Diurnal variability of the tropical tropopause: Significance of VHF radar measurements

VHF radars have been used for the detection of atmospheric stable layers such as the tropopause for quite sometime. This opened up a possibility of using VHF radars to study the short period variability of the tropopause with a better time and height resolution as compared to conventional radiosonde measurements. In the present study, a method to determine the tropopause height using VHF radar located at Gadanki (13.5°N, 79.2°E) is discussed and the results are compared with the simultaneous balloon observations. Comparison of radar measurements of cold-point tropopause height (CPT) with simultaneous balloon ascents on 58 occasions has shown a very good agreement with a correlation coefficient of 0.85. The data collected during four different campaigns, which were carried out by employing radar and radiosonde/GPS sonde observations to study the different aspects of lower atmospheric dynamics, are used for this purpose. The radar measurements are then extensively used to study the diurnal variation of tropopause height for the first time over this latitude. The results showed that the tropopause does respond to the diurnal oscillations, which are consistent with the earlier radar studies reported from other geographical locations. During one of the campaigns, the tropopause height has also exhibited the semidiurnal oscillation, which is first of its kind. The diurnal observations clearly demonstrated the usefulness of the radar-detected tropopause. Further, the oscillations in the tropopause height showed a phase delay of ∼1 h and 4–6 h with zonal and meridional wind tidal oscillations, respectively. The observed diurnal/semidiurnal oscillations in the tropopause height are interpreted as tidal manifestations. However, the diurnal variation of convective activity, which also contributes to the diurnal variation of tropopause height, is also accounted. The significance of the present study lies in showing the response of tropopause height to the propagating tides, which can provide some new insights in stratosphere-troposphere exchange processes.