Abstract
Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.
Highlights
The COPS (Convective and Orographically-induced Precipitation Study) field campaign was undertaken from 1 June to 31 August 2007 within the low mountain range of theBlack Forest, the Vosges Mountains and the Swabian Jura with the Rhine rift valley as a pronounced topographic lowland plain in between (Wulfmeyer et al, 2008)
The focus of the present study is to demonstrate the applicability of the EC method to the detection of free convection conditions (FCCs) in experimental data obtained during the COPS field campaign in the Kinzig valley (Black Forest), which was found in earlier studies (e.g., Meißner et al, 2007) to establish a pronounced diurnal thermally-induced valley wind system
FCCs were found to be triggered by a change of the local valley circulation system from down to up-valley winds in the morning hours and occurred on about half of the total 92 COPS days summing up days classified as “event” and “intermittent days”
Summary
The COPS (Convective and Orographically-induced Precipitation Study) field campaign was undertaken from 1 June to 31 August 2007 within the low mountain range of theBlack Forest, the Vosges Mountains and the Swabian Jura with the Rhine rift valley as a pronounced topographic lowland plain in between (Wulfmeyer et al, 2008). Rainfall in the COPS area is characterized by subgrid-scale convection initiation (CI) processes, e.g. orographically or thermallyinduced local circulation systems, triggered by the complex terrain, complicating the exact modeling and forecasting of precipitation events (Meißner et al, 2007; Barthlott et al, 2006). Together with diurnal mountain winds developing over mountainous terrain (Whiteman, 1990), land surface-atmosphere interactions and the related physical processes within complex terrain are the key to the local occurrence and timing of the initiation of convection, cloud formation and precipitation (Hanesiak et al, 2004; Pielke, 2001; Chen and Avissar, 1994; Rabin et al, 1990; Banta, 1990, 1984; Raymond and Wilkening, 1980).
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