High resolution VHF radar measurements of tropopause structure and variability at Davis, Antarctica (69° S, 78° E)

Abstract

Abstract. 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 = 106 m2 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.

Observations and fine‐scale model simulations of gravity waves over Davis, East Antarctica (69°S, 78°E)

“…The radar tropopause altitude is calculated every 2 h and is defined as the level of the maximum vertical gradient of radar echo return power [Gage and Green, 1979]. It corresponds closely to the −2 PVU (potential vorticity unit) dynamical tropopause surface (where 1 PVU = 10 6 m 2 s −1 K kg −1 ) [Alexander et al, 2013a]. The radar tropopause is generally ∼0.5 km lower than the lapse rate tropopause over these three days in February.…”

Stratospheric ozone intrusion events and their impacts on tropospheric ozone in the Southern Hemisphere

“…At Davis, ozonesondes are launched twice as frequently during the ozone hole season and preceding months (June-October) as at other times of year (Alexander et al, 2013). A summary of ozonesonde releases at each site can be seen in Table 1.…”

“…The ozone tropopause may misdiagnose the real tropopause altitude during stratosphere-troposphere exchange; however, it is useful at polar latitudes in winter, where the lapse rate definition may result in artificially high values for tropopause height (Bethan et al, 1996;Tomikawa et al, 2009;Alexander et al, 2013). We require lapse-rate-defined tropopauses to be at a minimum of 4 km altitude.…”

The radar tropopause at 78°N, 16°E: Characteristics of diurnal variation

“…[6] Although the tropopause is estimated for each hour of radar echoes as described by Hall et al [2009], for a variety of reasons, this estimation may not be successful. Reasons include instrument failure, poor signal-to-noise ratios, and also meteorological phenomena such as frontal passages and tropopause folds [Alexander et al, 2013;May et al, 1991;Nastrom et al, 1989]. Generally, though, most days of observation yield several tropopause altitude estimates, but by no means all possible 24.…”

“…Using more sophisticated radars, it is possible to identify the radar tropopause as the maximum in gradient of reflected power, but for the relatively simple system used to obtain echoes in this study, it is more reliable to identify the height of peak echo power itself. A very detailed review of the various definitions of tropopause is given by Alexander et al [2013] together with pros and cons of adopting the different definitions. Somewhat exhaustive comparisons have been made between the radar results which will be presented here and the tropopause determinations from radiosonde soundings nearby (Ny-Ålesund, 79°N, 12°E), showing a high degree of agreement in temporal behavior and that the altitude difference between the two determinations is almost invariant-1.8 ± 0.3 km-with a slight seasonal variation [Hall et al 2011;Hall, 2013] contained within the standard deviation.…”