Case studies in different synoptic situations

To distinguish the effect of pressure distribution in daily rainfall events in the period from 3/12/2000 to 25/3/2003, three synoptic situations will be selected. The synoptic situations have been selected such that they could represent different history of air masses that bring water vapour from different sources at the boundaries of the area in different seasons of the year. These three synoptic situations represent rainfall events during inverted V-shape situation in late winter from 10/2/2002 to 12/2/2002, during blocking situation from 6/1/2002 to 10/1/2002 and during wide spread instability on 30/11/2002. Besides, we will explain an abnormal event during Khamsin situation, which took place on 5/4/2001 at Ras-Benas.

3.2.1. Variation during an inverted V-Shape Situation

During the successive three days of this synoptic situation in the period from 10/2/2002 to 12/2/2002, the rainfall amounts have decreased gradually from 24.4 mm on the first day to 6.3 mm on the third day. The general distribution of geopotential heights at 1000 hPa level is depicted in Figure 9. It can be seen that there is an arm of equatorial trough extending along the Red Sea in a form of an inverted V-shape. The low pressure in the area of this inverted V-shape brings warm and humid air mass in lower layers from the Indian Ocean and Red Sea to the convergence zone over East Mediterranean and Egypt. Also, the existence of cold upper air trough in the middle troposphere at 500 hPa level (Fig. 10) caused severe instability at the first day on10/2/2002.

FIG. 9. Contour lines and field of wind vector at 1000 hPa associated with Inverted V-shape trough at surface layers and rainfall event on 10/2/2002 at Alexandria.

FIG. 10. Contour lines and field of wind vector at 500 hPa associated with Inverted V-shape trough at surface layers and rainfall event on 10/2/2002 at Alexandria.

The backward trajectories BA, CA, and DA on three levels at Alexandria are depicted in Figure 11.

The route BA indicates the backward trajectory at 1000 hPa level, which may represent the trajectory at surface in the period from 8/2/2002 to 12/2/2002. The route CA at the 850 hPa level in the same period may represent the trajectory near the top of Planetary Boundary Layer (PBL). The route DA may represent trajectory at the base of the free atmosphere.

FIG. 11. Backward trajectory during foregoing 72 hours from Alexandria at 1200 UT on 10/2/2002 at

PBL. Such situation causes severe instability and leads of falling of 24.4 mm of rainwater in one day at Alexandria. Also, it gave an explanation of the abnormal positive fractionation ratios of 2.17‰ and 33.25‰, which were found for δ¹⁸O and δ²H on 10/2/2002 respectively.

From the foregoing results, one can conclude that abnormal high positive values of fractionation ratios of δ¹⁸O and δ²H in rainwater over Egypt in late winter may be as result of evaporation of falling rainfall-drops during its course in the PBL. This evaporation increases the ratios of δ¹⁸O and δ²H dramatically, especially when the rainy cloud is a result of combing domes of cold air at upper layer with tropical air mass in the PBL that is originated from the Red Sea and Indian Ocean.

3.2.2. Rainfall events during a blocking situation

The blocking synoptic situation that has lasted for five days from 6/1/2002 to 10/1/2002 leaded to high values of d-excess in most days with absolute maximum of 27.29‰ on the first day of blocking followed by gradual decrease and ends by isotopic inverted V-shaped trend with its minimum value on 9/1/2002 where it ceased to 21.61‰ (Fig. 12). These relatively high values of deuterium excess may be referred to the existence of severe instability in air column in the vicinity of deep depression in east Mediterranean by that time at 500 hPa level (Fig. 13). In addition, high wind stress and shear near sea surface leads to lift sea spray upward and increase its concentration in lower parts of the PBL of the atmosphere. These high values of deuterium excess and isotopic trend are normally experienced in east and west Mediterranean in winter [13], [14]. Also, high values of Deuterium excess were found in daily water vapor samples from air aloft Mediterranean Sea surface in January [15].

The fields of wind vector at 500 hPa (Fig. 13) and at 1000 hPa (Fig. 14) indicate clearly that the associated air mass is of European source which, is remarked in such blocking situation. Form the above; one can conclude that blocking situations in winter are characterized by high deuterium excess associated with a V-shape trend and non significant changes in δ¹⁸O. This may due to low temperature associated with severe instability that mixes sea-spray, which is lifted to higher layers by wind shear and stress, with falling rainfall before reaching the ground especially near coastal stations.

FIG. 12. Variation of δ¹⁸O, δ²H and d-excess during blocking situation in the period from 6 to 10/1/2002 at Alexandria with daily rainfall amounts written on the upper ends of vertical arrows.

FIG. 13. Contour lines and field of wind vector at 500 hPa level on 6/1/2002 during the blocking situation lasting for successive days in the period from 6 to 10/1/2002 associated with 24.1 mm of rainfall and d- excess value of 27.29‰ at Alexandria.

FIG. 14. Contour lines and field of wind vector at 1000 hPa level on 6/1/2002 during the blocking situation lasting for successive days in the period from 6 to 10/1/2002 associated with 24.1 mm of rainfall and d-excess value of 27.29‰ at Alexandria.

3.2.3. Wide spread instability synoptic situation in autumn

Synoptic situation sometimes leads to unstable weather condition on west and east Mediterranean at the same time. Such situation occurred on 30/11/2002. This synoptic situation can be seen in the two maps of Fig. 15, which represent levels 500 hPa (15-a) and 700 hPa (15-b). On the same day, rainfall

.

FIG. 15. Contour lines and vector wind fields at 500 hPa (15-a) and 700 hPa (15-b Levels on 30/11/2002 where instability occurred simultaneously in east and west Mediterranean Basin associated with 8.7 mm of rainfall and d-excess of 6.96‰ at Alexandria (29.95^o E, 31.18^o N) that coincide with 13.9 mm of rainfall and ratio of 16.9‰ for deuterium excess at Alger (3.1^o E, 36.77^o N).

These two stations are apart from each other by about 26 degrees of longitudes. From the two maps in Figure 15, one can see that most parts of Mediterranean basin were affected by the same synoptic pattern on the same date. It is clear that the air mass that leads to develop rainfall events at different sites of the basin was originally from the same source, which was the Atlantic Ocean. The big difference between value of deuterium excess at Alexandria (6.96‰) and that at Alger (16.9‰) may refer to the successive changes in synoptic history of the precipitating air masses associated with the route of weather system from west to east Mediterranean. Such synoptic situation of the precipitating air masses was found to behave as if it were a fingerprint of the predominant factor, which controls the isotopic composition in precipitation [16], [17], [18].

3.2.4. The effect of spring situation on fractionation at Ras-Benas station

As stated in the introduction of this work, warming up of boundary layer in spring season may lead sometimes to formation of Khamsin depressions over north coast of Africa. After passing one of these

over Ras-Benas. The associated values of fractionation ratios of δ¹⁸O and δ²H were found to be 5.66‰

and 40.2‰ respectively. For this reason, the estimated d-excess equals -5.08‰. These abnormal values may be explained from the distribution of thermal and wind field at 700 hPa level as shown in figures 17. At 700 hPa level, which is situated directly above the top of boundary layer of the atmosphere; the thermal field is quasi barotropic (the isotherms are quasi parallel to the contour lines and wind vectors at the level). This means that the advected cold air has not yet reached the ground surface and the boundary layer has been still warm during rainfall event. This leads to successive evaporation of falling rain drops, which change the fractionation rations markedly. Also, this explains the small amount of rain in the rainfall sample on 5/4/2001 at Ras-Benas.

FIG. 16. Contour lines and field of wind vectors at 1000 hPa level on 5/4/2001 associated with 12.7 mm of rainfall and abnormal values of d-excess=-5.08‰, δ ¹⁸O =5.66, and δ²H=40.2‰, which were found as a result of relatively warm boundary layer at Ras-Benas and its surroundings.

FIG. 17. Contour Lines, thermal field and field of wind vectors at 700 hPa level on 5/4/2001

18

at Ras-Benas was explained in the foregoing section. Among these samples there are 21 daily rainfall samples, which have taken place on the same dates at Alexandria and Sidi-Barrani. Table 3 depicts these samples and the associated results of the analysis. From the table, one can see that values of δ¹⁸O, δ²H and deuterium excess are similar except on some dates. For example, on 23/12/2002 the difference between δ²H at the two stations makes deuterium excess be 18.9‰ at Alexandria and 27.7‰ at Sidi-Barrani. This difference may be due to the difference in thermal structure in the boundary layers at the two sites. The values of deuterium excess were reversed on 6/3/2003, where deuterium excess at Alexandria was 16.8‰ and 9.8‰ at Sidi-Barrani. The associated synoptic situation at 700 hPa with the last case is depicted in Figure 18. In spite of the two sites were affected by the same source of air mass, this difference in values of d-excess may refer to modification made by PBL on falling drops of rain. Since, The Planetary Boundary Layer (PBL) is very complex by that time of year, especially during Khamsin situations in spring season.

Table 3. Fractionation ratios of 21 synchronized daily samples at Alexandria and Sidi-Barrani stations in Egypt

FIG. 18. Contour, thermal and wind fields at 700 hPa level associated with synchronized rainfall events at Alexandria and Sidi-Barrani on 6/3/2003 giving 16.8‰ and 9.8‰ of deuterium excess at the two sites respectively.

5. Variations of fractionation ratios with climatic variables in upper layers above