Update on progress with the implementation of a new two-moment microphysics scheme: Model description and single-column tests Hugh Morrison, Andrew Gettelman, and Richard Neale National Center for Atmospheric Research CCSM Atmospheric Model Working Group Meeting 22 March, 2006 Boulder, CO

Description of the new scheme Two-moment, four class – current version predicts number concentration N and mixing ratio q of cloud droplets, cloud ice, rain, and snow (based on approach of Morrison et al. 2005, JAS). Assumes functional form for the particle size distributions: either gamma or exponential. Several detailed microphysical processes are included.

What are the potential advantages of using a detailed two-moment scheme? Better treatment of cloud-aerosol interactions – indirect aerosol radiative effects in particular. More physically-based treatment of microphysical processes. Consistent with microphysics schemes in mesoscale/cloud-resolving models with consideration of special needs for GCM (e.g., fractional cloudiness, long time step).

Microphysical processes in the new scheme Cloud Ice Snow Cloud Droplets Rain Heterogeneous, Homogeneous Freezing Melting Collection Autoconversion Accretion Heterogeneous, Homogeneous Freezing Melting Autoconversion Collection Sedimentation Water Vapor EvaporationSublimation Dep/SubEvap/Cond

Current microphysical processes Droplet/cloud ice cond./dep./evap./sub. – Saturation adjustment method of Zhang et al. (2003). Partitioning between liquid/ice is a linear function of temperature similar to old CAM microphysics scheme. Evaporation/sublimation of rain/snow – Morrison et al. (2005). Droplet autoconversion and accretion by rain – Khairoutdinov and Kogan (2000). Ice autoconversion – Truncation of cloud ice PSD over specified timescale (similar to Ferrier 1994). Heterogeneous freezing of droplets and rain (contact + immersion) – Morrison and Pinto (2005).

Current microphysical processes Homogeneous freezing of droplets and rain instantaneously at -40 C. Melting of cloud ice and snow instantaneously at 0 C. Collection of droplets by snow – continuous collection with size-dependent collection efficiency. Droplet activation – Abdul-Razzak and Ghan (2000). Ice nucleation – Function of temperature based on truncated Fletcher (1962) curve.

Preliminary results using SCAM Single-column runs using CAM3_2_50. ARM SGP IOPs: July 1995 and March Prescribed aerosol (bimodal lognormal SD, total concentration = 100 cm -3 ), sub-grid vertical velocity at cloud base of 0.8 m/s (needed for droplet activation). No tuning!

July 1995 IOP Stratiform cloud water dominated by detrainment from deep convection. Time-height plot of grid-scale avg cloud water mixing ratio qc, cloud ice mixing ratio qi, and local droplet and ice effective radius, using the new microphysics scheme.

March 2000 IOP Stratiform cloud water dominated by frontal systems with some detrainment associated with deep convection. Time-height plot of grid-box avg cloud water mixing ratio qc, cloud ice mixing ratio qi, and local droplet and ice effective radius, using the new microphysics scheme.

Major features: Values appear to be reasonable. Droplet effective radius < ice effective radius. Cloud droplet mixing ratio > cloud ice mixing ratio. Ice effective radius decreases with height, reflecting size sorting during sedimentation, and decreasing ice nuclei concentration with increasing temperature. Large cloud ice sizes in lower region of cloud result in rapid autoconversion – most total (cloud ice plus snow) mass is snow in this region. Time-height plot of grid-box avg cloud water mixing ratio qc, cloud ice plus snow mixing ratio qi+qs, for March 2000 IOP. qi+qs

Difference in results (new – old microphysics) March 2000 July 1995

cLWP cIWP PRECT g m -2 g m -2 mm day -1 March 2000 New Micro Old Micro Obs July 1995 New Micro Old Micro Obs Averages during IOPs

Sensitivity Tests – 10 x Aerosol # (March 2000) Mean cloud liquid water path increased by factor of ~ 2. Mean downwelling solar flux at surface decreased by 29.9 W m -2.

Sensitivity Tests – 0.1 x Ice Nuclei # (March 2000) Mean cloud ice water path decreased by factor ~ 5. Mean downwelling solar flux at surface increased by 4.8 W m -2.

Summary of results Scheme is stable and conserves water and energy. Cloud microphysical properties using the new scheme are physically reasonable. Similar results to old scheme, but some differences with the old microphysics scheme – more liquid water at low levels, less at mid levels. Need to look in detail at other case studies/locations.

Ongoing and potential improvements Diagnostic rather than prognostic precipitation (but keeping both mixing ratio and number). Main problem with prognostic precipitation appears to be the removal of most rain mass within one time step due to sedimentation – leading to limited accretion and large cloud LWPs. Diagnostic precipitation should mitigate this problem and hence reduce the LWP. Explicitly model the Bergeron-Findeisen process. Assume water saturation in mixed-phase clouds – explicitly calculate vapor deposition onto cloud ice and snow – similar to Rostayn (2000). Have microphysics code structure in place for link with independent column approach. Other issues to think about: oSupersaturation threshold for ice nucleation. oExplicit vapor deposition onto cloud ice and snow in ice clouds. oIce microphysics (fallspeed, mass-dimension relationship, etc.)