M. Kunz

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.

On the Performance of a Low-Cost K-Band Doppler Radar for Quantitative Rain Measurements

Abstract This paper deals with the ability of a vertically pointing, FM–CW Doppler radar to measure quantitatively raindrop size distributions and rainfall intensity. The wavelength of the radar is 12.5 mm (K band). To improve estimates of the rainfall intensity, radar-received noise, Mie scatter, and radar calibration corrections are applied to the radar data. The electronic noise correction rendered the radar particle size retrievals below 0.7-mm drop diameters invalid; exponential extrapolation of the spectrum below drop diameters of 0.7 mm decreased the rain intensity up to 20%. Proper Mie corrections to the particle backscatter cross sections decreased the rain intensity between 5% and 30%. Calibration of the radar against a Joss–Waldvogel disdrometer led to a decrease in rain intensity of 49%. The electronic noise, Mie scatter, and calibration corrections yielded a correlation coefficient of 0.94 in a comparison of the radar and disdrometer data for a case study. The daily rain sums over half a year...

High-resolution assessment of the hail hazard over complex terrain from radar and insurance data

A new method for the assessment of the hail hazard in high spatial resolution and its application for Southwest Germany are presented. By applying a cell tracking algorithm to 3D radar data, tracks of severe hailstorms are detected and merged with loss data from a building insurance company. Besides the detection of hailstorm tracks between 1997 and 2007, maximum reflectivity is projected on a 10 km x 10 km grid and analyzed by extreme value statistics. In doing so, radar reflectivities are related to statistical return periods. The results confirm a high spatial variability of both track density and hail hazard. Accordingly, severe hailstorms occur most frequently, and consequently also with highest intensities, in the region south of Stuttgart, whereas hail activity is lowest over the Rhine valley and the low mountain ranges of the Black Forest and Swabian Jura. Based on a conceptual approach, it is discussed how spatial differences in hail incidence can be explained plausibly by low Froude number flow (Fr < 1) around the mountains of Black Forest and the northwestern edge of the Swabian Jura, which causes downstream horizontal convergence.

Orographic Enhancement of Precipitation over Low Mountain Ranges. Part I: Model Formulation and Idealized Simulations

Abstract A diagnostic model for simulating orographic precipitation over low mountain ranges is presented. It is based on linear theory of hydrostatic flow over mountains and calculates condensation rates from vertical lifting at the different model layers. Several other physical processes, such as hydrometeor drifting, evaporation, and moisture loss, are incorporated in the model by simple parameterizations. Idealized simulations of precipitation with different model performances provide insight into the physical processes of orographic precipitation. Evaporation, in combination with hydrometeor drifting into descent regions, is identified as one of the key aspects that primarily determine the spatial distribution of precipitation. The variability in orographic precipitation that results from changes in model parameters and ambient conditions is investigated in sensitivity studies. Simulated intensities as well as their spatial distributions are very sensitive to the temperature T0 at the lowest layer an...

A review of multiple natural hazards and risks in Germany

Although Germany is not among the most hazard-prone regions of the world, it does experience various natural hazards that have caused considerable economic and human losses in the past. Moreover, risk due to natural hazards is expected to increase in several regions of Germany if efficient risk management is not able to accommodate global changes. The most important natural hazards, in terms of past human and economic damage they caused, are storms, floods, extreme temperatures and earthquakes. They all show a pronounced spatial and temporal variability. In the present article, a review of these natural hazards, associated risks and their management in Germany is provided. This review reveals that event and risk analyses, as well as risk management, predominantly focus on one single hazard, generally not considering the cascading and conjoint effects in a full multi-hazard and risks approach. However, risk management would need integrated multi-risk analyses to identify, understand, quantify and compare different natural hazards and their impacts, as well as their interactions.

Orographic Enhancement of Precipitation over Low Mountain Ranges. Part II: Simulations of Heavy Precipitation Events over Southwest Germany

Abstract A diagnostic precipitation model that combines linear theory of hydrostatic flow with parameterized microphysics is applied to several stratiform heavy precipitation events over the low mountain ranges of southwestern Germany. Model-simulated rainfall is in good agreement with observations in both magnitude and location, yielding correlation coefficients against observational data between 0.74 and 0.90. Two events that caused local flooding over and near the Black Forest mountains, on 11–13 December 1997 and on 28–29 October 1998, are discussed in detail. Results show that, in addition to orographic features, wind speed U, moist static stability Nm, and melting level are important parameters to describe the amount and spatial distribution of orographic precipitation. The effect of hydrometeor drifting significantly reduces the precipitation peaks near the crests, and the inclusion of evaporation decreases precipitation mainly in descent regions downstream of the mountains. Using the upslope appro...

Review of the flood risk management system in Germany after the major flood in 2013

Widespread flooding in June 2013 caused damage costs of €6 to 8 billion in Germany, and awoke many memories of the floods in August 2002, which resulted in total damage of €11.6 billion and hence was the most expensive natural hazard event in Germany up to now. The event of 2002 does, however, also mark a reorientation toward an integrated flood risk management system in Germany. Therefore, the flood of 2013 offered the opportunity to review how the measures that politics, administration, and civil society have implemented since 2002 helped to cope with the flood and what still needs to be done to achieve effective and more integrated flood risk management. The review highlights considerable improvements on many levels, in particular (1) an increased consideration of flood hazards in spatial planning and urban development, (2) comprehensive property-level mitigation and preparedness measures, (3) more effective flood warnings and improved coordination of disaster response, and (4) a more targeted maintenance of flood defense systems. In 2013, this led to more effective flood management and to a reduction of damage. Nevertheless, important aspects remain unclear and need to be clarified. This particularly holds for balanced and coordinated strategies for reducing and overcoming the impacts of flooding in large catchments, cross-border and interdisciplinary cooperation, the role of the general public in the different phases of flood risk management, as well as a transparent risk transfer system. Recurring flood events reveal that flood risk management is a continuous task. Hence, risk drivers, such as climate change, land-use changes, economic developments, or demographic change and the resultant risks must be investigated at regular intervals, and risk reduction strategies and processes must be reassessed as well as adapted and implemented in a dialogue with all stakeholders.

Changes in wind gust extremes over Central Europe derived from a small ensemble of high resolution regional climate models

Simulations of the regional climate models CCLM and REMO, forced by different runs of the global model ECHAM5, are used to quantify changes in the local winter gust extremes over the next decades. The horizontal resolution of the models is smaller than 20 km. The gust wind speed conditions are characterised through the comparison of gusts in the control (1971-2000) and the projection period (2021-2050) for the SRES scenarios A1B, A2 and B 1. Extreme value statistics is applied to calculate gust wind speeds for different return periods at each grid point. The results show that the individual regional model runs differ considerably in the change signal and the significance of the single simulations is limited to some main features. The spatial variability of the gusts is only marginally influenced by the return period, but the significance decrease with increasing return period. However, the global model determines the general features of the change signal in the individual regional simulations. Therefore a seven member ensemble provides the opportunity to study the main features of the changes in the gust wind speeds. Over Northern Germany, the change between the two periods in wind gust speed for a 10-year return period studied was found to lie between +6 % and -1.5 % with median, 25th and 75th percentile of 1.5 %, 0.7 % and 2.8 %, respectively. This is confirmed by the majority of the model runs. Over the rest of Germany, the results are not uniform. Nevertheless, over some parts over Southern and Central Germany the majority of the simulations projects a slight decrease of gust wind speeds. According to these results, the current high level of storm activity will not substantially drop until 2050 and will even rise in Northern Germany.

Development and application of a logistic model to estimate the past and future hail potential in Germany

This study investigates to what extent the frequency of hail events in the summer months has changed during the past decades and which changes are expected to occur in the future. To improve the diagnostics of hail events by considering various factors relevant for the formation of hail, a logistic hail model has been developed by means of a multivariate analysis method. This statistical model is based on a combination of appropriate meteorological parameters (convective parameter, moisture content, etc.) and synoptic weather types. The output of the model is a new index that estimates the potential of the atmosphere for hailstorm development, referred to as potential hail index. Validations with independent data sets confirm the reliability of the model results. For Germany, the logistic hail model applied to reanalysis data over the past decades shows a markedly north-to-south gradient with most of the potential hail days occurring in the south. Applied to an ensemble of seven regional climate model simulations, it is found that the potential for hail events will increase in the future (2021–2050) compared to the past (1971–2000), but only statistically significant in the northwest and south of Germany.

Sensitivity of flow dynamics and orographic precipitation to changing ambient conditions in idealised model simulations

Idealised numerical simulations using the non-hydrostatic weather prediction model of the Consortium for Small-scale Modeling (COSMO) in a three-dimensional (3D) configuration were conducted to investigate the relationship between ambient conditions, flow characteristics, and orographic precipitation patterns. By changing the model input parameters of wind speed, static stability, temperature, and relative humidity, different flow effects from conditionally unstable flow to upstream deceleration are considered. It is shown that latent heat release significantly delays the onset of flow stagnation, which can be understood using the moist stability concept. However, due to the vertical variations of saturated and unsaturated layers, it is not possible to apply this concept for determining gravity waves. Both the drying ratio, expressing the conversion of the upstream moisture flux into precipitation, and the location of the precipitation maxima can be described to a certain extent by the saturated nondimensional mountain height, M m = N m H/U, where H is the mountain height, N m is the saturated stability, and U is the undisturbed wind speed. In the flow around regime, the precipitation maxima are associated with the extended gravity wave and are located downstream of the mountain crest. With increasing direct mountain overflow (decreasing M m ), the precipitation maxima are shifted to a location upstream of the crest. The transition from purely stratiform precipitation to embedded convection occurs abruptly when M m becomes imaginary, indicating conditional instability.