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Evaluation of the diurnal cycle of precipitation, surface thermodynamics and surface fluxes in the ECMWF model using LBA data

The mean diurnal cycle of precipitation, near-surface thermodynamics and surface fluxes from short-term forecasts of the ECMWF model are compared with corresponding observations from the Large-Scale Biosphere-Atmosphere Experiment in the Amazonia wet season campaign in 1999 in Rondônia. Precipitation starts about 2 hours after sunrise in the model, several hours earlier than observed, because the model does not simulate well the morning growth of the non-precipitating convective boundary layer. However, the mean daily precipitation during the wet season compares well with observed rainfall. On most days, maximum early afternoon temperature and cloud base height are lower in the model than observed. Maximum equivalent potential temperature is close to that observed. The model surface evaporative fraction is higher than observed and rises to near unity in the late afternoon. Work is in progress to evaluate and integrate the parameterizations for shallow and deep convection.

Correction: October 24, 2012

I reviewed this paper, and the data it was based on, in October 2012 for publication by the ORNL DAAC. It appears that the model thermodynamic diurnal cycle in the left column of Figure 2 was incorrectly computed from the ECWMF model specific humidity, rather than from mixing ratio as stated in the legend. This means that while the potential temperature comparisons is correct, the model moisture-derived variables in the left column should be adjusted for the following systematic biases. The moisture variable q is low by about 0.3 g/kg, θE is low by about 1K and PLCL is high by about 4hPa. (The observational values were computed correctly.) All other comparisons of precipitation and surface fluxes are unaffected by this error. This correction does not affect the conclusions of the paper.

Archived dataset

The ORNL DAAC announces the release of a data set from the Physical Climate (PC-06) Science theme of LBA.

LBA-HMET PC-06 ECMWF Modeled Precipitation and Surface Flux, Rondonia, Brazil: 1999, http://daac.ornl.gov/cgi-bin/dsviewer.pl?ds_id=1141. Data set prepared by A.K. Betts and C. Jakob. This data set provides the mean diurnal cycle of precipitation, near-surface thermodynamics, and surface fluxes generated from short-term forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) model; and the hourly mean precipitation observations used for evaluation.

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Full Citation

Betts, A. K. and C. Jakob, (2002), Evaluation of the diurnal cycle of precipitation, surface thermodynamics and surface fluxes in the ECMWF model using LBA data. J. Geophys. Res., 107, 8045, doi:10.1029/2001JD000427