DYCOMS RF02 SCM Intercomparison

GCSS Boundary Layer Cloud WG

SCM intercomparison organizers: Chris Bretherton and Matt Wyant, University of Washington

Deadline for submission of revised results1 August 2005

Preliminary results were presented by Chris Bretherton and discussed at BLCWG breakout of pan-GCSS meeting., Athens Greece, 16-20 May 2005. This discussion led to several small modifications to the LES case that we are also adopting, including changed droplet conc., changed geostrophic winds, and specified surface heat and moisture fluxes.

Registered Participants

Please email Chris ( breth@atmos.washington.edu ) and cc Matt ( mwyant@atmos.washington.edu ) if you plan to participate. A list of participants who have already emailed that they plan to do the case is below.  Those in bold submitted preliminary results for the Athens meeting:

Discussion and Motivation

This single column model (SCM) intercomparison is a companion to the GCSS DYCOMS RF02 LES intercomparison, organized by Andy Ackerman of NASA-Ames.  The goal is to compare SCM simulations of a nocturnal drizzling stratocumulus-topped boundary layer with each other, with LES simulations, and with comprehensive observations taken during DYCOMS-II.  The RF02 LES intercomparison web page describes the case, corresponding observations and papers discussing them,  and some minor idealizations made to facilitate model intercomparison.

This case follows on our GCSS BLCWG intercomparison of a nocturnal non-precipitating stratocumulus-topped mixed layer observed in DYCOMS RF01.  The intercomparison (Stevens et al. 2005 for LES, Zhu et al. 2005 for SCM, both accepted by Mon. Wea. Rev.) showed that even in this particularly simple boundary layer, cloud liquid water path (LWP) varied by almost an order of magnitude between both SCM and LES models.  A few LES models (with subgrid turbulence and advection schemes that tended to suppress mixing across the strong inversion overlaying the stratocumulus)  did obtain approximately correct LWP for this case. However, a challenge in the drizzling case both for LES and SCM models is sorting out the effects of turbulent mixing and of drizzle in determining the boundary layer structure and LWP.  Another challenge for SCM models is that aerosol and cloud droplet concentrations were lower in RF02 than RF01, favoring drizzle production. This feedback is not represented in many SCMs.  However, given the potential importance of precipitation processes to both the mean albedo of stratocumulus in some regions, and the second aerosol indirect ('Albrecht') effect on climate, this is an important issue to tackle in an intercomparison format.

SCM Specifications

Setup

Sensitivity studies - do whichever you can.

Output (netcdf format)