HR: 09:15h
AN: H51G-06    [Abstracts]
TI: Origins of the Caribbean Bimodal Rainfall Pattern
AU: * Gonzalez, J E
EM: jgonzalezcruz@scu.edu
AF: Santa Clara University, Mechanical Engineering Department, Santa Clara, CA 95051, United States
AU: Angeles, M
EM: moisesangeles@hotmail.com
AF: University of Puerto Rico-Mayaguez, Industrial Engineering Department, Mayaguez, PR 00680, Puerto Rico
AU: Comarazamy, D
EM: comarazamy@me.uprm.edu
AF: University of Puerto Rico-Mayaguez, Industrial Engineering Department, Mayaguez, PR 00680, Puerto Rico
AU: Ramirez, N
EM: nazario@ece.uprm.edu
AF: University of Puerto Rico-Mayaguez, Industrial Engineering Department, Mayaguez, PR 00680, Puerto Rico
AU: Tepley, C
EM: ctepley@naic.edu
AF: National ArecibonIonospheric Observatory, Arecibo Observatory, Arecibo, PR , Puerto Rico
AB: The Caribbean region is geographically located along a relative land-free tropical band, where the Tropical Atlantic and Pacific equatorial regions influence the inter-annual variability of its rainy season. Its position in a middle tropical warm pool and the ageostrophic dynamic circulation also plays an important role in that variability. Past studies have ascertained that the Caribbean rainfall season has a bimodal nature, where the initial peak of this season, called the early rainfall season (ERS), begins in May and extends until June, with a brief dry period in July. The second half of the overall rainy season, or late rainfall season (LRS), spans from August to November. During the rainy season, the easterly waves and tropical storms begin to be frequent and the rainfall begins to increase. Intuitively, the rainfall should continue increasing, especially from the beginning of the rainy season until the end of this season. The actual data, however, shows an unexpected rainfall decrease in the month of July. The low rainfall peak during the rainy season defines the Caribbean bimodal behavior. The Caribbean rainfall bimodal structure has been reported in different studies over the Caribbean region. However, very little is known about this summer drought, its origins, and factors influencing it. It has been hypothesized that increases in giant aerosols concentration due to the Saharan Dust across the Caribbean in the summer months may result in precipitation suppression. AIn this paper, a multivariable analysis was carried to determine which climatological variables may correlate with the Caribbean summer drought that included Intertropical Convergence Zone (ITCZ), the North Atlantic Oscillation (NAO) index, the Vertical Wind Shear (VWS) and the aerosol particles (AP) coming from northern Africa. The analysis shows that the ITCZ and the SST are weakly correlated with the Caribbean bimodal precipitation; however, the VWS and aerosol particles revealed an important contribution to rainfall during the summer months. The multiple regression analysis also reveals an independent strong correlation of the VWS. Spectral analysis revealed that the NAO does not have periodicity leaving the VWS and AP as potential controlling variables. Numerical experiments are performed to uncouple the VWS and AP effects in the lower summer rainfall time. The numerical approach uses the regional atmospheric modeling system (RAMS) with a new cloud microphysics module which considers small and giant AP. A control run is carried out for July 2003 which has a VWS closer to the climatology and a second run for July, 2002, where a VWS anomaly is present. These numerical experiments supported the statistical result that the VWS effect uncoupled to the AP influences the rainfall production in July. Two additional numerical experiments are performed in which observed AP are ingested into RAMS. The first of these simulations uses observed Cloud Condensation Nuclei (CCN) and Giant CCN measured in July 2003, and in a fourth run the concentration of AP are increased maintaining the same atmospheric conditions as in the previous run. Results indicate that higher AP concentrations decreases further rainfall in July across the Caribbean. Possible variability of this bimodal trend with climate changes is also explored in the paper.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0429 Climate dynamics (1620)
DE: 0466 Modeling
DE: 1833 Hydroclimatology
SC: Hydrology [H]
MN: 2007 Joint Assembly