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Features of the Weather Modification Assesment Project in Southwest Region of Saudi Arabia

Paul Kucera, Duncan Axisa, Roelof P. Burger, Don R. Collins, Runjun Li, Michael Chapman, Rafael Posada, Terry W. Krauss, Ayman S. Ghulam

Abstract


This paper provides an overview of an ongoing rainfall assessment program that has been conducted in the southwest region of Saudi Arabia in the summers of 2008 and 2009 in conjunction with an intensive airborne measurement program. The goal of the study is to examine summertime convection that is observed over the mountainous region (often referred to as the "escarpment") that is adjacent to the Red Sea. The escarpment provides a focus for orographic precipitation as a result of complex interactions with the sea breeze and upper level thermodynamics.

The main interest in the study is to examine clouds that are observed on top of the escarpment. Seasonal precipitation results show two distinct peaks in this region: March-April and August. Evaluation of radar observations during these two peaks indicates the area has distinct characteristics in terms of the diurnal cycle and cell structure. Climatological evaluation indicates there are several distinct precipitation zones in the southwest region.

We have initially focused our airborne research program on summer clouds observed over the escarpment.  This paper presents observations of clouds measured by aircraft during an intensive study carried out in the summer of 2009 in the southwest region of Saudi Arabia. A total of 35 research flights were flown during the intensive field campaign during the period 5 August 2009 to 31 August 2009. These flights were conducted under the direction of a flight scientist that assisted the pilot in flight planning and in performing the necessary profiles to accomplish the measurement objectives. Research aircraft operations focused primarily on conducting measurements in clouds that are targeted for cloud top-seeding. Cloud measurements describing the evolution of droplet coalescence, supercooled liquid water, cloud ice and precipitation hydrometeors are necessary to the understanding of precipitation formation.

From this study, we describe the large annual variability in precipitation in the southwest region of Saudi Arabia. Based on our analysis, we have developed a new conceptual model that summarizes the mechanisms for summer precipitation formation in the southwest region. Observations indicate that convective cells tend to be short-lived with complicated microphysics; the presence and concentration of large cloud droplets suggest that GCCN broaden the cloud droplet spectrum; and ice-phase microphysics is important and seems to be efficient. These results have important ramifications for cloud seeding operations.

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