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This Quarterly Journal of the Royal Meteorological Society Special Issue gives an overview of the scientific results of the Convective and Orographically-induced Precipitation Study (COPS). It is the all-encompassing objective of COPS to identify the physical and chemical processes responsible for the deficiencies in Quantitative Precipitation Forecasting (QPF) over low-mountain regions with the target to improve their model representation. The overarching COPS goal can be summarized as:

Advance the quality of forecasts of orographically-induced convective precipitation by 4D observations and modelling of its life cycle.

Improving the skill of QPF is one of the major challenges in atmospheric sciences. On the mesoscale, progress in QPF has been slow for a paramount reason: the high degree of complexity involved in the understanding and simulation of the chain of processes leading to the initiation of convection, as well as in the development, organization and decay of clouds and precipitation. The relevance of particular processes within this causal chain strongly depends on the forcing conditions and the geographical region. This holds for factors such as land-surface exchange during airmass convection, as well as the presence of surface fronts.

It is not surprising that some of the most severe deficiencies in QPF have been identified in orographic terrain. These regions are characterized by a tendency for flash-flood events due to the enhancement of precipitation by lifting. Systematic errors include an overestimation and underestimation of precipitation on the windward and lee side of the mountains, respectively, and a phase error in the diurnal cycle of precipitation leading to an onset of precipitation several hours too early in forecasts. Also aerosol-cloud interaction, which is crudely parameterized in most of the current NWP models, can lead to significant errors in precipitation intensity and distribution. Generally, the skill of QPF decreases considerably with increasing amounts of precipitation resulting in an unacceptable performance during strong precipitation events for many end users.

COPS was performed from 1 June–31 August 2007 in a low-mountain area in southwestern Germany/eastern France covering the Vosges Mountains, the Rhine Valley and the Black Forest Mountains. COPS was initiated within the Priority Program PP 1167 PQP (Precipitationis Quantitativae Predictio) of the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft).

To address the ambitious goals of COPS a strong international collaboration was considered essential. This was successfully established by a series of concerted measures. A pivotal achievement was the endorsement of COPS as Research and Development Project (RDP) of the World Weather Programme (WWRP). Furthermore, COPS was interlinked with a number of international projects and programmes such as the PP 1167 General Observations Period, the first summertime European THORPEX Regional Campaign (ETReC2007), and the Transport and Chemistry of Convective Systems (TRACKS) campaign of the German Helmholtz Research Centres. Pan-national programmes such as COPS-France (supported by the Agence Nationale de la Recherche, the Institut National des Sciences de l'Univers and the Centre National d'Etudes Spatiales), UK-COPS (supported by the Natural Environment Research Council), COPS-Austria (supported by the Austrian Science Foundation) and the US ARM programme also contributed significantly to the project instrumental detachment. COPS received substantial contributions of DWD, Météo-France and MeteoSwiss, as well as ECMWF, and also greatly benefited from the skills and guidance of forecasters from these forecast centers.

A close interaction with the mesoscale modeling community was accomplished by coordinating COPS with the WWRP Forecast Demonstration Project (FDP) Demonstration of Probabilistic Hydrological and Atmospheric Simulation of flood Events in the Alpine region (D-PHASE). On the one hand, the output of an ensemble of forecast systems was applied for mission planning; on the other hand, a huge ensemble of mesoscale forecasts became available for process and verification studies.

During COPS, an unprecedented combination of in situ instruments and remote sensing systems was applied. This includes what is to our knowledge the largest combination of multi-wavelength passive and active remote sensing systems that has been deployed yet during a field campaign. At five supersites along a nearly west–east transect through the COPS domain, 14 lidar systems (11 ground-based, 3 airborne), 3 cloud radars, 10 precipitation radars, 8 microwave radiometers, 1 Fourier-transform infra-red radiometer and 5 sodars were operated in synergy with in situ measurements. Some of these instruments were successfully operated for the first time, such as the airborne Doppler lidar–water-vapour DIAL combination of DLR Oberpfaffenhofen and the ground-based, scanning water-vapour DIAL of the University of Hohenheim. A unique backbone of the measurements was the US ARM Mobile Facility (AMF), which was operated for 9 months (March–December) at one of the supersites in the COPS region.

Ten aircraft with different special sensors were operated in coordination with each other and with ground-based sensors in a region with high commercial aircraft density. The specific combination of sensors permitted the observation of atmospheric variables and aerosol-cloud microphysics in a larger domain and for closing gaps between the supersites and for upstream observations in connection with ETReC2007. COPS measurements were supported by a MSG 5-min Rapid Scan Service (RSS) of EUMETSTAT. The success of the RSS operation during COPS led to the operational RSS of MSG-8 which commenced on 13 May 2008.

New networks were set up and operated for the campaign, such as observations of soil moisture. Existing networks were densified, e.g. weather stations in the domain as well as the network of ground-based GPS receivers. A new high-resolution surface dataset, the so-called JDC (Joint D-Phase COPS) dataset, was collected and harmonized over central Europe during 2007. To our knowledge this is the densest surface dataset that has been collected so far in Europe, with data from more than 10 000 stations. An important backbone of the observations was radio soundings at all supersites, which were performed intensively during the IOPs. In total, 2700 soundings are available, 11 000 h of lidar data, 400 h of aircraft data and 10 000 model runs with 50 000 000 model fields and plots in the COPS domain.

The COPS data set covers the entire evolution of convective precipitation events during 18 Intensive Observations Periods under different combinations of forcing conditions. During theses IOPs, the formation and organization of convective precipitation systems in low-mountain regions has been investigated with unprecedented detail.

This Special Issue consists of a series of 21 papers analyzing these IOPs, measurement periods or continuous measurements during the entire COPS period. It starts with an overview of the COPS campaign and of first scientific results (Wulfmeyer et al). A comprehensive forcing concept is introduced for understanding the key drivers for the development of precipitation. Process studies from land-surface exchange, to convection initiation, to the formation and microphysics of clouds and precipitation are introduced. Furthermore, data assimilation studies and research on model predictive skill and predictability are presented. Weckwerth et al discuss convection initiation and convective enhancements statistics in the summer of 2007 and compare them to a 8-year climatology developed using radar reflectivity data in the COPS region for the period of May–August in the years 2000–2006 and 2008. Hauck et al use data from the newly installed soil moisture monitoring network comprising 47 stations with soil moisture sensors installed at three different depths within the COPS area in southwest Germany to analyse soil moisture variability and its influence on convective precipitation over complex terrain. Eigenmann et al discuss the effect of land use and orography on turbulent fluxes measured at different locations within the COPS area as well as the surface energy balance and turbulence response to a frontal passage. The dependence of convection-related parameters on surface and boundary-layer conditions over complex terrain is investigated by Kalthoff et al, who provide evidence that the frequency of low CIN (convective inhibition) was higher in the Black Forest mountains than in the Rhine valley, which facilitates convection initiation over the mountain sites. Behrendt et al detail the processes leading to deep convection over the Black Forest during COPS IOP 8b (15 July 2007) using a suite of state-of-the-art research instruments, namely of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and high-resolution gridded data of weather stations as well as Global Positioning System (GPS) integrated-water-vapour data. The forecasting skill of the Meso-NH model regarding summer convection over the Black Forest is investigated during IOP 8b by Richard et al with a focus on the ability of the model to reproduce the dynamical forcing. The same case is also investigated using an ensemble of mesoscale models by Barthlott et al, highlighting the importance of boundary-layer convergence features for quantitative precipitation forecasts in mountainous terrain. The complex multi-scale interactions between processes driving deep convection over the Black Forest during IOP 9c are examined by Corsmeier et al This contribution provides a thorough analysis of the complex interplay of the small and large scales that cause convective initiation. The impact of assimilating conventional and GPS zenith total delay data over France into the Weather Research and Forecasting (WRF) model is investigated during IOP 9c by Schwitalla et al Bennett et al study the processes leading to the initiation of convection over the Black Forest mountains during IOP15a by comparing Doppler-On-Wheels radar observations with high-resolution WRF simulations. Their comparisons demonstrate the capacity of the WRF model to detect the location of precipitation. The first-ever vertical profiles of latent heat flux measurements obtained over complex terrain from a combination of airborne water vapour and wind lidars are presented by Kiemle et al Van Baelen et al investigate the relationship between water vapour field evolution and the life cycle of precipitation systems using GPS tomography and the C-band POLDIRAD weather radar. The influence of upstream wind conditions on the initiation of convection in the vicinity of the Vosges Mountains is examined by Hagen et al Chaboureau et al inspect long-range transport of Saharan dust and its radiative impact on precipitation forecast over France during IOP 13a. A suite of instruments housed at the top of the Hornisgrinde Mountain in the Black Forest region provided datasets that allow an investigation into the physical, chemical and hygroscopic properties of the aerosol particles sampled during COPS (Jones et al). Junkermann et al examine a plume of ultrafine particles observed downwind of the Karlsruhe city and industrial area during the COPS/TRACKS Lagrangian airborne experiment in summer 2007. A combination of modelling studies and ground-based and aircraft measurements is used to examine the development of ice particles in convective clouds observed over the Black Forest mountains on 11 and 15 July (Huang et al). Predictive skill of a subset of models participating in D-PHASE in the COPS region is analyzed by Bauer et al Long-term cloud statistics and cloud radiative effects for a low-mountain site are examine using observations from the Atmospheric Radiation Measurement Mobile Facility (Ebell et al). Finally, Bhawar et al present an intensive water vapour intercomparison effort, involving airborne and ground-based water vapour lidar systems intended to provide accurate error estimates for the six systems deployed during the experiment.