Climate Effects/Controls on Mesoscale Processes
NOAA/NWS/International Activities Office – Evaluation and Adaptation of a Regional Climate Model for the Horn of Africa
Segele, Lamb (primary – CIMMS at OU), Leslie
Funding Type: CIMMS Task III (Program Manager – Rob Masters)
Objectives
Evaluate the Abdus Salam International Center for Theoretical Physics
(ICTP) version 3 RCM (RegCM3) to reproduce the observed rainfall amounts
and distribution over the topographically varied region of the Horn
of Africa.
Accomplishments
Regional climate models (RCMs) increasingly are being applied in various
parts of the world to examine regional climate patterns and processes
and to downscale seasonal climate predictions. This study evaluates
the ability of the Abdus Salam International Center for Theoretical
Physics (ICTP) version 3 RCM (RegCM3) to reproduce the observed rainfall
amounts and distribution over the topographically varied region of
the Horn of Africa. Simulations are performed for the widespread very
dry 1984 and locally very wet 1996 years using all of the alternative
convective schemes available in the ICTP RegCM3 -- the Modified Anthes-Kuo
scheme, the Grell scheme with the Arakawa-Schubert closure, the Grell
scheme with Fritsch-Chappell closure (default RegCM3 convective scheme),
and the Emanuel scheme. Extensive comparisons of the simulations for
1984 and 1996 reveal that the Emanuel scheme best captures the rainfall
patterns and interannual variations for the two extreme years over
the Horn of Africa. This scheme therefore is selected for specific
adaptation for the region, despite its excessive rainfall estimation
there and its demonstrated deficiencies over other regions of Africa
not of interest in this study.
To reduce rainfall overestimation by the selected Emanuel scheme, several sensitivity experiments are performed by varying the key parameters that control the rate of convective mass flux adjustment, the fraction of condensed water converted to precipitation, and the heating and moistening characteristics of the environment. Results show that the amount of condensed water that ultimately falls out as rain, which also governs the net heating and the vertical distribution of moistening, crucially affects simulated rainfall amounts. With an appropriate adjustment to this key parameter, the excessive rainfall amount produced by the Emanuel scheme is reduced substantially. This successful parameter modification results in root mean square error (bias) reductions, relative to observations, of 46-58% (58-83%) from those for the default Emanuel simulations over Ethiopia, East Africa, and Central Africa. Furthermore, evaluation of RegCM3 simulations for 1982-99 shows that the modified Emanuel convective scheme not only reproduces the 18-year average rainfall realistically, but also captures the interannual variability adequately over the Horn of Africa. The correlation between the modified Emanuel-simulated and Ethiopian station rainfall is quite strong (+0.66), whereas the counterpart correlation for the RegCM3’s default Grell scheme is -0.05. Despite this success, the modified Emanuel scheme still overestimates the observed Ethiopian station rainfall by 26% for 1982-99. This customized ICTP RegCM3 model now can be used with confidence for the Horn of Africa to study regional rainfall processes and variability and to dynamically downscale seasonal rainfall forecasts. To date, this customized model has been employed to examine the effects of SST variations in the Atlantic and Indian Oceans on Horn of Africa rainfall, and also to assess the likely impacts of changes in local vegetation coverage on that rainfall. The results of these investigations are being reported in separate papers.
This project is ongoing.
Publications
Segele, Z.T., L. M. Leslie, and P. J. Lamb, 2007: Evaluation
and adaptation of a regional climate model for the Horn of Africa: Rainfall
climatology and interannual variability. Intl. J. of Climatology, submitted.
Interannual variations of Ethiopian June-September standardized rainfall rate anomalies for 1982-99. (a) Modeled with the standard default Grell convective scheme (GrFC; blue) and rain gauge (red). (b) Rain gauge (red), CRU (blue), CMAP (green), and modeled with modified Emanuel convective scheme (black).