Lydia Dümenil

A rainfall–runoff scheme for use in the Hamburg climate model

Abstract With a view to increasing the realism of the description of land surface processes in the Hamburg climate model, a scheme is introduced that accounts for the partition of rainfall at the ground between infiltration into the soil and surface runoff. The scheme was adapted and calibrated for the catchment of the river Arno and takes the heterogeneity of the land surface within a grid area into account. For the use in the general circulation model it has been expanded to account for stronger runoff components in regions of mountainous terrain. The GCM simulated annual globally averaged runoff is found to be in good agreement with observations.

A model of river runoff for use in coupled atmosphere-ocean models

Abstract A model for simulating continental river runoff is presented. The local runoff is transported by a linear advection scheme. The advection rates are quasi-objectively determined as a function of the slope of the orography. The river runoff is incorporated in the general circulation model ECHAM. The results of a 20 year simulation are presented. Qualitatively, observations are well produced.

Variability of the Indian Monsoon in the ECHAM3 Model: Sensitivity to Sea Surface Temperature, Soil Moisture, and the Stratospheric Quasi-Biennial Oscillation

Abstract The variability of the monsoon is investigated using a set of 90-day forecasts [MONEG (Tropical Ocean Global Atmosphere Monsoon Numerical Experimentation Group) experiments] and a set of AMIP-type (Atmospheric Model Intercomparison Project) long-term simulations of the atmospheric circulation with the ECHAM3 model. The large-scale aspects of the summer monsoon circulation as represented by differences of dynamical quantities between the two extreme years 1987 and 1988 were reproduced well by the model in both kinds of experiments forced with observed sea surface temperature (SST). At the regional scale the difference of precipitation over India during summer 1987 and 1988 was well reproduced by the model in the 90-day forecasts using interannually varying SSTs; however, similarly good results were achieved in forecasts using climatological SSTs. The long-term simulations forced with interannually varying SST at the lower boundary of the atmosphere over a period of 14 years, on the other hand, onl...

COMPARING THREE LAND SURFACE SCHEMES USED IN GENERAL CIRCULATION MODELS

Abstract Land-surface processes are part of the lower boundary conditions for the atmosphere. The schemes used in general circulation models (GCM) to represent this forcing are relatively simple but they play an important role in the simulation of climate. This paper compares three different land-surface schemes used in GCM. All were run with the same imposed atmospheric conditions in order to analyse the differences in the hydrological cycle. The intercomparison showed that simulated evaporation, runoff and soil moisture were different in all three cases. In order to get a better understanding of the mechanisms at work in the models, features from two of the schemes were included in the other scheme. With these modifications we were able to reduce the wide range between model results. We showed that the surface resistance and field capacity were essential parameters in determining the annual cycle of evaporation and that a representation of subgrid-scale variability of soil moisture had an important impact on runoff.

Evaluation of daily precipitation in China from ECMWF and NCEP reanalyses

Correlation analysis, hidden period analysis and complex Morlet wavelet transform were used with daily rainfall in China from observation, ECMWF (European Centre for Medium-Range Weather Forecast) and NCEP (National Center for Environmental Prediction) reanalyses to evaluate the validation of precipitation estimates. The results showed that Fishers test and wavelet analysis, specially the latter, are useful tools for statistical analysis of daily precipitation datasets. Daily rainfall data obtained from ECMWF reanalysis are obviously better than those from NCEP reanalysis in terms of long period daily mean, local correlation, variation amplitude, fluctuation pattern and frequency. Although there is still room for improvement, ECMWF reanalysis is the best available dataset with global coverage and daily variability. In both of the reanalyzed daily mean precipitation fields, the higher estimations of Sichuan Basin rainfall are most likely caused by the topography of the basin, where small scale mountains in the southeast could not be represented by the reanalysis grid points, the typhoon and summer monsoon rainbelt could reach this region.

Variability of daily precipitation in China (1980–1993): PCA and wavelet analysis of observation and ECMWF reanalysis data

In this study, principal component analysis (PCA) and complex Morlet wavelet transform were used with daily rainfall in China for the period 1980–1993 (1 May–31 Dec.) from observation and ECMWF reanalysis to study its variability and eaaluate the validation of reanalyzed precipitation. The results showed that northward movement of the summer rain belt was a wavelike propagation, which was always accompanied by rainfall breaks and could be treated as one event under time scale of about 1 month only. The first 4 EOFs accounted for 28% and 35% of total variance from observation and reanalysis, respectively, and were roughly consistent with each other. The first and third EOFs for observation mainly represented interweekly, interseasonal and interannual variations and contained some summer intraseasonal fluctuations also. The second and fourth ones mainly represented some rather strong summer intraseasonal fluctuations for a paticular year and contained interweekly, interseasonal and interannual variations also. Although there is still room for improvement, the ECMWF reanalysis is the best available dataset with global coverage and daily variability.