Adel M. Awad

Synoptic features associated with dust transition processes from North Africa to Asia

Aerosol index data from the total ozone mapping spectrometer satellite and reanalysis data from the National Center for Environmental Prediction and the National Center for Atmospheric Research are useful in the study of synoptic properties of the dust storms that carry dust from North Africa to Asia during the spring season for the period 1979 to 2006. In this study, we analyzed the synoptic properties of dust cases that pass through the transition zone between North Africa and Asia. We identified the dust cases to study by looking, inside transition zone, at events with an aerosol index greater than 2. We then divided the identified cases, depending on the spread and strength of the dust inside the transition zone, into seven categories ranging from weak events to moderate events to violent events. We found the common synoptic characteristics in all these categories as follows: The high pressure belt located over northern Africa allows the low pressure belt located over the South African Sahara to move northward; a pressure gradient between these two atmospheric systems directs from south to north; an increase in the pressure gradient leads to increased in both of the event’s dust and the amount of dust moves to North Africa from the Sahara; an additional pressure gradient between the western Azores high pressure system and the low pressure system located over the Arabian Peninsula directs from west to east; the stronger the pressure gradient, the greater the amount of dust in the event and moving a large amount of dust from Northeast Africa to Asia. To verify that these characteristics capture the essence of dust events from North Africa to Asia, we checked if they were also common to two additional extremes categories and two extremes events. The results confirmed the continued existence of these common characteristics.

Characteristics of the internal and external sources of the Mediterranean synoptic cyclones for the period 1956–2013

This paper investigates the main sources and features of the Mediterranean synoptic cyclones affecting the basin, using the cyclone tracks. The cyclones’ tracks are identified using sea level pressure (SLP) from the NCEP/NCAR reanalysis data for the period 1956–2013. The identified cyclones are classified into two categories: basin affected and basin non-affected. Most of the basin-affected (non-affected) cyclones are internal (external), i.e., generated inside (outside) the Mediterranean basin. This study reveals four (five) main sources of internal (external) cyclones. These four (five) main sources generated about 63.76% (57.25%) of the internal (external) cyclones. Seasonal analysis shows that most of the basin-affected internal (external) cyclones were generated in the winter (spring) season. The lowest number of cyclones were found in the summer. Moreover, the synoptic study of the atmospheric systems accompanied the highest- and lowest-generated years demonstrates that the deepening of the north Europe cyclones and the relative positions of Azores- and Siberian-high systems represent the important factors that influence the number of internal cyclones. Essential factors influencing the external cyclones are the strength of the maximum upper wind, Azores high, Siberian high, and orientations of their ridges.

Climatology of the autumn Red Sea trough

In this study, the Sudan low and the associated Red Sea trough (RST) are objectively identified using the mean sea level pressure (SLP) data from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset covering the period 1955–2015. The Sudan low was detected in approximately 60.6% of the autumn periods, and approximately 83% of the detected low-pressure systems extended into RSTs, with most generated at night and during cold months. The distribution of the RSTs demonstrated that Sudan, South Sudan and Red Sea are the primary development areas of the RSTs, generating 97% of the RSTs in the study period. In addition, the outermost areas affected by RSTs, which include the southern, central and northern Red Sea areas, received approximately 91% of the RSTs originating from the primary generation areas. The synoptic features indicated that a Sudan low developed into an RST when the Sudan low deepened in the atmosphere, while the low pressures over the southern Arabian Peninsula are shallow and the anticyclonic systems are weakened over the northern Red Sea. Moreover, stabile areas over Africa and Arabian Peninsula form a high stability gradient around the Red Sea and the upper maximum winds weaken. The results of the case studies indicate that RSTs extend northward when the upper cyclonic and anticyclonic systems form a high geopotential gradient over Arabian Peninsula. Furthermore, the RST is oriented from the west to the east when the Azores high extends eastward and the Siberian high shrinks eastward or shifts northward.