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Initial results of a new ice particle size parameterization in Racmo. Gerd-Jan van Zadelhoff CloudNet : April 2005 Co-authors: Dave Donovan, Erik van Meijgaard
ObservedR’effdistributions Cabauw ARM • R’eff (T) not the same for the ARM and Cabauw sites. • R’eff (DZ) similar for DZ < 3.5 at the ARM and Cabauw site.
Comparing mean profiles to normalized cloud thickness ARM Cabauw - Reff(DZ, complex polycrystals) ARM-SGP & Cabauw results are the same within the errors.
Parameterization of Reff versus cloud thickness Reff=A+B(DZ/H)+C(DZ/H)2 ARM Cabauw ARM C(H) B(H) A(H) - Reff(DZ, complex polycrystals): Use parabolic fits to the observed values
Current Reff(T) relationship in Racmo vs. Obs Current Racmo parameterization Mean Reff-T relationship observed, using Complex polycrystals as habit.
Version 1: Reff parameterization & distribution. First version of the Reff parameterizations based on cloud-thickness. Above a threshold cloud-fraction of 0.1 maximum overlap is assumed.
Differences due to changes in parameterization Differences in the mean net short wave flux at the surface and cloud cover. Comparing the new parameterization with the current operational one.
Version 2: Reff parameterization & distribution. Assume random-maximum overlap and add fractional cloud-thickness according to the minimum cloud fraction.
Resulting Reff-T distribution due to Parameterization The Reff-T distribution changes. Note however that the mean stays the same.
Work in progress: Current RACMO Reff > Observed Reff and misses a distribution. As radiation of the models is reasonable tuned, the optical depth should be reasonable as well t = IWC/Reff IWC should be too large as well. IWC(Z,T) Feed observed IWC profiles and the Reff parameterization to a single column radiation module. To check if the Cabauw radiation observations are similar to the calculated ones.
Example of radiation profiles and surface values for an observed IWC profile and a parameterized Reff. There are large differences between the different calculated fluxes. Changing from Reff(T) to Reff(Dz) is the first step but getting the cloud thickness right is as important.
IWC(Z,T) 25 April 2004 In case of an optically thin ice cloud with low IWP there is not much difference between the different parameterizations, but in all cases the Reff(DZ) seems to go in the right direction.
9 June 2004 19 July 2004
The Reff has been parameterized according to depth into cloud and total cloud thickness resulting in large differences in SW-flux, Cloud cover etc. • Using this parameterization the resulting Reff-T distribution looks very similar to the one observed. • The cloud-thickness calculated using the Racmo vertical resolution results in too thick clouds. • Using observed IWC profiles and Reff(Dz) gives far better result to the observed SW-flux compared to the original Reff(T) parameterization. Conclusions…