Barbara Früh

Evaluation of the precipitation for South-western Germany from high resolution simulations with regional climate models

Precipitation data from long-term high-resolution simulations with two regional climate models (CLM and REMO) are evaluated using a climatology based on observations for south-western Germany. Both models are driven by a present day climate forcing scenario from the global climate model ECHAM5. The climatological evaluation shows a strong seasonal dependence of the model deficiencies. In spring and summer there are relatively small differences between simulation results and observations. But during winter both the regional models and ECHAM5 strongly overestimate the precipitation. The frequency distributions of the model results agree well with observed data. An overestimation of the precipitation at the upwind sides of mountainous areas occurs in the regional simulations. We found that the coupling of the regional models to the driving model is stronger in winter than in summer. Therefore, in winter the large scale model have a larger impact on the performance of the regional simulations. During summer the benefit of regional climate simulations is higher.

Estimation of Climate-Change Impacts on the Urban Heat Load Using an Urban Climate Model and Regional Climate Projections

Abstract A pragmatic approach to estimate the impact of climate change on the urban environment, here called the cuboid method, is presented. This method allows one to simulate the urban heat load and the frequency of air temperature threshold exceedances using only eight microscale urban climate simulations for each relevant wind direction and time series of daily meteorological parameters either from observations or regional climate projections. Eight representative simulations are designed to encompass all major potential urban heat-stress conditions. From these representative simulations, the urban-heat-load conditions in any weather situation are derived by interpolation. The presented approach is applied to study possible future heat load in Frankfurt, Germany, using the high-resolution Microscale Urban Climate Model in three dimensions (MUKLIMO_3). To estimate future changes in heat-load-related climate indices in Frankfurt, climate projections from the regional climate models Max Planck Institute ...

Comparison of observed and simulated NO2 photodissociation frequencies in a cloudless atmosphere and in continental boundary layer clouds

To validate radiative transfer models for the actinic flux and photodissociation frequencies, it is essential to compare model results with observed data. For this purpose, we used in situ vertical profile measurements of thermodynamic, aerosol particle, and cloud drop properties in order to compare simulated photodissociation frequencies of NO2, J(NO2), with respective measurements. Two case studies in cloudless and four cases in cloudy conditions were investigated. We found that the deviations between the simulated J(NO2) profiles and the measured data are less than 10% in all cases which is in the range given by the measurement uncertainties. For quantifying the sensitivity of the simulated J(NO2) to the different input parameters the impact of experimental uncertainties on the measurement of aerosol and drop microphysical parameters was investigated. In cloudless conditions the largest effect on the modeled J(NO2) is due to the uncertainty of the aerosol volume (about 2% at ground level). In cloudy atmosphere the uncertainty of the observed liquid water content (about 20% at ground level) is of largest importance. Furthermore, the influence of different quantum yield and absorption cross section spectra of NO2 was examined: The different spectra result in changes of ±5% for the computed J(NO2). Additionally, we investigated the effect of different solution methods for the radiative transfer equation on J(NO2). The delta-four-stream method was shown to be a very fast and accurate technique with errors of less than 2%. On the other hand, the delta-two-stream method may cause relative errors of up to 10 to 20%.

Verification of precipitation from regional climate simulations and remote-sensing observations with respect to ground-based observations in the upper Danube catchment

An evaluation of precipitation fields for four selected months simulated by the regional climate model At-moMM5 and provided by the satellite retrieval method AtmoSat is presented. As reference, observations at 5 km resolution on a daily and monthly basis are used. We applied conventional verification tools (root mean square error, grid-point based categorical error scores, etc.) as well as the new error score SAL, which separately considers aspects of the structure, amplitude and location of the precipitation field in a predefined area. We also discussed the advantages and disadvantages of each of the scores. The aim of our evaluation was to unfold the strengths and weaknesses of AtmoMM5 and AtmoSat to calculate daily and monthly high resolution precipitation. As a result we found that the catchment averaged monthly mean precipitation is simulated with an acceptable accuracy by both methods. The spatial pattern of the monthly precipitation (typically with a precipitation maximum in the alpine foreland) can only be reproduced by AtmoMM5. Regarding the daily precipitation, our evaluation revealed that both methods still need improvement. The deviations to the observations increase with decreasing precipitation amount resulting in large uncertainties in case of very dry conditions. Overall, we can conclude that AtmoMM5 is better suited to simulate precipitation at 5 km resolution on a daily basis than AtmoSat.

Determination of precipitation return values in complex terrain and their evaluation.

To determine return values at various return periods for extreme daily precipitation events over complex orography, an appropriate threshold value and distribution function are required. The return values are calculated using the peak-over-threshold approach in which only a reduced sample of precipitation events exceeding a predefined threshold is analyzed. To fit the distribution function to the sample, the L-moment method is used. It is found that the deviation between the fitted return values and the plotting positions of the ranked precipitation events is smaller for the kappa distribution than for the generalized Pareto distribution. As a second focus, the ability of regional climate models to realistically simulate extreme daily precipitation events is assessed. For this purpose the return values are derived using precipitation events exceeding the 90th percentile of the precipitation time series and a fit of a kappa distribution. The results of climate simulations with two different regional climate models are analyzed for the 30-yr period 1971–2000: the so-called consortium runs performed with the climate version of the Lokal Modell (referred to as the CLM-CR) at 18-km resolution and the Regional Model (REMO)–Umweltbundesamt (UBA) simulations at 10-km resolution. It was found that generally the return values are overestimated by both models. Averaged across the region the overestimation is higher for REMO–UBA compared to CLM-CR.

A pragmatic approach for downscaling precipitation in alpine-scale complex terrain

A statistical method is presented to downscale precipitation from a mesoscale atmospheric model simulation. The algorithm consists of two steps. First, local subscale variability is estimated based on a high resolution observed climatology. Second, there is a bias correction, which constrains the downscaled model climatology to be equal to the observed climatology on the coarse grid. Combining both steps results in a local scaling factor for each day of the climatological year. The method is applied to the upper Danube catchment which encompasses part of the European Alps and which is characterized by highly complex orography. The subgrid-scale variability described by the first part of the algorithm partly reflects the underlying orography, especially the narrow alpine valleys. The bias correction leads to a redistribution of precipitation on the catchment scale and accounts for the model deficiency producing too much precipitation in the inner alpine regions and too little at the edges of the Alps. An evaluation with regard to the simulated and observed daily precipitation indicating the significant potential of the method is presented.

Three-dimensional solar radiation effects on the actinic flux field in a biomass-burning plume

[1] Three-dimensional (3-D) solar radiative transfer models describe radiative transfer under inhomogeneous atmospheric conditions more accurately than the commonly used one-dimensional (1-D) radiative transfer models that assume horizontal homogeneity of the atmosphere. Here results of 3-D radiative transfer simulations for a biomass-burning plume are presented and compared with local one-dimensional (l-1-D) simulations, i.e., 1-D simulations in every column of the model domain. The spatial distribution of the aerosol particles was derived from a 3-D atmospheric transport simulation. We studied the impact of 3-D radiative effects on the actinic flux within the plume center. The differences in the actinic flux between results from the 3-D and the l-1-D simulations are considerable, ranging from � 40% to more than +200%, depending on the wavelength, solar zenith angle, and the absorbing properties of the aerosol. The reason for this discrepancy is the neglect of horizontal photon transport in the 1-D simulation. These large 3-D effects on the actinic flux have the potential to influence significantly the in-plume photochemistry. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution— urban and regional (0305); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: biomass burning plume, actinic flux, 3-D solar radiative transfer simulations, aerosol absorption

Ground-Based Measured and Calculated Spectra of Actinic Flux Density and Downward UV Irradiance in Cloudless Conditions and their Sensitivity to Aerosol Microphysical Properties

Ground-based spectral measurements of actinic flux density (300–660 nm wavelength) and downward UV irradiance (300–324 nm) under cloudless conditions have been compared with the results of one-dimensional radiative transfer calculations employing concurrent airborne vertical profile measurements of aerosol particle size distributions. Good agreement (within ±10%) between measured and calculated spectra was found. The remaining differences were explained by uncertainties inherent in the aerosol particle microphysical input data and the column ozone content. A respective sensitivity analysis of the calculated spectra, which was based on the observed variability of microphysical properties, has shown that the particle number concentration is the most crucial input uncertainty for both the actinic flux density and the UV irradiance. For the wavelength range investigated, the uncertainty of the column ozone content is of minor importance for both spectral quantities.

A Pragmatic Approach to Build a Reduced Regional Climate Projection Ensemble for Germany Using the EURO-CORDEX 8.5 Ensemble

AbstractThe application of an ensemble reduction technique to the European branch of the World Climate Research Program Coordinated Regional Downscaling Experiment (EURO-CORDEX) ensemble at resolution “EUR-11” (~12.5 km) under the RCP8.5 scenario is presented. The technique is based on monthly mean changes between a reference and two future time periods, calculated for eight regions in Germany, of the parameters near-surface air temperature (tas), precipitation totals (pr), contribution of precipitation from very wet days to precipitation totals (R95pTOT), near-surface specific humidity (huss), and surface downwelling shortwave radiation (rsds). The sensitivity of the reduction procedure with respect to a number of tuning parameters is investigated. When the optimal combination of tuning parameters is applied, the technique allows the reduction from 15 to 7 ensemble members, while the reduced ensemble reproduces about 94% of the spread of the full ensemble. Keeping in mind that climate projection ensemble...

Precipitation and Temperature

Climatological studies indicate that climate change lead to an increase in the mean global temperature of around 0.5 °C until the end of the twentieth century. This warming impacts the atmospheric humidity, wind, radiation, and precipitation. However, the magnitude of changes is not equally distributed over the globe but differs markedly with regions, making a regionalization of the global information essential. The GLOWA-Danube project follows such a downscaling approach with the focus on the drainage basin of the Upper Danube River.