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The formation of a large summertime Saharan dust plume: Convective and synoptic-scale analysis.

Roberts AJ, Knippertz P - J Geophys Res Atmos (2014)

Bottom Line: Emerging from this region the MCS has intermittent triggering of new cells, but later favorable deep layer shear produces a mesoscale convective complex.The unusually large size of the resulting dust plume (over 1000 km long) is linked to the longevity and vigor of the MCS, an enhanced pressure gradient due to lee cyclogenesis near the Atlas Mountains, and shallow precipitating clouds along the northern edge of the cold pool.Dust uplift processes identified are (1) strong winds near the cold pool front, (2) enhanced nocturnal low-level jet within the aged cold pool, and (3) a bore formed by the cold pool front on the nocturnal boundary layer.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds Leeds, UK.

ABSTRACT

Haboobs are dust storms produced by the spreading of evaporatively cooled air from thunderstorms over dusty surfaces and are a major dust uplift process in the Sahara. In this study observations, reanalysis, and a high-resolution simulation using the Weather Research and Forecasting model are used to analyze the multiscale dynamics which produced a long-lived (over 2 days) Saharan mesoscale convective system (MCS) and an unusually large haboob in June 2010. An upper level trough and wave on the subtropical jet 5 days prior to MCS initiation produce a precipitating tropical cloud plume associated with a disruption of the Saharan heat low and moistening of the central Sahara. The restrengthening Saharan heat low and a Mediterranean cold surge produce a convergent region over the Hoggar and Aïr Mountains, where small convective systems help further increase boundary layer moisture. Emerging from this region the MCS has intermittent triggering of new cells, but later favorable deep layer shear produces a mesoscale convective complex. The unusually large size of the resulting dust plume (over 1000 km long) is linked to the longevity and vigor of the MCS, an enhanced pressure gradient due to lee cyclogenesis near the Atlas Mountains, and shallow precipitating clouds along the northern edge of the cold pool. Dust uplift processes identified are (1) strong winds near the cold pool front, (2) enhanced nocturnal low-level jet within the aged cold pool, and (3) a bore formed by the cold pool front on the nocturnal boundary layer.

No MeSH data available.


Related in: MedlinePlus

Cross section through cold pool traveling into the desert showing equivalent potential temperature (color shading), virtual potential temperature (thin black contours), outline of cloud including liquid and ice (thick black contour), and outline of precipitation (thick blue contours) at 2100 9 June. The position of the cross sections are shown in Figure 8.
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fig11: Cross section through cold pool traveling into the desert showing equivalent potential temperature (color shading), virtual potential temperature (thin black contours), outline of cloud including liquid and ice (thick black contour), and outline of precipitation (thick blue contours) at 2100 9 June. The position of the cross sections are shown in Figure 8.

Mentions: Another factor that might explain the extent of the cold pool ingress into the desert is the generation of additional convective cells along the leading edge of the cold pool. When considering the movement of an MCS, the limitations on the distance traveled are not related to a theoretical maximum outflow distance of the initial cold pool. If new convective cells are triggered along a section of the leading edge traveling into the desert, the system is no longer similar to a single release density current as seen in laboratory experiments. Figure 11 is a cross section through the head of the cold pool positioned roughly perpendicular to its direction of travel (solid black line marked in Figure 8c). Contours of θe and θv show the position of different air masses, vectors show winds parallel to the cross section, and thick black and blue contours outline regions of cloud and precipitation, respectively. Shallow convective cells near the edge of the cold pool (between 3.5 and 7 km above the surface and 20 to 30 km across) produce precipitation that evaporates in the boundary layer. Cells producing precipitation which reaches the surface can also be seen close to the northern edge of the cold pool in Figure 8c. These have the potential to alter the expected behavior of a density current by refreshing the cooled air in the cold pool. Clouds similar to those shown in Figure 11 are likely to be overlooked in satellite imagery as they are shallow, and their tops do not significantly differ in temperature to the lofted dust below (Figure 12). The clouds shown in Figure 12 have a limited horizontal extent (approximately 3 km), this means that they are difficult to see even in 1 km pixel geostationary satellite imagery (Figures 12a and 12b). These clouds are evident in Figure 12c in the 250 m resolution MODIS true color imagery while they are difficult to see in either the NASCube thermal anomaly or SEVIRI dust imagery due to limitations associated with their dependence on BT and BT differences and the similar temperature of cloud and lofted dust. The grid spacing of the WRF simulation is too coarse to model the horizontal scale of clouds seen in Figure 12c; however, it is thought that the production of shallow, precipitation producing convective cells is a realistic process and could help to force cold pool air into the desert.


The formation of a large summertime Saharan dust plume: Convective and synoptic-scale analysis.

Roberts AJ, Knippertz P - J Geophys Res Atmos (2014)

Cross section through cold pool traveling into the desert showing equivalent potential temperature (color shading), virtual potential temperature (thin black contours), outline of cloud including liquid and ice (thick black contour), and outline of precipitation (thick blue contours) at 2100 9 June. The position of the cross sections are shown in Figure 8.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4379907&req=5

fig11: Cross section through cold pool traveling into the desert showing equivalent potential temperature (color shading), virtual potential temperature (thin black contours), outline of cloud including liquid and ice (thick black contour), and outline of precipitation (thick blue contours) at 2100 9 June. The position of the cross sections are shown in Figure 8.
Mentions: Another factor that might explain the extent of the cold pool ingress into the desert is the generation of additional convective cells along the leading edge of the cold pool. When considering the movement of an MCS, the limitations on the distance traveled are not related to a theoretical maximum outflow distance of the initial cold pool. If new convective cells are triggered along a section of the leading edge traveling into the desert, the system is no longer similar to a single release density current as seen in laboratory experiments. Figure 11 is a cross section through the head of the cold pool positioned roughly perpendicular to its direction of travel (solid black line marked in Figure 8c). Contours of θe and θv show the position of different air masses, vectors show winds parallel to the cross section, and thick black and blue contours outline regions of cloud and precipitation, respectively. Shallow convective cells near the edge of the cold pool (between 3.5 and 7 km above the surface and 20 to 30 km across) produce precipitation that evaporates in the boundary layer. Cells producing precipitation which reaches the surface can also be seen close to the northern edge of the cold pool in Figure 8c. These have the potential to alter the expected behavior of a density current by refreshing the cooled air in the cold pool. Clouds similar to those shown in Figure 11 are likely to be overlooked in satellite imagery as they are shallow, and their tops do not significantly differ in temperature to the lofted dust below (Figure 12). The clouds shown in Figure 12 have a limited horizontal extent (approximately 3 km), this means that they are difficult to see even in 1 km pixel geostationary satellite imagery (Figures 12a and 12b). These clouds are evident in Figure 12c in the 250 m resolution MODIS true color imagery while they are difficult to see in either the NASCube thermal anomaly or SEVIRI dust imagery due to limitations associated with their dependence on BT and BT differences and the similar temperature of cloud and lofted dust. The grid spacing of the WRF simulation is too coarse to model the horizontal scale of clouds seen in Figure 12c; however, it is thought that the production of shallow, precipitation producing convective cells is a realistic process and could help to force cold pool air into the desert.

Bottom Line: Emerging from this region the MCS has intermittent triggering of new cells, but later favorable deep layer shear produces a mesoscale convective complex.The unusually large size of the resulting dust plume (over 1000 km long) is linked to the longevity and vigor of the MCS, an enhanced pressure gradient due to lee cyclogenesis near the Atlas Mountains, and shallow precipitating clouds along the northern edge of the cold pool.Dust uplift processes identified are (1) strong winds near the cold pool front, (2) enhanced nocturnal low-level jet within the aged cold pool, and (3) a bore formed by the cold pool front on the nocturnal boundary layer.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds Leeds, UK.

ABSTRACT

Haboobs are dust storms produced by the spreading of evaporatively cooled air from thunderstorms over dusty surfaces and are a major dust uplift process in the Sahara. In this study observations, reanalysis, and a high-resolution simulation using the Weather Research and Forecasting model are used to analyze the multiscale dynamics which produced a long-lived (over 2 days) Saharan mesoscale convective system (MCS) and an unusually large haboob in June 2010. An upper level trough and wave on the subtropical jet 5 days prior to MCS initiation produce a precipitating tropical cloud plume associated with a disruption of the Saharan heat low and moistening of the central Sahara. The restrengthening Saharan heat low and a Mediterranean cold surge produce a convergent region over the Hoggar and Aïr Mountains, where small convective systems help further increase boundary layer moisture. Emerging from this region the MCS has intermittent triggering of new cells, but later favorable deep layer shear produces a mesoscale convective complex. The unusually large size of the resulting dust plume (over 1000 km long) is linked to the longevity and vigor of the MCS, an enhanced pressure gradient due to lee cyclogenesis near the Atlas Mountains, and shallow precipitating clouds along the northern edge of the cold pool. Dust uplift processes identified are (1) strong winds near the cold pool front, (2) enhanced nocturnal low-level jet within the aged cold pool, and (3) a bore formed by the cold pool front on the nocturnal boundary layer.

No MeSH data available.


Related in: MedlinePlus