GEWEX Cloud System Study (GCSS) cirrus cloud working group: development of an observation-based case study for model evaluation

Abstract

Abstract. The GCSS working group on cirrus focuses on an inter-comparison of model simulations ranging from very detailed microphysical and dynamical models through to general circulation models (GCMs). The past GCSS cirrus cloud inter-comparison highlighted the wide range in modelling results that was a surprise to the modelling community. That inter-comparison was idealised and, therefore, a key issue was that it did not benefit from observations to help distinguish between model performances.In this work, we aim to address this key issue by developing an observationally based case study to be used for the GCSS cirrus modelling inter-comparison study. We focused on developing a case that had sufficient observations with which to evaluate models, to help identify which models in the inter-comparison are performing well and highlight areas for model development. Furthermore, it will provide a base case for future model comparisons or testing of new or updated models. This paper outlines the modelling case development and the inter-comparison results will be presented in a follow-on paper.The case was based on the 9 March 2000 Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) during an intensive observation period (IOP). The case was developed utilising various observations including ARM SGP remote sensing including the MilliMeter Cloud Radar (MMCR), radiometers, radiosondes, aircraft observations, satellite observations, objective analysis and complemented with results from the Rapid Update Cycle (RUC) model as well as bespoke gravity wave simulations used to provide the best estimate for large scale forcing. The retrievals of ice water content, ice number concentration and fall velocity provide several constraints to evaluate model performances. Initial testing of the case has been reported using the UK Met Office Large Eddy Simulation Model (LEM) which suggests the case is appropriate for the model inter-comparison study. To our knowledge, this case offers the most detailed case study for cirrus comparison available and we anticipate this will offer significant benefits over past comparisons which have mostly been loosely based on observations.

A flexible three-dimensional stratocumulus, cumulus and cirrus cloud generator (3DCLOUD) based on drastically simplified atmospheric equations and the Fourier transform framework

“…Figure 7 also shows the power spectra computed following the x and y directions and then averaged, for BRAMS and 3DCLOUD optical depth fields. The 3DCLOUD optical depth spectral slope is close to −5/3 in the L out : 1/ (2 x) m −1 wave number range, as expected, because of the statistical adjustment (2000) and Yang et al (2012). performed in the second step of the 3DCLOUD algorithm. By contrast, the BRAMS optical depth spectral slope is close to −5/3 only in the 2 × 10 −3…”

Supersaturation Variability and Cirrus Ice Crystal Size Distributions

“…Small-scale fluctuations of ice supersaturation Synoptic regimes conducive to cirrus formation vary widely, defining the large-scale environment in which ice cloud particles nucleate and develop. Cirrus clouds exhibit significant internal meso-and microscale structure (Sassen et al 1989), including vertical air motion variability (Yang et al 2012). Nonetheless, little is known about the characteristics of small-scale supersaturation variability in cirrus clouds and its impact on cirrus microphysical properties.…”