Initiation of deep convection at marginal instability in an ensemble of mesoscale models: a case‐study from COPS

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194

136

Within the framework of the international field campaign COPS (Convective and Orographically-induced Precipitation Study), a large suite of state-of-the-art meteorological instrumentation was operated, partially combined for the first time. This includes networks of in situ and remote-sensing systems such as the Global Positioning System as well as a synergy of multi-wavelength passive and active remote-sensing instruments such as advanced radar and lidar systems. The COPS field phase was performed from 01 June to 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. The collected data set covers the entire evolution of convective precipitation events in complex terrain from their initiation, to their development and mature phase until their decay. Eighteen Intensive Observation Periods with 37 operation days and eight additional Special Observation Periods were performed, providing a comprehensive data set covering different forcing conditions. In this article, an overview of the COPS scientific strategy, the field phase, and its first accomplishments is given. Highlights of the campaign are illustrated with several measurement examples. It is demonstrated that COPS research provides new insight into key processes leading to convection initiation and to the modification of precipitation by orography, in the improvement of quantitative precipitation forecasting by the assimilation of new observations, and in the performance of ensembles of convection-permitting models in complex terrain.

Section: Model Verification and Predictabilitymentioning

confidence: 99%

The Convective and Orographically‐induced Precipitation Study (COPS): the scientific strategy, the field phase, and research highlights

Wulfmeyer

Behrendt

Kottmeier

et al. 2011

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194

136

“…In contrast to COSMO-DE, the MESO-NH model was successful in simulating convective precipitation in this case . A comparison study of modelling results for IOP 8b with a suite of convection-resolving models was discussed by Barthlott et al (2011). An idealized simulation of this case with even higher cloud-resolving resolution (minimum horizontal grid spacing of 100 m) was discussed by Kirshbaum (2011) who showed that the existence and vigour of the convection is highly sensitive to small changes in the background wind speed.…”

Section: Introductionmentioning

confidence: 99%

Observation of convection initiation processes with a suite of state‐of‐the‐art research instruments during COPS IOP 8b

Behrendt

Pal

Aoshima

et al. 2011

107

In the afternoon of 15 July 2007, a thunderstorm was initiated within a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographicallyinduced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary-layer structure, lids and the water-vapour field in the pre-convective environment of the event are discussed. For this purpose, we investigated measurements of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated-water-vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large-scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent-heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area in which CI took place.

“…The solid line shows the unmodified control run, in the simulation shown by the dashed line the initial soil moisture field was increased by 25%, and in the simulation shown by the dot-dashed line the soil moisture field was increased by 25% and an additional 3 g kg −1 of moisture was added into the lowest 1 km of the domain. The grey lines show the average values of potential temperature, specific humidity and boundary layer top for the three models that simulated the storm in Barthlott et al (2011). Also, from Figure 7(a) we see that at Achern, where there was good agreement between the simulated and observed boundary layer moisture profiles (Figure 4), the simulation shows a local maximum in low-level moisture which could explain the favourable comparison.…”

Section: Original Ensemblementioning

confidence: 81%

“…This convergence zone was apparently responsible for initiating the observed storm. Barthlott et al (2011) performed an intercomparison of eight high-resolution convection-permitting simulations of this event from different numerical models. The models produced a wide range of results: only five of them simulated convective precipitation in the COPS region, with only three simulating an intense storm in approximately the correct location.…”

Section: Observationsmentioning

confidence: 99%

“…The simulated boundary layer grows to ∼2.5 km in depth and has ∼8 g kg −1 of moisture in the well-mixed layer. Although no observations are available at this location to verify the simulated profiles, we refer to Figure 13 of Barthlott et al (2011), which shows the profiles at this time and location for the eight participating models. The models that simulated the observed cell (Arome, Meso-NH and WRF UK) had boundary layer depths of ∼1.4 km and 11-14 g kg −1 of moisture at this time.…”

Section: Original Ensemblementioning

confidence: 99%

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Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

Hanley

Kirshbaum

Belcher

et al. 2011

Convective-scale ensemble simulations with perturbed initial conditions are performed to investigate the physical mechanisms, sensitivities and predictability of convective precipitation for a well-observed event from the Convective and Orographically induced Precipitation Study (COPS). On this day an isolated thunderstorm developed over the Black Forest mountains in southwest Germany that was initiated by thermally driven low-level convergence. With the default ensemble configuration, none of the members simulated any deep convection. A soil moisture and atmospheric moisture deficiency was found to be responsible for a lack of boundary layer moisture in the analysis. Correction of this moisture bias yielded an ensemble in which some members produced a realistic convective storm. In contrast, other members only produced shallow convection. A robust feature of the members that simulated the storm was that an impinging upper-level disturbance to the west had progressed further east towards the COPS region. This led to increased westerly winds aloft, which when mixed down into the boundary layer acted to feed a deeper layer of moist valley air into the convergent mountain circulation. The simulations reveal a very strong sensitivity to uncertainties in the large-scale flow as well as the boundary layer structure and stability.