean jet core heights during pre-monsoon, monsoon, post-monsoon (October–November), and winter (December–February) seasons are found to be 687m, 691m, 593m, and 586m respectively. Seasonal mean jet core speeds during pre-monsoon and monsoon are higher than those during winter. There are some occasions during monsoon season when hourly mean jet speeds during nighttime are as high as 15–20ms−1. Horizontal wind directions in the NLLJ during different seasons are consistent with the seasonal mean flow over the tropical Indian region. Most frequently occurring jet core height is in the height range 600–700m with almost 65% of the cases having jet core heights<700m and maximum frequency of occurrence of jet speed is in the range 9–11ms−1. Large east–west temperature gradients, inertial oscillations, stability in the lower atmosphere seem to be some of the factors that play significant role in the formation and sustenance of NLLJ over the location during different seasons.