Comments on “Droplets to Drops by Turbulent Coagulation”

Riemer and Wexler (2005, henceforth RW05) have utilized the turbulent collision kernel model of Zhou et al. (2001) to study the effect of turbulence on the initiation and development of raindrops from cloud droplets. They concluded that, for a cloud dissipation rate at 300 cm s , turbulent coagulation can move 96% of the droplet mass to sizes over 100 m in radius in 30 min, as compared to only 7% without turbulence. This result shows that turbulence is capable of rapidly transforming droplets to sizes for which the gravitational coagulation can operate effectively, thus overcoming the size-gap bottleneck for rain initiation. Under the assumption that the turbulent collision kernel of Zhou et al. (2001) can be extrapolated to atmospheric Reynolds numbers, RW05 found that the turbulent coagulation kernel is several orders of magnitude larger than the sedimentation kernel for droplets smaller than 100 m. We believe that the effects of turbulence have been grossly overestimated in RW05 for reasons to be discussed below. First, we would like to point out an error in RW05 that led to an overestimation of the rms velocity u by a factor of 3 and thus an overestimation of the Taylor microscale Reynolds number R by a factor of 3. RW05’s estimations were based on the rms velocity u and the average cloud dissipation rate on the in-cloud measurements by MacPherson and Isaac, shown in Table 1 of MacPherson and Isaac (1977). The cloud turbulence is anisotropic and a rough estimate of u for equivalent isotropic turbulence would be