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when a mature thunderstorm cell moves into a region with a weak updraft, the cloud water content itself may rise as the rate of precipitation formation increases. as the updraft flow is weakened, the cloud water content will tend to increase until it reaches a critical value that is determined by the vapor transport efficiency. at this point, the cloud water content will begin to decrease, generating drizzle. this is because the cloud column contains a balance of vapor and rainwater transport that is determined by the cloud-top level and the precipitation formation rate. if this balance is disturbed by the absence of cloud water at the cloud top, the balance of water transport changes. the vapor transport becomes dominated by the rainwater transport, reducing the amount of vapor that can be lifted from the surface. the location of the drizzle/cloud-top transition is controlled by the balance of vapor and rainwater transport across the system, which, in turn, is determined by the cloud top, the precipitation formation rate, the cloud water content, and the temperature of the environmental air.
viray et al. (2011) developed, tested, and applied a cloud-scale model for convective boundary-layer (cbl) storms. the model, hucm, was validated against field measurements of extreme rain rate at schauberger, germany, and was then run for a 6-year period for a few storms in order to understand the impact of ice-precipitation patterns on the statistics of extreme rain rates.
this model has been benchmarked in various manners to other models that are based on subfreezing nucleation theory (burton and joyce 1990 ; ebert et al. 2005 ; panofsky and wilchins 2000). in particular, mcmurtry et al. (2006) compared the hucm with other multivariate statistical model-simulated severe precipitation (sp) events to validate it in this sense.
this is consistent with the existence of large refreezing nuclei in the lip which have been suggested by several authors ( bintz 1994, demott et al. 2010). as found by bringi et al. (1997), these nuclei act as seeds of freezing raindrops when they are further from the environmental 0c level down to the lip depth. in the ice channel, where zh is larger than 5kdb, ice embryos are found with z > 5kdb ( not shown in fig. 4 ) up to the lip depth. this explains the decrease in the rotational relaxation time of ice. as the refreezing nuclei are large (>1m), they are considered to play the role of ccn for hail growth. smith et al. (1999) have described the various aspects of hail growth, and kennedy et al.
in this work we extend previous qens studies on water confined in cnts to investigate the dynamical properties of this peculiar molecular arrangement, reporting results for the first time in the very low-temperature regime. to this end we performed a series of neutron scattering experiments at ill, in the q-range from 0.25 to 2.9 å -1. in particular, we have focused on the dynamics of water confined in a (10, 10) swcnt in the temperature interval from 300 to 10k. this carbon nanotube was selected because it presents the lowest (10, 10) swcnt-to-cnt distance of the entire array, d(10, 10) =8.7 å, and because the hexagonal ice sheet formed by each (10, 10) swcnt acts as a channel along the nanotube axis, and thereby its role is evident. the same isotopic water sample was used in all the experiments. moreover, to perform the qens measurements we have used a new experimental set-up, as reported in the previous section. quasi-elastic scattering data are reported in fig. 2 and fig. 3 for the (10, 10) swcnt at the selected temperatures, while dynamical data, i.e. the scattering functions and the self-diffusion parameters, are reported in fig. 4. our results support and confirm the complete compatibility of the findings already reported in the literature for the (10, 10) swcnt 6, 16 , except for the sole aspect of the rotation dynamics, where, by using a new experimental set-up, we have now demonstrated for the first time that at temperatures as low as 10k the dynamics of water in a confined geometry is characterized by the onset of the quasi-free rotation of molecules. in the following we report the features of the obtained dynamical data and their interpretation in terms of the dynamics of the confined water.