High resolution VHF radar measurements of tropopause structure and variability at Davis, Antarctica (69° S, 78° E)
Two years of Very High Frequency (VHF) radar echo power observations are used to examine the structure and variability of the tropopause at Davis, Antarctica. Co-located radiosonde and ozonesonde launches provide data with which to calculate the lapse-rate and chemical tropopauses. The radar tropopause, defined as the maximum vertical gradient of echo return power, can be used as a definition of the Antarctic tropopause throughout the year under all meteorological conditions. During the extended summer period of December–April (DJFMA) inclusive, radar tropopauses are (0.2 ± 0.4) km lower than radiosonde lapse-rate (i.e. the World Meteorological Organisation – WMO) tropopauses and during the extended winter period of June–October (JJASO) inclusive, the radar tropopauses are (0.8 ± 1.0) km lower. A potential vorticity tropopause is defined as the altitude of the −2 PVU surface (where 1 PVU = 10 6 m 2 s −1 K kg −1). This is (0.3 ± 0.5) km lower than the radar tropopause during DJFMA and (0.5 ± 1.0) km lower during JJASO. The radar, potential vorticity and ozone tropopauses decrease in altitude during increasingly strong cyclonic conditions, in contrast to the radiosonde WMO tropopause which remains nearly constant. During strong JJASO cyclonic conditions, there are large (several km) differences between WMO tropopause altitudes and radar tropopause altitudes. A seasonal cycle in tropopause fold occurrence is observed, with approximately a three-fold increase during JJASO.