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Abstract
A Conceptual model of the Saharan Air Layer (SAL) and easterly wave disturbance is presented in light of diagnostic analyses of dust outbreaks.
Numerical simulations of the SAL were carried out to 5 days for two case studies using the Penn State/NCAR limited-area tropical model. The region of simulations encompasses North Africa and the eastern tropical Atlantic Ocean. One set of simulations used a horizontal resolution of 220 km. Analysis of the simulations emphasize the structure of the SAL and easterly wave disturbance and evaluation is made with reference to available observations and a conceptual model. Because both cases are similar, emphasis of the sensitivity tests is placed on the August 1974 case only. These tests include the effect of enhancing the SAL in the initial conditions, the role of surface sensible heating, the role of latent heating in the atmosphere, and the effect of heating due to radiative warming of the aerosol. A fine-mesh simulation of 110 km was also made to resolve the mesoscale features of the SAL.
Topics treated in the discussion include 1) the interaction of the SAL with attendant easterly wave disturbances, 2) the frontal structure of the SAL along the leading and southern boundary of the SAL, 3) forcing of vertical motions and the transverse/vertical circulations in the SAL front, 4) the nature of the anticyclonic curvature of the SAL plume along the coast of Africa and 5) the role of aerosol radiative heating in preserving the characteristics of the SAL as it moves toward the west. A significant conclusion is that the SAL contributes to forcing of vertical motions and cumulus convection and is therefore important (if not necessary) in the initial development of some easterly wave disturbances. Without surface heating over the Sahara or a proper initialization of the desert mixing layer, atmospheric forcing tends to be very much weaker than for the cam where a deep SAL is present.
Abstract
A Conceptual model of the Saharan Air Layer (SAL) and easterly wave disturbance is presented in light of diagnostic analyses of dust outbreaks.
Numerical simulations of the SAL were carried out to 5 days for two case studies using the Penn State/NCAR limited-area tropical model. The region of simulations encompasses North Africa and the eastern tropical Atlantic Ocean. One set of simulations used a horizontal resolution of 220 km. Analysis of the simulations emphasize the structure of the SAL and easterly wave disturbance and evaluation is made with reference to available observations and a conceptual model. Because both cases are similar, emphasis of the sensitivity tests is placed on the August 1974 case only. These tests include the effect of enhancing the SAL in the initial conditions, the role of surface sensible heating, the role of latent heating in the atmosphere, and the effect of heating due to radiative warming of the aerosol. A fine-mesh simulation of 110 km was also made to resolve the mesoscale features of the SAL.
Topics treated in the discussion include 1) the interaction of the SAL with attendant easterly wave disturbances, 2) the frontal structure of the SAL along the leading and southern boundary of the SAL, 3) forcing of vertical motions and the transverse/vertical circulations in the SAL front, 4) the nature of the anticyclonic curvature of the SAL plume along the coast of Africa and 5) the role of aerosol radiative heating in preserving the characteristics of the SAL as it moves toward the west. A significant conclusion is that the SAL contributes to forcing of vertical motions and cumulus convection and is therefore important (if not necessary) in the initial development of some easterly wave disturbances. Without surface heating over the Sahara or a proper initialization of the desert mixing layer, atmospheric forcing tends to be very much weaker than for the cam where a deep SAL is present.
Abstract
A combined longwave and shortwave radiative transfer model was used to determine effects of Saharan dust on the radiative fluxes and heating/cooling rates in the atmosphere. Cases are treated for cloud-free and overcast conditions over the ocean and for cloud-free sky over desert.
A benchmark comparison, made for the cloud-free ocean case between our calculations and those from Wiscombe’s detailed model, yielded results which were in close agreement. For moderately heavy dust amounts commonly measured over the Sahara and the eastern tropical Atlantic Ocean, typical calculated aerosol heating rates for the combined longwave and shortwave spectrum were in excess of 1 K day−1 for all three cases for most of the atmosphere beneath the top of the dust layer (500 mb). For the ocean case, maximum heating rates are found near the level of maximum concentration (700 mb), and also near the surface beneath the Saharan air layer (below 900 mb).
Net fluxes determined at the top of the atmosphere for the ocean cloud-free case were very insensitive to changes in dust optical depth. For the cloudy oceanic and desert cases, the reflectivity of the earth-atmosphere system diminished with increasing dust optical depth and approached that for the ocean case at large optical depth. In all three cases, the dust reduced the downward radiative flux into the ocean or desert while at the same time it increased the heating in the atmosphere, thus indicating a stabilizing effect by dust on the temperature lapse. However, further speculation concerning climatological significance of these results must be tempered by a need for further study of interactions between aerosol heating and atmospheric circulations, and between aerosols themselves and cloud microphysical processes.
Abstract
A combined longwave and shortwave radiative transfer model was used to determine effects of Saharan dust on the radiative fluxes and heating/cooling rates in the atmosphere. Cases are treated for cloud-free and overcast conditions over the ocean and for cloud-free sky over desert.
A benchmark comparison, made for the cloud-free ocean case between our calculations and those from Wiscombe’s detailed model, yielded results which were in close agreement. For moderately heavy dust amounts commonly measured over the Sahara and the eastern tropical Atlantic Ocean, typical calculated aerosol heating rates for the combined longwave and shortwave spectrum were in excess of 1 K day−1 for all three cases for most of the atmosphere beneath the top of the dust layer (500 mb). For the ocean case, maximum heating rates are found near the level of maximum concentration (700 mb), and also near the surface beneath the Saharan air layer (below 900 mb).
Net fluxes determined at the top of the atmosphere for the ocean cloud-free case were very insensitive to changes in dust optical depth. For the cloudy oceanic and desert cases, the reflectivity of the earth-atmosphere system diminished with increasing dust optical depth and approached that for the ocean case at large optical depth. In all three cases, the dust reduced the downward radiative flux into the ocean or desert while at the same time it increased the heating in the atmosphere, thus indicating a stabilizing effect by dust on the temperature lapse. However, further speculation concerning climatological significance of these results must be tempered by a need for further study of interactions between aerosol heating and atmospheric circulations, and between aerosols themselves and cloud microphysical processes.
Abstract
The vertical growth, diameters and trajectories of cumulus clouds observed over the San Francisco Peaks near Flagstaff, Ariz., were determined from photogrammetric measurements. New clouds formed in a small, well defined area. Their trajectories indicate the complex nature of orographic effects on the wind field. Active cloud elements were found to have growth characteristics similar to thermals observed in laboratory experiments. In particular, thermals broadened with height along a cone whose interior angle was about 30°, increasing their volumes by an order of magnitude prior to destructive erosion. Computed values of buoyancy decreased from a maximum initially, approaching zero or becoming negative near the erosion level, depending on the form of the momentum equation used.
Abstract
The vertical growth, diameters and trajectories of cumulus clouds observed over the San Francisco Peaks near Flagstaff, Ariz., were determined from photogrammetric measurements. New clouds formed in a small, well defined area. Their trajectories indicate the complex nature of orographic effects on the wind field. Active cloud elements were found to have growth characteristics similar to thermals observed in laboratory experiments. In particular, thermals broadened with height along a cone whose interior angle was about 30°, increasing their volumes by an order of magnitude prior to destructive erosion. Computed values of buoyancy decreased from a maximum initially, approaching zero or becoming negative near the erosion level, depending on the form of the momentum equation used.
Abstract
Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly waves.
The model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places. Bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day. An elevated layer of dust developed over the ocean as the northeast trade winds advected clean air underneath the dust-laden air as it moved westward. The size and spatial distributions of aerosol in the marine layer depended upon the undercutting process, the amount of background mineral aerosol present, and transport across the marine layer inversion by sedimentation and turbulent mixing.
Abstract
Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly waves.
The model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places. Bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day. An elevated layer of dust developed over the ocean as the northeast trade winds advected clean air underneath the dust-laden air as it moved westward. The size and spatial distributions of aerosol in the marine layer depended upon the undercutting process, the amount of background mineral aerosol present, and transport across the marine layer inversion by sedimentation and turbulent mixing.
