Observation of a tropopause fold by MARA VHF wind-profiler radar and ozonesonde at Wasa, Antarctica: comparison with ECMWF analysis and a WRF model simulation

Abstract

Abstract. Tropopause folds are one of the mechanisms of stratosphere-troposphere exchange, which can bring ozone rich stratospheric air to low altitudes in the extra-tropical regions. They have been widely studied at northern mid-or high latitudes, but so far almost no studies have been made at mid-or high southern latitudes. The Moveable Atmospheric Radar for Antarctica (MARA), a 54.5 MHz wind-profiler radar, has operated at the Swedish summer station Wasa, Antarctica (73 • S, 13.5 • W) during austral summer seasons from 2007 to 2011 and has observed on several occasions signatures similar to those caused by tropopause folds at comparable Arctic latitudes. Here a case study is presented of one of these events when an ozonesonde successfully sampled the fold. Analysis from European Center for Medium Range Weather Forecasting (ECMWF) is used to study the circumstances surrounding the event, and as boundary conditions for a mesoscale simulation using the Weather Research and Forecasting (WRF) model. The fold is well resolved by the WRF simulation, and occurs on the poleward side of the polar jet stream. However, MARA resolves fine-scale layering associated with the fold better than the WRF simulation.

Tropopause fold occurrence rates over the Antarctic station Troll (72° S, 2.5° E)

“…As was shown by previous studies (in the Northern Hemisphere Rao and Kirkwood, 2005, and in the Southern Hemisphere Mihalikova et al, 2012), folds can be positively identified in radar data by static-stability features which agree closely with layers of enhanced ozone measured by colocated in situ measurements by ozonesonde. This makes radar observations a powerful source of information about the occurrence rates of tropopause folds in areas without the possibilities of targeted ozonesonde measurement campaigns.…”

“…Folding-like events, when measured wind speeds over the observation site do not reach decided 30 m s −1 , can in reality be tropopause folding events, which bring ozone-rich air mass from the lower stratosphere or extratropical transition layer (see e.g. Gettelman et al, 2011) to significantly lower altitudes, as suggested by radar measurements, ozonesonde observation and high-resolution modelling for a summer-fold at Wasa, Antarctica (Mihalikova et al, 2012).…”

“…The radar observations suggest that the deepness of the tropopause fold is more than the ECMWF −2 pvu level would suggest. Previously it has been shown by high-resolution modelling that, with improved spatial resolution of a model, the model representation of a radar-observed fold is significantly improved and matches also the observed vertical profile of ozone concentration in detail (Mihalikova et al, 2012). This may have implications for future model-based studies (e.g.…”

“…This can mean that, although the models are representing the processes leading to formation of tropopause folds, the folds were not resolved with high enough resolution in the models, thus leading to underestimation of fold occurrence rates. For the Southern Hemisphere the study by estimated occurrence rates of tropopause folds in high latitudes to be 0-4 %, with nearly 0 % occurrence rates in summer months (December, January, February) Mihalikova et al (2012) showed an agreement between ozonesonde observations and radar measurements as well as improved representation of a tropopause fold by high-resolution modelling. Since December 2011, MARA has been located at Troll station, and it operates continuously throughout the year.…”

“…It was run during subsequent summer season months until the season 2010/2011. During this time a tropopause fold observation was successfully made and reported (Mihalikova et al, 2012). However, the database was insufficient to infer any information about tropopause fold occurrence rates.…”