Comments on ‘‘Atlantic Tropical Cyclogenetic Processes during SOP-3 NAMMA in the GEOS-5 Global Data Assimilation and Forecast System’’

摘要

Considerable attention has been given to the potential negative impacts of the Saharan air layer (SAL) in recent years (Dunion and Velden 2004; Jones et al. 2007; Wu 2007; Dunion and Marron 2008; Reale et al. 2009, hereafter RL1; Shu and Wu 2009; Sun et al. 2008, 2009). Braun (2010) recently raised questions about the negative impacts of Dunion and Velden (2004) and other studies in terms of storms that reached at least tropical storm strength and suggested that the SAL was an intrinsic part of the tropical cyclone environment for both storms that weaken after formation and those that intensify. Braun (2010) also suggested that several incorrect assumptions underlie many of the studies on the negative impacts of the SAL, including assumptions that most low-to-midlevel dry tropical air is SAL air, that the SAL is dry throughout its depth, and that the proximity of the SAL to storms struggling to intensify implies some role in that struggle. The recent paper RL1 is an example of the problems inherent in some of these assumptions. In their paper, RL1 analyze a simulation from the Global Earth Observing System (GEOS-5) global model and describe an extensive tongue of warm, dry air (see the white and yellow shading in their Fig. 5) that stretches southward from at least 308N (the northern limit of their plots) and wraps into a low pressure system during the period 26–29 August 2006, suppressing convection and possibly development of the African easterly wave associated with that low pressure system. They attributed the warm, dry tongue to the SAL (i.e., heating of the air mass during passage over the Sahara and radiative warming of the dust layer). Whether it was their intention, the implication is that this entire feature is due solely to the SAL and not to other possible sources of dry air or warmth. In addition, they suggested that a cool tongue of air in the boundary layer located directly beneath the elevated warm, dry tongue (forming a thermal dipole) was possibly the result of reduced solar radiation caused by an overlying dust layer. They stated that ‘‘the cool anomaly in the lower levels does not have any plausible explanation relying only on transport.’’