Wednesday, 9 July 2014
In-situ measurements associated with the passage of a cold front over the UK on 29 November 2011 are used to initialize a Lagrangian parcel model for the purpose of calculating instantaneous rates of diabatic heating and cooling associated with the phase changes of water along the aircraft flight track. The front is observed to be transitioning from ana to kata type as it passes over the UK mainland. The parcel model calculations are performed with both bin-resolved and bulk treatments of microphysical processes. The in-situ data from this case study reveal the prevalence of liquid water, although planar ice crystals were detected at cloud top, as well as columnar crystals produced by rime splinter ejection within the pre-frontal warm sector. Analysis of ice crystal images taken by the cloud probes reveal the presence of vapour grown pristine ice (both plates and columns) up to ~1mm in size, as well as larger aggregates and some heavily rimed snow. We show that the diabatic terms of largest magnitude are associated with the liquid phase, i.e. from condensation within the updraughts at the leading edge of the front, in line with previous idealized modeling studies. The diabatic effects of ice phase processes (deposition/sublimation and melting) were typically found to be an order of magnitude smaller than for liquid processes, although they tend to cover a wide horizontal extent, and in the case of melting, are concentrated within a shallow vertical layer. Precipitating ice is shown to produce a cooling effect via sublimation and melting which can influence the mesoscale evolution of the front by modifying the temperature profile below cloud base. Based on this case study, we conclude that in terms of PV generation, the role of the ice phase is most important during the ana phase of the front, where sublimation and melting can directly influence the strength of the cold air inflow. Then as the system matures and evolves into a kata front, the diabatic influence of the ice phase is reduced as a consequence of the dry intrusion aloft, which pushes the ice into the warm sector ahead of the cold front. Therefore we argue that the strong dynamical forcing of this wintertime cyclone acts to control the extent to which ice phase processes contribute to the mesoscale evolution. We also use the parcel model framework in conjunction with the observations to assess the suitability of existing bulk microphysical treatments, of the kind used in operational weather forecast models. We find that the assumption of spherical ice crystals, and the use of negative exponential functions to describe ice particle size distributions, can lead to an overestimation of local diabatic heating and cooling rates by a factor of two or more. Our findings suggest that an adaptive treatment of ice crystal habit is necessary to capture realistic diabatic heating and cooling profiles associated with ice phase processes. We also recommend that the existing categories of cloud ice and snow, as used in many bulk microphysics schemes, be redefined to allow for representation of large pristine ice crystals such as those observed in the in-situ data.
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