The roles of dynamical variability and aerosols in cirrus cloud formation

[摘要] The probability of occurrence of ice crystal number densities in young cirrus clouds is examinedbased on airborne measurements. The observations have been carried out at midlatitudes in bothhemispheres at equivalent latitudes (52-55°N/S) during the same season (local autumnin 2000). The in situ measurements considered in the present study include temperatures, verticalvelocities, and total ice crystal concentrations, the latter determined with high precision andaccuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities(1-10 cm^-3) of small (diameter <20 mm) ice crystals. This mode dominates the probability distributionsand is shown to be caused by rapid cooling rates associated with updraft speedsin the range 10-100 cm s^-1. A second mode containing larger crystals extends from~1 cm^-3 to low concentrations close to the detection threshold(~3 x 10^-4 cm^-3) and could be associated with lower updraft speeds.Results of a statistical analysis provide compelling evidence that the dynamical variability ofvertical air motions on the mesoscale is the key factor determining the observed probabilitydistributions of pristine ice crystal concentrations in cirrus. Other factors considered arechanges of temperature as well as size, number, and ice nucleation thresholds of the freezingaerosol particles. The variability in vertical velocities is caused by atmospheric gravity waves leading to small-scaletemperature fluctuations. Inasmuch as gravity waves are widespread, mesoscale variability invertical velocities can be viewed as a universal feature of young cirrus clouds. Large-scalemodels that do not account for this subgrid-scale variability yield erroneous predictions of thevariability of basic cirrus cloud properties. Climate change may bring about changes in the globaldistribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work,these changes could significantly modify the probability distribution of cirrus ice crystalconcentrations. This study emphasizes the key role of vertical velocities and mesoscale variabilityin vertical velocities in controlling cirrus properties. The results suggest that, in any effort toascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes incloud formation should be done before conclusions regarding the role of other anthropogenic factors,such as changes in aerosol composition, are made.

[学科分类] 大气科学