Forecasting techniques for seedable storms over the Western Hajar mountains in the Sultanate of Oman

Abstract

Oman faces a water resources crisis as the demand of fresh water increases day by day. Most of the renewable water resources in the country are directly or indirectly dependent on the rainfall. The Western Hajar Mountains extend to the borders with the UAE. This area is a very important source of ground water for both countries. A collaborative program to investigate the feasibility of rainfall enhancement over the Omani mountains by means of hygroscopic cloud seeding was implemented in the UAE during the summer of 2003 and 2004. Due to the complicated logistics and astronomical cost involved in the cloud seeding experiment, it is crucial that timely and accurate forecasts are made for these mesoscale storms. However, convective clouds of the Al Hajar Mountains are notoriously difficult to predict as they result from mesoscale circulation. This study developed forecasting techniques for seedable storms over the Western Hajar Mountains in the Sultanate of Oman. A period of 10 days (5th-14th of July 2004) was investigated in order to describe the differences in the atmospheric parameters between days when convection occurred and days which remained dry. The main ingredient for convective development is the influx of surface moisture from southeast over the Arabian Sea. This moisture often results from the circulation around the surface low over the central part of the Arabian Peninsula but may occasionally be caused by the sea breeze circulation. The northeasterly sea breeze moves in from the Gulf of Oman to the mountains where it converges with the southeasterly moist flow and this may result in the occurrence of convective clouds. Localized wind convergence zones near the mountains are good indicators of the onset of convection. The Oman Convection Index (OCI) was developed to replace the K-index (KI) and was found to provide a good indication of convective development over the mountains. A forecasting decision tree table for predicting convective storms over the Al Hajar Mountains is proposed where circulation criteria are stipulated as well as critical values for meteorological variables. The ingredients includes the location of the heat low over the centre of Arabian Peninsula, latent instability of the atmosphere, and critical values of mixing ratio and the OCI. This research results have shown that careful investigation and manipulation of the available data allow for increased accuracy in the forecast of convection. The OCI was developed to describe the conditions favorable for summer convection over the mountains of Oman specifically. The results show that the OCI fairs considerably better than the KI in identifying days when significant convection is likely to occur. However, the OCI is only one element of the forecasting techniques table. For truly significant convection to occur there are at least four other circulation criteria which should be satisfied. The forecasting decision tree table provides a systematic approach to the forecasting of convection of the mountains and therefore, for the first time in Oman, objective verification is possible and opens the door for improving these and other forecasting techniques. The techniques developed here may therefore contribute to future cloud seeding projects in the mountains of Oman.