Saharan Air Layer Dust Loading: Effects on Convective Strength in Tropical Cloud Clusters

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Monday, 5 January 2015
Randall J. Hergert, University of South Florida, Tampa, FL; and J. M. Collins, J. P. Dunion, and C. H. Paxton

Numerous factors play a role in the development and maintenance of North Atlantic hurricanes as they originate and cross the Main Development Region, including sea-surface temperatures, relative humidity, vertical wind shear, etc. One key player that can modulate many of these factors is the Saharan Air Layer (SAL) which has been a source for study for nearly 5 decades. The interplay between dust loading within the Saharan Air Layer and the development of African Easterly Waves (AEWs) has been repeatedly noted in several previous studies. The cumulative indirect effect of the dust on African Easterly Waves however remains unknown. On a case by case basis, the SAL has been shown to negatively influence the development of specific AEWs though entrainment of dry air into the low to mid-levels and suppression of convection within the storm. Positive influences on AEW development have also been attributed to the SAL, namely its enhancement of the African Easterly Jet (AEJ) which in turn helps produce positive vorticity that AEWs tap into for energy. Further study is indeed warranted to try to fully understand whether or not the SAL has a positive or negative influence on the development of AEWs. A polarized view may be inadequate, as the SAL's role could very well be positive, negative or neutral depending on the storm characteristics and environmental conditions present at that specific time. This study examined the role of dust loading on the mixing between the SAL and the moist marine boundary layer directly beneath the base of the SAL from sea-level to ~800-850 hPa. We also explored the possibility of dust being entrained into tropical cyclones and acting as cloud condensation nuclei, in turn enhancing the convective strength of these systems. The study area covers much of the eastern North Atlantic extending from 0o 30oN, 10o 65oW. An aerosol optical thickness (AOT) dataset derived from the Advanced Very High Resolution Radiometer (AVHRR) Pathfinder Extended dataset (PATMOS-x) is correlated with total precipitable water (TPW) derived from microwave satellites and provided by Remote Sensing Systems (REMSS). In this correlation, gridded map data on a 1x1 degree resolution was used to find a significant relationship between dust loading and TPW. This relationship was used to determine whether or not increased dust loading within the SAL leads to increased mixing with the moist marine boundary layer underlying the SAL, indicated by increased TPW values. A second correlational study was conducted with the AOT data with Infrared brightness temperature (IR BT) measurements constructed from GridSat and confirmed on IBTrACS. IR BT measurements from individual storms were averaged on a monthly basis and set to gridded map data in order to bring it into temporal and spatial consistence with the AOT data. This portion of the study looked at the possibility that increased dust loading could lead to increased convective strength within developing tropical systems, indicated by reduced IR BTs.