Outstanding features of pressure systems in rainy season

The whole area, including Egypt, is situated in the subtropical zone. The zone is semi arid in most places, so there is no source of water vapour to make instability arise inspite of the existence of warm air in the boundary layer. The area could be regarded as a sink of the water vapour that must be brought by the air flow from the boundaries. To attain instable weather conditions in the zone, water vapour, topographic effect and frontal activity would be required. To know the different air masses that invade the area, we will display the important features of pressure distribution that play the major role in the weather situation in each season.

1.2.1. Outstanding pressure features in winter season

There are four main features in the winter season. They are explained briefly in the following items.

(a) The Sahara high pressure

It is the extension of the Azores anticyclone that belongs to the so called subtropical high belt around the globe at about 30^o N. This high is associated with a major subsidence, which leads to a local dynamical heating of the air and drying it up. This situation leads to absolute stability in most places.

(b) Traveling depressions over Mediterranean

Instability arises only when a moving low pressure system invades the Mediterranean. In front of these depressions, the surface wind are south to southwesterly. These lows cause rain, showers, and sometimes thunderstorms along the African coast. But in some cases, when a low reaches Egypt the polar air associated with it is modified and becomes dry. When the low becomes already in East Mediterranean, the orography around east Mediterranean and the warm air flow from the south reinforce the depression and instability arises again.

when upper trough is associated. This situation is normally associated with a barometric minimum at Cyprus Island in East Mediterranean.

(d) The formation of Cyprus Low

When this low is formed, successive cold fronts are remarkable on the weather charts.

According to El-Fandy [1] and our daily experience at Cairo Main Analysis Center, the associated weather is generally analogous to squall phenomena rather than to the development of a depression. However, there are two main currents, which accompanied this low. The first is the relatively warm southeast currents over Iraq, which takes the form of the orography to the east and north of Iraq. The second current is the polar current from Balkan in the rear of the low (Fig. 1). Besides to these two currents, an upper trough is also necessary to make the low persist.

Due to the polar air mass in the rear of the low, successive cold fronts travel from north to south over the Mediterranean and then from west to east over land. Such cold fronts lead to the coldest weather in the Middle East area. As a result of blocking situation, the low may persist for a week.

FIG. 1. Warm and cold currents associating the formation of Cyprus depression, which is reinforced by terrain feature in eastern and northern parts of east Mediterranean.

1.2.2. Outstanding features in spring season

The whole pressure system begins to move northward associated with decreasing of the intensity of the Sahara high pressure. A new inverted trough is developed as an extension to the Sudan Monsoon thermal low. This inverted trough may extend to the south of Balkan. The air mass, which is dominant

depression attacks Egypt, dust storms or sandstorms occur. The wind associated with Khamsin depression takes different names in different countries of North Africa. For example, it is called Chili in Tunisia, and Gibli in Libya. In most cases after passing Khamsin depression, small amounts of rainfall are experienced. The small amounts of associated rainfall may be due to the evaporation of rainfall drops in the vicinity of warm boundary layer in the area. Such evaporation and isotopic exchange with the atmospheric moisture below the base of rainy cloud affects fractionation ratios of stable isotopes in rainwater [2], [3], [4], [5]. However, according climatic studies of [6], [7], [8], [9], Khamsin Depressions can be classified into two main types, which will be explained briefly in the following sections.

(b) Winter-like depressions

These depressions are similar to those attack south Mediterranean. They may be independent of well developed secondaries, coming from the Atlantic or forming in the Mediterranean. The most favorable part is that in which the depressions are formed in Gulf of Genoa on one of the lagging cold fronts, or when strong northeast winds blow around the southern end of the Alps to the Gulf of Lyons. Scherhag [10] has attributed the formation of such depression to topographic nature of this part of Mediterranean basin. However, this type of Khamsin depressions is formed in late winter and early spring.

(c) Desert depressions

They form in the lee side of Atlas Mountains or over the desert of Libya. These depressions are mostly fast and vigorous. However, rainfall is rarely associated with this type of depressions.

Sometimes, this type continues moving for seven days along its track from the point of formation at the lee side of Atlas Mountains up to north Iraq (Fig. 2). However, the depression during its motion at the surface is associated with an upper air cold trough, which is bounded from south by the subtropical jet stream in upper layer of the atmosphere.

FIG. 2. Mean Track of Khamsin depressions that are frequently formed at the lee side of Atlas Mountains in spring.

But, they are weak and slower in their eastward motion than those of the spring season. Also, the humidity in this season is higher than that of spring, which leads to higher frequency of rain and thunder storm, a fact that is especially true in November [11]. Flash floods and thunderstorms may happen over the eastern parts of Egypt especially in the months of October and November. This happens when a cooled upper air trough are superimposed over an inverted surface trough, which is associated with southerly surface winds along the Red Sea.

2. Seasonal variations of stable isotopes and associated meteorological elements at