A Brief UAE Climatology
Past climatological studies have identified the winter season (Dec-March) as accounting for the bulk of rain in the UAE. These studies were based on reporting stations located primarily along the northern coast or in the Gulf. Troughs, depressions and the occasional front, moving into the region from the west and northwest, result in large-scale systems that provide significant rainfall. During strong frontal conditions, rainfall can be experienced throughout the country but reaches a peak over the mountains due to additional uplift as airflow is forced over the mountains. Often times these systems have embedded convective elements or even isolated convective cells as the frontal system passes through the region. Strong systems with convective instability occur infrequently, perhaps once or twice a year, while weaker systems occur much more often, resulting in several days of cloud cover and light rain or drizzle. No major strong synoptic events occurred during the 2001 winter and studies were extended into 2002 to supplement the 2001 data. However, no strong systems occurred in the winter of 2002 either, emphasizing the fact that the annual rainfall in the UAE is highly variable - the standard deviation being larger than the mean.
Convective rainfall over the Oman Mountains during the summer season is a phenomenon that is widely known to local meteorologists but is not described adequately in climatological studies. Although there is some indication of the existence of a summer convective component in the Al Ain rainfall records (Figure 3 on the left), the frequency and importance of mountain rainfall is not well documented. During the summer, the UAE region is under the influence of upper level easterly to northeasterly flow associated with the tropical easterly belt, enhanced by the thermal low over the sub-Asian continent. This circulation can provide some moisture from the Arabian Sea. However, the flow at low levels is often from the northwest during the daytime on the UAE side of the mountains due to a sea breeze circulation, forced by surface temperature differences between the desert and the Arabian Gulf. The mountains often initiate convection under these conditions, depending on the wind flow and the thermodynamic profile of the atmosphere. Relatively small changes in the wind flow and thermodynamic structure can result in large changes in cloud development.
Rainfall Patterns in Al Ain
The monthly mean precipitation (µ) for the Al Ain station and includes the standard deviation (s) and the coefficient of variation (cv = s/µ). The fact that the monthly standard deviation exceeds the monthly average precipitation for the wettest months (January through March) is revealing, as the year-to-year variation in precipitation is quite extreme in this region. These results emphasize the need for caution in basing conclusions about the potential for rain enhancement on data collected for only one or two seasons. Also evident the mean precipitation figure is that although there seems to be a secondary rainfall peak in July in Al Ain, most of the intense rainfall is not captured. The most intense rainfall occurs over the mountains east of Al Ain in Oman, where there are no rainfall measurements. It is important to monitor the rainfall over the mountains in order to fully understand the hydro-meteorological interactions that occur in the UAE/Oman region. This monitoring is best done with quantitative meteorological radars in the UAE, using surface weather stations to calibrate the radar-derived rainfall measurements.
Atmospheric Environment
The winter and summer conditions are delineated by distinctly different meteorological regimes, while May and October are transition months with rarely any rainfall. Mean soundings from nine years of data for January, February, and March were plotted. The soundings for each month are very similar and show generally dry, stable conditions typical of most days in the winter. The influence of strong mid-latitude westerly flow is also obvious, which occasionally brings rain-producing westerly troughs and frontal systems.
Mean sounding conditions for the summer to late summer season were also plotted. During the summer months (June to September), the region is under the influence of weak upper-level tropical easterly flow. The beginning of the transition period is evident in the slight changes of the mean September sounding. Although the atmosphere is still generally dry in the summer, tropical moisture is sometimes advected into the region from the Gulf of Oman and Arabian Sea. Hot surface temperatures and a deeper unstable layer in the lower atmosphere often causes convection to occur as air is forced up over the mountains, developing thunderstorms over the Oman Mountains and into eastern UAE. These thunderstorms are generally short-lived but can produce intense precipitation over small regions and times.
The atmosphere in the UAE is generally very dry due in part to the drying effects of upper-level subsidence and limited sources of upper-level moisture. This moisture comes almost exclusively from frontal systems in the winter and tropical monsoon systems in the summer. However, due to the latitude of the UAE, only the fringes of the frontal systems in the winter and the monsoon systems in the summer affect the weather in the UAE. In both situations, the mountains play an important role in the distribution of rainfall over the area.
