Christian Barthlott

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

A Numerical Sensitivity Study on the Impact of Soil Moisture on Convection-Related Parameters and Convective Precipitation over Complex Terrain

AbstractThe impact of soil moisture on convection-related parameters and convective precipitation over complex terrain is studied by numerical experiments using the nonhydrostatic Consortium for Small-Scale Modeling (COSMO) model. For 1 day of the Convective and Orographically Induced Precipitation Study (COPS) conducted during summer 2007 in southwestern Germany and eastern France, initial soil moisture is varied from −50% to +50% of the reference run in steps of 5%. As synoptic-scale forcing is weak on the day under investigation, the triggering of convection is mainly due to soil–atmosphere interactions and boundary layer processes. Whereas a systematic relationship to soil moisture exists for a number of variables (e.g., latent and sensible fluxes at the ground, near-surface temperature, and humidity), a systematic increase of 24-h accumulated precipitation with increasing initial soil moisture is only present in the simulations that are drier than the reference run. The time evolution of convective p...

Observations of Kinematics and Thermodynamic Structure Surrounding a Convective Storm Cluster over a Low Mountain Range

Abstract Measurements of a convective storm cluster in the northern Black Forest in southwest Germany have revealed the development of a warm and dry downdraft under its anvil cloud that had an inhibiting effect on the subsequent development of convection. These measurements were made on 12 July 2006 as part of the field campaign Prediction, Identification and Tracking of Convective Cells (PRINCE) during which a number of new measurement strategies were deployed. These included the collocation of a rotational Raman lidar and a Doppler lidar on the summit of the highest mountain in the region (1164 m MSL) as well as the deployment of teams carrying radiosondes to be released in the vicinity of convective storms. In addition, an aircraft equipped with sensors for meteorological variables and dropsondes was in operation and determined that the downdraft air was approximately 1.5 K warmer, 4 g kg−1 drier, and therefore 3 g m−3 less dense than the air at the same altitude in the storm’s surroundings. The Raman...

Soil moisture impacts on convective indices and precipitation over complex terrain

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields.

Emergence and Secondary Instability of Ekman Layer Rolls

Abstract The authors revisit the idealized scenario by which long-lived rolls are believed to emerge in the neutral planetary boundary layer, that is, through the saturation of the shear instability of the neutrally stratified Ekman flow. First, the nonlinear stages of the primary instability are studied, using a constant turbulent viscosity with Reynolds numbers up to 1000. Two-dimensional equilibrated rolls are found to exist, as predicted earlier based on a weakly nonlinear expansion. However, the flow may not saturate into those equilibrated rolls if the turbulent Reynolds number is too high. Second, a linear stability analysis of these equilibrated rolls is performed, which finds that they are subject to a three-dimensional instability. The growth rate of the most unstable mode is comparable to the growth rate of the primary instability; the selected horizontal length scale is about 4 times shorter. The unstable mode draws its energy by interacting with both across-roll and along-roll shear, the latt...

Aerosol- and Droplet-Dependent Contact Freezing: Parameterization Development and Case Study

AbstractA parameterization for contact freezing is presented that combines theoretical expressions for determining the collision efficiency with experimentally determined freezing efficiency results. The parameterization has dependencies on aerosol and cloud droplet physical properties, including electric charges, as well as ambient temperature and humidity. The highest freezing rate is obtained at large aerosol and large cloud droplet sizes, and at cold temperatures and low relative humidities, with typical dust aerosol and droplet properties. The number concentration of ice nucleating particles (INPs) in the contact freezing mode are generally lower than those in the immersion freezing or deposition nucleation mode; however, under certain conditions contact INP concentrations can exceed those of the other modes. The new parameterization is used in a high-resolution, semi-idealized simulation of a deep convective cloud, and a number of sensitivity studies are performed. Results indicate the greatest sens...

La campagne Cops : genèse et cycle de vie de la convection en région montagneuse

The Convective and Orographically- induced Precipitation Study (COPS) is a coordinated international project, comprised of an observational field campaign and a research programme aiming to advance the quality of fore- casts of orographically-induced pre- cipitation by four-dimensional obser- vations and modelling of its life cycle. The COPS field campaign took place during June-July-August 2007 over eastern France and south-western Germany. Its main objective was to provide an unprecedented comprehen- sive set of in situ and remotely-sensed meteorological observations of the entire depth of the troposphere. After a short overview of the project, the paper focuses on the French contribution to the COPS field phase, describes the experimental setup and highlights some key observations.

Influence of high-frequency radiation on turbulence measurements on a 200 m tower

Turbulence measurements conducted by means of Solent Gill ultrasonic anemometers at several altitudes on the 200 m tower at the Research Center Karlsruhe showed white noise behaviour at the high-frequency end of the spectra with increasing measurement height. In a number of data sets with a time resolution of 48 ms, the computed power spectra of the velocity components and temperature converge into white noise and the decline in the inertial subrange expected theoretically is hidden. The noise covers a broad range of the spectrum, which greatly increases variance and makes further data analysis dife cult. The cause of this parasitic noise is found in high-frequency radiation from regional longwave transmitters in the frequency range of 150 to 200 kHz, which interferes with the transducer crystals. The resonance frequency of the transducer crystals is 180 kHz. It is found that a thin, grounded mesh wire around the sensor head acts like a Faradays cage, protecting the transducers from the radiation. Negative side effects, like eddy production from the cage or a modie ed mean wind speed can be excluded from results gained by different ultrasonic anemometers, operated simultaneously close to the ground. The mesh wire shield thus is a permanent solution to these problems in case of longwave transmitters surrounding the measurement site. Zusammenfassung