Analysis and Prediction of West African Moist Events during the Boreal Spring of 2009. (Under the Analysis and Prediction of West African Moist Events during the Boreal Spring
Mera, Roberto Javier
2009
unpublished
direction of Fredrick H.M. Semazzi). Weather and climate in Sahelian West Africa are dominated by two major wind systems, the southwesterly West African Monsoon (WAM) and the northeasterly (Harmattan) trade winds. In addition to the agricultural benefit of the WAM, the public health sector is affected given the relationship between the onset of moisture and end of meningitis outbreaks. Knowledge and prediction of moisture distribution during the boreal spring is vital to the mitigation of
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... itis by providing guidance for vaccine dissemination. The goal of the present study is to a) develop a climatology and conceptual model of the moisture regime during the boreal spring, b) investigate the role of extra-tropical and Convectively-coupled Equatorial Waves (CCEWs) on the modulation of westward moving synoptic waves and d) determine the efficacy of a regional model as a tool for predicting moisture variability. Medical reports during 2009, along with continuous meteorological observations at Kano, Nigeria, showed that the advent of high humidity correlated with cessation of the disease. Further analysis of the 2009 boreal spring elucidated the presence of short-term moist events that modulated surface moisture on temporal scales relevant to the health sector. The May moist event (MME) provided insight into interplays among climate anomalies, extra-tropical systems, equatorially trapped waves and westward-propagating synoptic disturbances. The synoptic disturbance initiated 7 May and traveled westward to the coast by 12 May. There was a marked, semi-stationary moist anomaly in the precipitable water field (kg m-2) east of 10°E through late April and early May, that moved westward at the time of the MME. Further inspection revealed a mid-latitude system may have played a role in increasing the latitudinal amplitude of the MME. CCEWs were also found to have an impact on the MME. A coherent Kelvin wave propagated through the region, providing increased monsoonal flow and heightened convection. A Ttropical Depression-type (TD-type) system developed on May 7 at 20°E and traveled westward with the MME. As this system progressed westward it induced important changes in surface moisture. The TD-type and Kelvin waves underwent phase coupling over central Nigeria (8°E), strengthening the westward-moving feature on May 9. Further evidence is presented that an ER wave also contributed to the development of the TD-type system. The Weather Research and Forecasting Model (WRF) was employed to simulate the environment during 2009 in seasonal and real-time forecast modes. WRF was configured during the 2006 boreal spring, given the increase in meteorological information through the Africa Monsoon Multidisciplinary Analyses project. The model simulated the moist events but tended to have a dry bias and a 2-day delay of the MME for the seasonal simulation. Real-time simulations were able to simulate the MME better than the seasonal run, temporally and spatially. The ensemble simulations served as a testbed for a new tool for the analysis of ensemble prediction skill called the extended ROC (EROC) method. The EROC retains the appealing simplicity of the traditional ROC method and the ability of the EV method to provide evaluation of the performance of an ensemble climate prediction system (EPS) for a hypothetical end user defined by the cost-loss ratio (µ= C/L). Seasonal simulations varied in their useable skill, with Bamako (Mali) as the location with the highest value. This study has revealed that moist events could be of crucial importance to meningitis mitigation. The systems constituting the MME represent predictable phenomena that can be forecasted days in advance. Real-time model simulations were able to diagnose the event 10 days in advance.
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