[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 a)S6Z  
observed from Chilbolton in southern England by a 94 GHz Doppler radar, a 905 nm lidar, a dual-wavelength ep5`&g]3  
microwave radiometer and also by four radiosondes. It is important to test and evaluate such simulations with
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observations, since there are significant differences between results from different cloud-resolving models for ice _'y`hKeI[  
clouds. Simulating the Doppler radar and lidar data within the LEM allows us to compare observed and modelled _@)-#7  
quantities directly, and allows us to explore the relationships between observed and unobserved variables. For >*B59+1P  
general-circulation models, which currently tend to give poor representations of mixed-phase clouds, the case dqBN_P%  
shows the importance of using: (i) separate prognostic ice and liquid water, (ii) a vertical resolution that captures nRX'J5Q m<  
the thin layers of liquid water, and (iii) an accurate representation the subgrid vertical velocities that allow liquid `DG
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water to form. ^XYK }J  
It is shown that large-scale ascents and descents are significant for this case, and so the horizontally averaged @+Nf@LJ  
LEM profiles are relaxed towards observed profiles to account for these. The LEM simulation then gives a o+k*ia~Fa  
reasonable cloud, with an ice-water path approximately two thirds of that observed, with liquid water at the C %j%>X`  
cloud top, as observed. However, the liquid-water cells that form in the updraughts at cloud top in the LEM have !%(kMN  
liquid-water paths (LWPs) up to half those observed, and there are too few cells, giving a mean LWP five to ten pIpd
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times smaller than observed. In reality, ice nucleation and fallout may deplete ice-nuclei concentrations at the #Ddo` >`&  
cloud top, allowing more liquid water to form there, but this process is not represented in the model. Decreasing >ZgV8X:  
the heterogeneous nucleation rate in the LEM increased the LWP, which supports this hypothesis. -+2A@kmEJ  
The LEM captures the increase in the standard deviation in Doppler velocities (and so vertical winds) with @!ja/Y^  
height, but values are 1.5 to 4 times smaller than observed (although values are larger in an unforced model run, 3TV4|&W;  
this only increases the modelled LWP by a factor of approximately two). The LEM data show that, for values larger 'Zex/:QS  
than approximately 12 cm s−1, the standard deviation in Doppler velocities provides an almost unbiased estimate PD^ 6Ywn>s  
of the standard deviation in vertical winds, but provides an overestimate for smaller values. Time-smoothing the B )\;Ja  
observed Doppler velocities and modelled mass-sq .. {ktwX\z  
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