Orographically-induced Precipitation Study Research Articles

The role of orography in the regeneration of convection: A case study from the convective and orographically-induced precipitation study

The role of orography in the regeneration of convection: A case study from the convective and orographically-induced precipitation study

An Observational and Modeling Study of the Processes Leading to Deep, Moist Convection in Complex Terrain

Abstract A case study of orographic convection initiation (CI) that occurred along the eastern slopes of the Vosges Mountains in France on 6 August 2007 during the Convective and Orographically-Induced Precipitation Study (COPS) is presented. Global positioning system (GPS) receivers and two Doppler on Wheels (DOW) mobile radars sampled the preconvective and storm environments and were respectively used to retrieve three-dimensional tomographic water vapor and wind fields. These retrieved data were supplemented with temperature, moisture, and winds from radiosondes from a site in the eastern Rhine Valley. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to further investigate the physical processes leading to convective precipitation. This unique, time-varying combination of derived water vapor and winds from observations illustrated an increase in low-level moisture and convergence between upslope easterlies and downslope westerlies along the eastern slope of the Vosges Mountains. Uplift associated with these shallow, colliding boundary layer flows eventually led to the initiation of moist convection. WRF reproduced many features of the observed complicated flow, such as cyclonic (anticyclonic) flow around the southern (northern) end of the Vosges Mountains and the east-side convergent flow below the ridgeline. The WRF simulations also illustrated spatial and temporal variability in buoyancy and the removal of the lids prior to convective development. The timing and location of CI from the WRF simulations was surprisingly close to that observed.

Characterization of the planetary boundary layer height and structure by Raman lidar: comparison of different approaches

Abstract. The planetary boundary layer (PBL) includes the portion of the atmosphere which is directly influenced by the presence of the earth's surface. Aerosol particles trapped within the PBL can be used as tracers to study the boundary-layer vertical structure and time variability. As a result of this, elastic backscatter signals collected by lidar systems can be used to determine the height and the internal structure of the PBL. The present analysis considers three different methods to estimate the PBL height. The first method is based on the determination of the first-order derivative of the logarithm of the range-corrected elastic lidar signals. Estimates of the PBL height for specific case studies obtained through this approach are compared with simultaneous estimates from the potential temperature profiles measured by radiosondes launched simultaneously to lidar operation. Additional estimates of the boundary layer height are based on the determination of the first-order derivative of the range-corrected rotational Raman lidar signals. This latter approach results to be successfully applicable also in the afternoon–evening decaying phase of the PBL, when the effectiveness of the approach based on the elastic lidar signals may be compromised or altered by the presence of the residual layer. Results from these different approaches are compared and discussed in the paper, with a specific focus on selected case studies collected by the University of Basilicata Raman lidar system BASIL during the Convective and Orographically-induced Precipitation Study (COPS).

Vertical velocity observed by Doppler lidar during cops - A case study with a convective rain event

Convective and Orographically-Induced Precipitation Study (COPS), conducted in the Black Forest region in Southern Germany and Eastern France during the summer of 2007. From the 13 June to the 16 August 2007, the National Centre for Atmospheric Science (NCAS), Facility for Ground-based Atmospheric Measurement (FGAM) 1.5 lm scanning Doppler lidar was deployed at Super Site R, Achern, in the Rhine Valley, in order to contribute to the extensive COPS observation campaign. The FGAM Doppler lidar system provides measurements of radial wind and aerosol backscatter in the layer 100–1500 m. Profiles of horizontal wind velocity are presented, these being derived from performing azimuth scans. Profiles of vertical velocity, its variance and skewness derived from the vertical scans are also presented and discussed in the paper. Knowledge of vertical velocity skewness is important for the understanding of the structure and origin of turbulent convection in the atmospheric boundary layer (ABL). The skewness of vertical velocity can provide a measure of the asymmetry in the distribution of vertical velocity perturbations within the ABL and can be estimated using the Doppler lidar. In addition, we investigate the behaviour of the boundary layer using data from the FGAM Doppler lidar and Automatic Weather Station (AWS), the University of Basilicata Raman lidar (BASIL) and the DLR’s Poldirad C-band radar. A case study event on the 6 th August 2007 is selected and investigations of possible causes of layers with positive and negative skewness are presented, along with comparisons with output from the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting (WRF) model to assess the accuracy of the model output, including location and timing of rainfall onset.

Comparison of IASI water vapour products over complex terrain with COPS campaign data

In this work, we compare IASI-retrieved vertical water vapour profiles and related precipitable water over a complex region, namely the Rhine Valley area, during the pre-operational period of IASI exploitation (June?August 2007). Both IASI water vapour mixing ratio profiles and integrated water vapour content are retrieved from L1C radiances spectra using two techniques and compared with water vapour related observations acquired during the Convective and Orographically-induced Precipitation Study (COPS) field campaign that took place in this area at that time (i.e. lidars, radiosoundings and a global positioning system - GPS - station network). This work addresses the issue of IASI vertical spatial resolution and its capability to detect two-layer water vapour structures such as those observed in a mountainous area and which play an important role in convective initiation or inhibition. We found that this capability mostly relies on the type of a-priori background vector (climatology or space-time colocated ECMWF analysis), which is used within the retrieval scheme. Systematic comparison of water vapour products derived from 71 IASI spectra confirms that IASI can retrieve water vapour amounts in 2 km width layers, in the lower troposphere, with an accuracy of approximately 10%.

Precipitation on the lee side of the Vosges Mountains: Multi-instrumental study of one case from the COPS campaign

In this article, we focus on one case study from the Convective and Orographically-induced Precipitation Study (COPS), which took place in north-eastern France and south-western Germany during the summer of 2007, in a low mountain area. We investigate lee side precipitation due to shallow and deep convection during one Intensive Observation Period (IOP) of COPS which have been well documented by all the instruments. For that aim, we use a set of observations from radars, radiosoundings, satellite, and a network of Global Positioning System (GPS) receivers, as well as meteorological analyses and dedicated model simulation results. The combination of these measurements with GPS tomography results suggests the role of low level water vapour accumulation and convergence as a precursor to the convective initiation. The origin of this moistening and wind convergence seems to be linked to a slight change in the wind direction in the north-west part of the COPS domain. Using a high resolution X band radar, we also describe how small scale orography affects the precipitation locations, and we show the role of hills near the mouths of the valleys in convective enhancement. This observation is confirmed by model simulation showing that convection is no longer enhanced when the hills are suppressed. The further intensification of one convective cell over the Rhine Valley, which is climatologically frequent, is also discussed.

The Regional Atmospheric Water Budget over Southwestern Germany under Different Synoptic Conditions

Abstract This study addresses the question of how complex topography in a low-mountain region affects the partitioning and the variability of the atmospheric water budget components (WBCs) as a function of synoptic-scale flow conditions. The WBCs are calculated for regions of different size and location in southwestern Germany and the summer months from 2005 to 2009 using the high-resolution regional climate model COSMO-CLM driven by Global Model (GME) analyses. Comparisons with observations from the Convective and Orographically-induced Precipitation Study (COPS) performed in summer 2007 show that the model is capable of simulating the atmospheric water budget reasonably (absolute mean error between 0.1 and 0.7 kg m−2 day−1). To investigate the influence of synoptic weather conditions, the daily WBCs are classified based on the inflow direction of the air masses and the cyclonality at 500 hPa. Using statistical tests, four groups out of the six synoptic conditions have significantly different distributions of the WBCs. This can be explained by differences in the air mass features and the influence of high/low pressure systems. The sensitivity of the modeled WBCs to topography and land cover is investigated by comparing a region in the flat upper Rhine Valley with one in the mountainous Black Forest/Swabian Jura. Compared to the Rhine Valley, increases of evapotranspiration (+5% to +16%), precipitation (+26% to +57%), and moisture convergence (+24% to +93%) are noticeable in the low-mountain region. Local modifications of the synoptic-scale flow, thermally induced winds, and land use cause this intensification of the atmospheric water budget, especially on the windward slopes of the mountains.

High-resolution representation of the mechanisms responsible for the initiation of isolated thunderstorms over flat and complex terrains: analysis of CSIP and COPS cases

Two important measurement campaigns took place in Europe in the last years, the Convective Storm Initiation Project (CSIP) and the Convective and Orographically-induced Precipitation Study (COPS) to gain a better understanding of why deep convection develops. In both campaigns, a dense network of instruments was deployed for detailed observation of the boundary layer characteristics. This paper investigates the usefulness of the combination of high-resolution surface, GPS, and radiosonde measurements to ascertain the likelihood of deep convection in particular for the complex terrain of the COPS domain. Two convection episodes were analysed for this purpose, one from the CSIP campaign and one from the COPS experiment. This study shows that despite the high spatial resolution of the radiosonde network in comparison to current observations, it was necessary to ascertain higher-resolution data sets illustrating the spatial variability of humidity, atmospheric stability, and convective inhibition. GPS and radiosonde data were used to determine high-resolution humidity fields, yielding high-resolution convection-related parameters’ fields representing atmospheric instability and inhibition in the area. The surface data provided a high-resolution representation of the near-surface convergence zones, which acted as triggering mechanism. The use of GPS measurements and surface stations, in addition to radiosonde measurements, turned out to be especially helpful to determine the regions of increased likelihood of deep convection.

Detailed flow, hydrometeor and lightning characteristics of an isolated thunderstorm during COPS

Abstract. The three-hour life-cycle of the isolated thunderstorm on 15 July 2007 during the Convective and Orographically-induced Precipitation Study (COPS) is documented in detail, with a special emphasis on the rapid development and mature phases. Remote sensing techniques as 5-min rapid scans from geostationary satellites, combined velocity retrievals from up to four Doppler-radars, the polarimetric determination of hydrometeors and spatio-temporal occurrences of lightning strokes are employed to arrive at a quantification of the physical parameters of this, during the COPS period, singular event. Inner cloud flow fields are available from radar multiple Doppler analyses at four consecutive times separated by 15 min-intervals. They contain horizontal winds of around 15 m s−1 and updrafts exceeding 5 m s−1, the latter collocated with lightning strokes. Reflectivity and polarimetric data indicate the existence of hail at the 2 km level around 14:40. Furthermore, polarimetric and Doppler radar variables indicate intense hydrometeor variability and turbulence corresponding to an enhanced variance of the retrieved 3-D wind fields. Profiles of flow and hydrometeor statistics over the entire cloud volume provide reference data for high-resolution numerical weather prediction runs in research mode. The study embarks from two movie-loops of geostationary satellite imagery (as Supplement), which provide an intuitive distinction of six phases making up the entire life-cycle of the thunderstorm. It concludes with a triple-image loop, juxtaposing a close-up of the cloud motion as seen by Meteosat, simulated brightness temperature (as a proxy for clouds seen by the infrared satellite channel), and a perspective view on the model generated system of cloud cells. The simulation suggests that several updrafts fed from a low level convergence line eventually removed the convective inhibition and set deep convection in motion. A shear line in the radial velocity relative to the Feldberg radar site shows good agreement beween observation and simulation, whereas the onset location of deep convection exhibits a horizontal discrepancy of 15 km. A quantitative schematic of the isolated thunderstorm synthesizes all retrieved characteristics.

Lidar and radar measurements of the melting layer: observations of dark and bright band phenomena

Abstract. Multi-wavelength lidar measurements in the melting layer revealing the presence of dark and bright bands have been performed by the University of BASILicata Raman lidar system (BASIL) during a stratiform rain event. Simultaneously radar measurements have been also performed from the same site by the University of Hamburg cloud radar MIRA 36 (35.5 GHz), the University of Hamburg dual-polarization micro rain radar (24.15 GHz) and the University of Manchester UHF wind profiler (1.29 GHz). Measurements from BASIL and the radars are illustrated and discussed in this paper for a specific case study on 23 July 2007 during the Convective and Orographically-induced Precipitation Study (COPS). Simulations of the lidar dark and bright band based on the application of concentric/eccentric sphere Lorentz-Mie codes and a melting layer model are also provided. Lidar and radar measurements and model results are also compared with measurements from a disdrometer on ground and a two-dimensional cloud (2DC) probe on-board the ATR42 SAFIRE. Measurements and model results are found to confirm and support the conceptual microphysical/scattering model elaborated by Sassen et al. (2005).

Raman lidar observations of a Saharan dust outbreak event: Characterization of the dust optical properties and determination of particle size and microphysical parameters

The Raman lidar system BASIL was operational in Achern (Black Forest) between 25 May and 30 August 2007 in the framework of the Convective and Orographically-induced Precipitation Study (COPS). The system performed continuous measurements over a period of approx. 36 h from 06:22 UTC on 1 August to 18:28 UTC on 2 August 2007, capturing the signature of a severe Saharan dust outbreak episode. The data clearly reveal the presence of two almost separate aerosol layers: a lower layer located between 1.5 and 3.5 km above ground level (a.g.l.) and an upper layer extending between 3.0 and 6.0 km a.g.l. The time evolution of the dust cloud is illustrated and discussed in the paper in terms of several optical parameters (particle backscatter ratio at 532 and 1064 nm, the colour ratio and the backscatter Angström parameter). An inversion algorithm was used to retrieve particle size and microphysical parameters, i.e., mean and effective radius, number, surface area, volume concentration, and complex refractive index, as well as the parameters of a bimodal particle size distribution (PSD), from the multi-wavelength lidar data of particle backscattering, extinction and depolarization. The retrieval scheme employs Tikhonov’s inversion with regularization and makes use of kernel functions for randomly oriented spheroids. Size and microphysical parameters of dust particles are estimated as a function of altitude at different times during the dust outbreak event. Retrieval results reveal the presence of a fine mode with radii of 0.1–0.2 μm and a coarse mode with radii of 3–5 μm both in the lower and upper dust layers, and the dominance in the upper dust layer of a coarse mode with radii of 4–5 μm. Effective radius varies with altitude in the range 0.1–1.5 μm, while volume concentration is found to not exceed 92 μm3 cm−3. The real and imaginary part of the complex refractive index vary in the range 1.4–1.6 and 0.004–0.008, respectively.

Comparison of Raman Lidar Observations of Water Vapor with COSMO-DE Forecasts during COPS 2007

Abstract Water vapor measurements with the multiwavelength Raman lidar Backscatter Extinction Lidar-Ratio Temperature Humidity Profiling Apparatus (BERTHA) were performed during the Convective and Orographically-induced Precipitation Study (COPS) in the Black Forest, Germany, from June to August 2007. For quality assurance, profiles of the water vapor mixing ratio measured with BERTHA are compared to simultaneous measurements of a radiosonde and an airborne differential absorption lidar (DIAL) on 31 July 2007. The differences from the radiosonde observations are found to be on average 1.5% and 2.5% in the residual layer and in the free troposphere, respectively. During the two overflights at 1937 and 2018 UTC, the differences from the DIAL results are −2.2% and −3.7% in the residual layer and 2.1% and −2.6% in the free troposphere. After this performance check, short-range forecasts from the German Meteorological Service’s (Deutscher Wetterdienst, DWD) version of the Consortium for Small-Scale Modeling (COSMO-DE) model are compared to the BERTHA measurements for two case studies. Generally, it is found that water vapor mixing ratios from short-range forecasts are on average 7.9% drier than the values measured in the residual layer. In the free troposphere, modeled values are 9.7% drier than the measurements.

Use of Meteosat Second Generation optimal cloud analysis fields for understanding physical attributes of growing cumulus clouds

This study develops an understanding on how retrieved cloud parameter fields from the Optimal Cloud Analysis (OCA) algorithm, operating on Meteosat Second Generation (MSG), Spinning Enhanced Visible and Infrared Imager (SEVIRI) data, behave at 5-min time resolutions for growing cumulus clouds. Fields retrieved by the OCA algorithm include cloud optical thickness (τ), cloud-top particle effective radius (r e), cloud-top pressure (p c), and cloud-top phase. OCA is based on a one-dimensional optimal estimation methodology, and a measure of radiance fit, the cost function (J m), is a quantity developed as part of the retrieval process and is shown to be useful in delineating mixed phase clouds; it too is evaluated (at 5-min intervals) for the information it provides. Data for 94 growing cumulus cloud events are processed. An “event” is defined as a cumulus cloud that is monitored at 5-min intervals with OCA, as it grows from the “fair weather” or “towering cumulus” stage to near the cumulonimbus stage when precipitation begins. The hypothesis is that OCA products are of high-enough quality to provide unique information about microphysical processes occurring at and near cloud top. The goal through analysis of the 94 events is to identify consistent, repeating patterns in OCA fields during cloud growth that can be in turn used to infer physical processes. Data from the Convective and Orographically-induced Precipitation Study (June and July 2007) and in four regions of Europe on 25 May 2009 are used. The validity of the OCA data is presented with a comparison to CloudSat Precipitation Radar and MODerate resolution Imaging Spectroradiometer retrieved cloud properties, showing good statistical agreements. Subsequently, results from the analysis of OCA fields for all events show that as cumuli deepen, r e values tend to increase, and then decrease in size as cloud tops glaciate and particle settling begins. The τ magnitudes generally increase as clouds deepen, while p c values and cloud-top temperatures fall as expected. The J m values exhibit the pattern of spiking in magnitude (over a 5–10-min period), which indicates the increase “misfit” within OCA during the mixed phase, at about the time τ values increase substantially as clouds deepen.

Cloud statistics and cloud radiative effect for a low‐mountain site

AbstractIn 2007, the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) was operated for a nine‐month period in the Murg Valley, Black Forest, Germany, in support of the Convective and Orographically‐induced Precipitation Study (COPS). The synergy of AMF and COPS partner instrumentation was exploited to derive a set of high‐quality thermodynamic and cloud property profiles with 30 s resolution. In total, clouds were present 72% of the time, with multi‐layer mixed phase (28.4%) and single‐layer water clouds (11.3%) occurring most frequently. A comparison with the Cloudnet sites Chilbolton and Lindenberg for the same time period revealed that the Murg Valley exhibits lower liquid water paths (LWPs; median = 37.5 g m−2) compared to the two sites located in flat terrain. In order to evaluate the derived thermodynamic and cloud property profiles, a radiative closure study was performed with independent surface radiation measurements. In clear sky, average differences between calculated and observed surface fluxes are less than 2% and 4% for the short wave and long wave part, respectively. In cloudy situations, differences between simulated and observed fluxes, particularly in the short wave part, are much larger, but most of these can be related to broken cloud situations. The daytime cloud radiative effect (CRE), i.e. the difference of cloudy and clear‐sky net fluxes, has been analysed for the whole nine‐month period. For overcast, single‐layer water clouds, sensitivity studies revealed that the CRE uncertainty is likewise determined by uncertainties in liquid water content and effective radius. For low LWP clouds, CRE uncertainty is dominated by LWP uncertainty; therefore refined retrievals, such as using infrared and/or higher microwave frequencies, are needed. Copyright © 2011 Royal Meteorological Society

Initiation of convection over the Black Forest mountains during COPS IOP15a

AbstractDoppler‐On‐Wheels radar observations made during the Convective and Orographically‐induced Precipitation Study (COPS) on 12 August 2007 showed that precipitating clouds only developed between the north–south orientated Murg and Nagold Valleys of the northern Black Forest. The clouds produced moderate precipitation. The Weather Research and Forecasting (WRF) model run at 700 m horizontal resolution in the inner domain was able to simulate the location of the precipitation. Insight is therefore gained into the physical mechanisms responsible for the initiation and development of the convection. Convergence lines resulting from thermally driven flows constrained the initial location of the convection within warm and moist cores produced by heating on elevated surfaces. The heaviest precipitation was later produced by secondary convection, which was initiated within the cores at the boundary between cold‐pool outflows and thermal flows. Copyright © 2011 Royal Meteorological Society

Development of ice particles in convective clouds observed over the Black Forest mountains during COPS

AbstractA 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 during the Convective and Orographically‐induced Precipitation Study (COPS). High concentrations of relatively small ice particles were observed in the weaker northern cell that developed on convergence lines over the mountains in the much‐studied 15 July 2007 case. The conditions in the cloud were not conducive for the Hallett–Mossop process. Instead, the explanation for such high concentrations of ice is likely associated with the type of ice nuclei ingested into the cloud. Biological nuclei, oxidised organic aerosol particles in the polluted air vented from the Murg valley into the cloud base, and desert dust are all possible candidates. A model sensitivity test with biological nuclei produced similar concentrations of ice particles to the observations. In contrast, the high concentration of ice particles measured in clouds that advected over the Black Forest mountains on 11 July 2007 were likely due to the Hallett–Mossop process. The deep convective cell at the southern end of the COPS domain on 15 July 2007 developed in less polluted air than the shallower northern cloud. A model sensitivity test with lower aerosol loading produced a more vigorous cloud with a higher top, more precipitation, and greater reflectivity, more similar to the radar observations. The results suggest that aerosol particles vented out of the valleys could have a significant impact on orographically induced precipitation. Copyright © 2011 Royal Meteorological Society and Crown copyright, the Met Office

Reconciliation of measurements of hygroscopic growth and critical supersaturation of aerosol particles in central Germany

Abstract. Aerosol physical, chemical and hygroscopic properties were measured in a range of airmasses during COPS (Convective and Orographically-induced Precipitation Study) ground-based in June and July of 2007 at the Hornisgrinde mountain site in the Black Forest, Southwest Germany. Non-refractory aerosol composition was measured with an Aerosol Mass Spectrometer, simultaneous to hygroscopic growth factors at 86% relative humidity and CCN activity measurements for particles of dry (<20%) diameters 27 to 217 nm, with particle water uptake exhibiting substantial variability with time and with particle size. Variability in the measurements of hygroscopic growth factor and critical supersaturation for particles of similar sizes indicates significant compositional impact on particle water affinity. Critical supersaturation prediction using a single parameter hygroscopicity approximation derived from measured HTDMA mean growth factors deviate, beyond measurement uncertainties, from critical supersaturations derived from CCN measurements. These led to differences averaging around 35% in the number of CCN (NCCN) for the most reliable measurements depending on averaging methodology, often very much larger for individual time periods. This indicates aspects of water uptake behaviour unresolved in this experiment by the single parameter representation which, depending on its origin, may have important consequences on its generalised use.

Spatio-temporal soil moisture variability in Southwest Germany observed with a new monitoring network within the COPS domain

Within the ‘Convective and Orographically-induced Precipitation Study’ (COPS) 2007 in Southwest Germany and Northeast France a soil moisture monitoring network was installed. The aim of the network is to identify the interaction between the temporal and spatial variability of the soil moisture field and its influence on the energy balance and the moisture availability in the planetary boundary layer. The network is comprised of a large number of newly developed low-cost soil moisture sensors based on the frequency-domain reflectometry method (FDR). In total 47 soil moisture stations within the COPS domain were each equipped with two to four sensors simultaneously measuring vertical profiles of soil moisture and soil temperature down to 50 cm depth. This contribution describes the soil moisture network, its installation procedure and the calibration of the sensor output signal. Furthermore we discuss the soil texture distribution within the study area and present first analyses of the spatio-temporal soil moisture variability during a 13 month period from June 2007 till June 2008 based on regional differences and site specific properties (altitude and soil texture). Results show that the altitude plays a key role for the overall soil moisture pattern relative to the area mean due to the direct linkage to precipitation patterns. Soil texture controls the vertical soil moisture gradient relative to the near surface soil moisture, as their properties control water storage and drainage characteristics. Both factors significantly influence regional soil moisture patterns in Southwest Germany. Zusammenfassung –

Generation of free convection due to changes of the local circulation system

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.

Model representation of boundary-layer convergence triggering deep convection over complex terrain: A case study from COPS

Abstract An isolated thunderstorm from the Convective and Orographically-induced Precipitation Study in southwest Germany and east France in 2007 is analyzed. On July 15, deep convection developed east of the Black Forest crest, although convective available potential energy (CAPE) was only moderate and convective inhibition (CIN) was high. Data analysis revealed that convection was triggered by updrafts penetrating the capping inversion of the planetary boundary layer as a result of low-level convergence. Although the numerical weather prediction model COSMO-DE of the German Weather Service (2.8 km grid resolution) simulated a convergence line and the evolution of a line of low clouds in good agreement with radar and satellite observations, no precipitating deep convection developed from this line of clouds. For an improved representation of orographic effects, simulations with a finer grid resolution of 1 km were performed. Despite almost optimal conditions, i.e. moderate amount of CAPE and almost vanishing CIN, the updrafts required to overcome CIN were not reached in both model configurations. Although both simulations did not initiate deep convection, the results suggest that in an air mass convection situation without mid-tropospheric forcing, the simulated location and timing of convergence lines with coexistent large values of CAPE and low values of CIN can be used as diagnostic parameters for deep convection nowcasting.