How representative were the meteorological conditions during the COPS field experiment in summer 2007?

Abstract

This study investigates the meteorological conditions in Europe during summer 2007 (June-August) and their representativity compared to climatology, based upon (re-)analysis data from the European Centre for Medium-range Weather Forecasts (ECMWF) for the years 1979-2007. Several parameters have been analyzed locally in the region of the Convective and Orographic Precipitation Study (COPS), e.g., low-tropospheric temperature and humidity and thermodynamic stability, and on the continental scale, e.g., frequency of surface lows and upper-level stratospheric cut-offs, Eady growth rate, and air mass pathways. It has been quantified whether the statistical distributions of these parameters during summer 2007 differ from climatology. It is shown that cyclones were extraordinarily frequent during summer 2007 over the British Isles. At upper levels, they typically went along with quasi-stationary stratospheric cut-offs that supported the transport of relatively cool air masses from the North Atlantic to France and warm air masses from the Mediterranean into Eastern Europe. In the lowest troposphere, the COPS area, situated in between these anomalous airflows, experienced fairly normal temperatures and enhanced humidity values compared to climatology. As a consequence, thermodynamic indices indicate slightly increased potential instability. Consistent with this feature-based analysis, trajectory calculations indicate reduced large-scale advection from Scandinavia and the western North Atlantic, and increased transport from southern France and northern Spain in 2007 compared to the previous almost 30 years. Although warm and humid conditions dominated during the Intense Observation Periods (IOPs) of COPS, the variability of ambient meteorological parameters during IOPs was large. This should be taken into account when interpreting the results from a particular IOP and considering their representativity in a climatological context. It is suggested that a similar approach could also be fruitful for other field experiments and for a feature-based characterization of climatologically extreme seasons like, e.g., the European heat summer 2003.