An Observational Study on the Impact of Saharan Aerosol Layer on Thermodynamical Parameters in the Mid-Atlantic during the AEROSE 2007 Campaign

Saharan dust can be lifted by convection over hot desert areas. After being elevated to high altitudes, Saharan dust can be transported worldwide by winds and can cover great distances. The dust combined with the extremely hot and dry air of the Sahara Desert forms an atmospheric layer called the Saharan Air Layer (SAL). The AEROSE project, the Saharan Dust AERosols and Ocean Science Expeditions, is series of trans-Atlantic field campaigns conducted onboard the NOAA Ship Ronald H. Brown. One of the objectives it to identify air mass outflows from Africa, and their impacts on climate, weather, and environmental health. Besides the ship’s onboard instrumentation that measures sea temperature, wind, and thermodynamical parameters, several other instruments were installed to reach its goals. For instance, during the 2007 campaign solar (Eppley, model 8-48) and terrestrial radiation (Eppley, model PIR) were measured, aerosol optical depth (AOD) was calculated from a hand-held multi-spectral sunphotometer (Microtops, model II), about 4 radiosondes (Vaisala, model RS92 SGPD) were launched daily, and a ceilometer (Vaisala, CL31) was collecting aerosol backscatter and cloud ceiling.

This study will discuss the impact of the Saharan Aerosol Layer (SAL) into several observed meteorological parameters during the 2007 campaign. During this trip, the ship passed through a dust storm during two days, in which was carefully monitored. For instance, during the dust front, at the surface solar radiation was diminished, even though its diurnal variation looked like as for a clear day, and high values of AOD. These observations were supported by high values of aerosol backscatter profile within the lowest 5 km of the atmosphere. Potential temperature profiles show a poor defined gradient at the top of the Planetary Boundary Layer, with a weak lapse rate at the free atmosphere above it. On the other hand, the specific humidity profile showed a very sharp gradient at the Lift Condensation Level (LCL), with a dry layer intruding at the top of the PBL. The subsequent days seemed to be also affected by the SAL. The AOD decreased to background values, but there was a thin layer of strong back scatter values at the top of the PBL. The solar radiation absorption and/or the heat advection provoked a local maxima of the air temperature just above this thin layer. There was a presence of a well defined potential temperature gradient at the top of the PBL for the next three days following the dust front. The dry layer intrusion increased its depth for the next seven days. Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) values were also calculated from each radiosonde sounding. CAPE values were high during the first day of the dust front, showing strong convective activity. Then, during the second day, as CAPE values started to decrease, CIN values started to rise from about close to zero to 400. After the dust storm, this trend continued for more two days, as CAPE values decreased and CIN values increased. Then for the next seven days, both CAPE and CIN values were about zero, indicating that there were no conditions for a sustainable convection. This case study shows that dust and dry anomalies of the SAL reduce thermal cooling at the top of the PBL and help maintain low CAPE values and high CIN values, suppressing deep convection activity in the mid-Atlantic region.

Figure 1: Top: positive and negative buoyancy for AEROSE 2017. Bottom: CAPE (black) and CIN (red) values derived from the above figure. Green arrows indicate the Dust Storm period.