Comments on “Smoking Rain Clouds over the Amazon” by M.O. Andreae, D. Rosenfeld, P. Artaxo, A.A. Costa, G.P. Frank, K.M. Longo and M.A.F. Silva-Dias
Earle
Williams, Massachusetts Institute of Technology, earlew@ll.mit.edu
(Presenting)
Recently published observations during the dry-to-wet transition season in Rondonia, Brazil, characterized by large-scale subsidence, show evidence that enhanced aerosol concentrations are invigorating moist convection, causing “intense thunderstorms, large hail, and greater likelihood for overshooting cloud tops into the stratosphere”. These results are puzzling because they stand in apparent contradiction with earlier findings in the same location in the same meteorological season (Williams et al, 2002). In particular, Williams et al could not discern differences in measured peak total lightning flash rate, daily cloud-to-ground lightning totals, or in the daily lightning yield per unit rainfall on several highly polluted days in October 1999 and several ‘clean’ days in November 1999.
Unfortunately, Andreae et al (2004) did not explore these parameters for cases they discuss, and their macroscopic evidence (in contrast with their in situ cloud microphysical evidence) for differences in convective vigor is mainly qualitative. Furthermore, extraordinarily intense thunderstorms with stratospheric penetrations were observed in November 1999 with radar by Williams et al (2002) when the aerosol concentrations were also “almost as low as over the ocean”. It has previously been established that the transition season surface air is hotter and hence more unstable (Williams and Stanfill, 2002) than the following wet season, supporting the notion that thermodynamics is playing the primary role in setting the vigor of the convection. A possible factor in these apparent discrepancies is the different degree of control over key observables (aerosol, instability, cloud microphysics, rainfall and lightning) in the two studies, and these aspects will be initially addressed toward resolution of the discrepancy. Andreae et al (2004) also conclude with multiple global implications for their local findings—in particular, “substantial effects on the …global circulation systems” and “enhancing planetary scale upper-level waves that affect global climate”. No quantitative results in this regard are shown. In judging the validity of such claims, it is important to note that the local regime explored in Brazil in both studies is characterized by pronounced subsidence of the Hadley circulation. As a consequence, from the standpoint of total latent heating, the variations in this regime are decidedly minor players in the general circulation. Radical effects of aerosol are therefore needed to have “substantial” global effects. Suggestions are made for transition season rainfall measurements in years with contrasting smoke prevalence (independently measured), toward quantifying these effects.
Submetido por Lorena Cordeiro Brewster em 01-ABR-2004