http://journals.ametsoc.org/doi/full/10.1175/1520-0493%282002%29130%3C0549%3ACOCMIT%3E2.0.CO%3B21) Sahara. The strengthening of the meridional temperature gradient over northern Africa in spring favors the development of Sahara depressions. Their impacts on the Mediterranean region justify the increased interest in these lows, formed mostly on the lee of the Atlas Mountains. They frequently bring strong winds and sandstorms (Alpert and Ziv 1989), with dust transported long distances, affecting the southern parts of Spain, France, and Italy, or Libya and Egypt (Moulin et al. 1998).
The geopotential height anomaly fields for spring Saharan lows are shown in Fig. 15a for PminP. There are two minima near the surface; one over Iberia and the deepest south of the Atlas Mountains. The depression aloft is centered over the Iberian Peninsula and extends southward over the northwestern tip of Africa. Although the criteria to choose the composite cases were based on 1000-hPa height, the composites of relative vorticity (Fig. 15b) suggest the development of relatively strong cyclogenesis south of the Atlas Mountains, extending well into the troposphere. Although not shown, the analysis of 10-m wind (available from NCEP–NCAR 12-hourly reanalyses) also shows signs of diurnal fluctuations in the strength of Saharan lows.
As in the case of Genoa cyclones, Saharan lows seem to be associated with a preexisting depression, developed through the whole troposphere, located northward of a mountainous barrier, in this case the Atlas Mountains (e.g., Egger et al. 1995). The vertical separation of the isentropes on the lee side, and the associated negative σ anomalies (Fig. 15b), are, however, more pronounced in the generation and maintenance of Saharan cyclones. As Fig. 15b clearly shows, the surface cyclone develops within a region with extremely weak static stability up to 650 hPa, and with strong low-level baroclinicity. These seem to be the optimal conditions, found mainly during late afternoon, to trigger the growth of a Saharan low, when an upper trough, such as that observed in the geopotential composites, is perturbed by the mountainous barrier and interacts with the low-level troposphere.
The absorption and multiple scattering of radiation by the airborne dust may significantly amplify the heating of the boundary layer, creating a deep mixed layer and increasing the sea–land temperature gradient. Attempts to quantify the heating rates of the lower atmosphere due to dust during Saharan storms lead to values between 10°C day−1 (Carlson and Benjamin 1980) and 6°C day−1 (Alpert et al. 1998). In the Sahara and other desert regions the role dust plays in the radiative processes of the lower troposphere seems to overcome the lack of moisture and release of latent heat, leading to unexpectedly strong cyclogenesis over these regions (Chen et al. 1995).
A number of different situations may contribute to Saharan cyclogenesis. Although this study is concerned with general considerations, it is worth mentioning the Thorncroft and Flocas (1997) case study where interaction between the polar jet, strongly deflected toward northern Africa with a pattern similar to the upper troughs found in our geopotential composites (Fig. 15a), leads to the highly mobile subtropical jet triggering a cyclone south of the Atlas Mountains. This type of interaction between large-scale features may well be reinforced by local radiative processes to favor the growth of Saharan lows.
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