[仿真]Evaluation of a large-eddy model simulation of a mixed-phase altocumulus

Using the Met Office large-eddy model (LEM) we simulate a mixed-phase altocumulus cloud that was yz?q(]  
observed from Chilbolton in southern England by a 94 GHz Doppler radar, a 905 nm lidar, a dual-wavelength L[rpb.'FG  
microwave radiometer and also by four radiosondes. It is important to test and evaluate such simulations with G+U3wF],  
observations, since there are significant differences between results from different cloud-resolving models for ice i^WIr h3a  
clouds. Simulating the Doppler radar and lidar data within the LEM allows us to compare observed and modelled "%VbI P  
quantities directly, and allows us to explore the relationships between observed and unobserved variables. For O}QFq14<+  
general-circulation models, which currently tend to give poor representations of mixed-phase clouds, the case (^LS']ybc  
shows the importance of using: (i) separate prognostic ice and liquid water, (ii) a vertical resolution that captures &&WDo(r3  
the thin layers of liquid water, and (iii) an accurate representation the subgrid vertical velocities that allow liquid +semfZ)  
water to form. m4@MxQm  
It is shown that large-scale ascents and descents are significant for this case, and so the horizontally averaged 7,*%[#-
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LEM profiles are relaxed towards observed profiles to account for these. The LEM simulation then gives a >8|+%pK8<  
reasonable cloud, with an ice-water path approximately two thirds of that observed, with liquid water at the 402x<H  
cloud top, as observed. However, the liquid-water cells that form in the updraughts at cloud top in the LEM have jeq:  
liquid-water paths (LWPs) up to half those observed, and there are too few cells, giving a mean LWP five to ten u,eZ6  
times smaller than observed. In reality, ice nucleation and fallout may deplete ice-nuclei concentrations at the lV\lj@  
cloud top, allowing more liquid water to form there, but this process is not represented in the model. Decreasing SG:bM7*1'  
the heterogeneous nucleation rate in the LEM increased the LWP, which supports this hypothesis. Hd*}k6  
The LEM captures the increase in the standard deviation in Doppler velocities (and so vertical winds) with 7:TO\0]2n  
height, but values are 1.5 to 4 times smaller than observed (although values are larger in an unforced model run, Lj8)'[K"  
this only increases the modelled LWP by a factor of approximately two). The LEM data show that, for values larger 4}i*cB`  
than approximately 12 cm s−1, the standard deviation in Doppler velocities provides an almost unbiased estimate Q[uAIyv0  
of the standard deviation in vertical winds, but provides an overestimate for smaller values. Time-smoothing the 'seyD
  
observed Doppler velocities and modelled mass-sq .. Ra[>P _  
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