A44B-01 INVITED
A Two-moment Stratiform Cloud Microphysics Scheme in the Community Atmosphere Model
A new stratiform cloud microphysics scheme for the Community Atmosphere Model Version 3 (CAM) General Circulation Model is presented. The goal is to provide a more robust treatment of the moist physics, improve flexibility and self-consistency, and allow for coupling between the cloud microphysics and aerosol. The scheme predicts number concentrations and mixing ratios of cloud droplets and cloud ice, and diagnoses mixing ratios and number concentrations of rain and snow. Droplet activation from aerosol is treated using a diagnosed sub- grid vertical velocity. Global results using the new scheme are compared to the old CAM microphysics scheme as well as observations. The new scheme produces relatively small differences in winds, temperature, and precipitation, but significantly reduces liquid water path in mid-latitude storm tracks and improves the cloud radiative forcing. More detailed tests of the microphysics scheme including sensitivity to vertical resolution and time step are also described.
A44B-02 INVITED
Clouds in GEOS-5
The GEOS-5 atmospheric model is being developed as a weather-and-climate capable model. It must perform well in assimilation mode as well as in weather and climate simulations and forecasts and in coupled chemistry-climate simulations. In developing GEOS-5, attention has focused on the representation of moist processes. The moist physics package uses a single phase prognostic condensate and a prognostic cloud fraction. Two separate cloud types are distinguished by their source: "anvil" cloud originates in detraining convection, and large-scale cloud originates in a PDF-based condensation calculation. Ice and liquid phases for each cloud type are considered. Once created, condensate and fraction from the anvil and statistical cloud types experience the same loss processes: evaporation of condensate and fraction, auto-conversion of liquid or mixed phase condensate, sedimentation of frozen condensate, and accretion of condensate by falling precipitation. The convective parameterization scheme is the Relaxed Arakawa-Schubert, or RAS, scheme. Satellite data are used to evaluate the performance of the moist physics packages and help in their tuning. In addition, analysis of and comparisons to cloud-resolving models such as the Goddard Cumulus Ensemble model are used to help improve the PDFs used in the moist physics. The presentation will show some of our evaluations including precipitation diagnostics.
A44B-03 INVITED
Cloud microphysics in the GFDL AGCM: current status and future developments
Cloud microphysics impacts various aspects of an atmospheric general circulation model (AGCM). An overview of the cloud microphysics parameterizations in the current version of the GFDL AGCM will be presented. Representation of microphysical processes are included in the convective parameterizations (shallow and deep) as well as the stratiform cloud scheme. Current convection schemes incorporate highly simplified formulations for the microphysical processes leading to the formation of precipitation. These convection schemes are being replaced by more sophisticated ones with explicit prediction of cloud plumes vertical velocities. This allows for the inclusion of single- or double-moment microphysics schemes as part of the convective parameterizations. The stratiform cloud scheme currently incorporates a single-moment microphysics and is being updated to a partly double-moment formulation. Of particular importance for the next version of the GFDL AGCM is the treatment of the aerosol indirect effect. Activation of cloud condensation nuclei to form cloud droplets is determined by local supersaturation, which is primarily controlled by cloud vertical velocities. Such velocities are not readily available in a large-scale model. Avenues under consideration for the parameterization of aerosol activation in the AGCM convective and stratiform parameterizations will be discussed.
A44B-04
The Challenges of Explicitly Representing Cloud Droplet and Ice Particle Nucleation on Aerosols in CRM, NWP, and GCM Models
In recent years we have incorporated increasingly sophisticated representations of aerosol effects on clouds and precipitation in RAMS. Our approach is to incorporate explicit aerosol activation in a bulk microphysics model that emulates a bin-resolving model. Most of our simulations have been in the context of cloud resolving simulations in which cloud-scale vertical motions, which impact droplet and ice particle nucleation, are explicitly simulated. In this talk I will briefly review our experience simulating aerosol affects on wintertime orographic clouds, Arctic stratus, deep convective clouds and even tropical cyclones. I then discuss what will be needed to incorporate a model like ours in NWP models and in climate models. Especially important to NWP is not only the engineering and physical parameterizations required, but also the infrastructure needed to predict/retrieve cloud nucleating aerosol sources, sinks, and transports.
A44B-05
Recent Improvements to the "Rutledge and Hobbs" Bulk Microphysics Scheme in the GCE Model
The Goddard Cumulus Ensemble (GCE) model is a cloud-resolving model (CRM) and has a variety of different options for cloud microphysics parameterizations. The most commonly used scheme is a bulk 3-class ice scheme wherein the third class of ice is moderate density graupel. This scheme has been and continues to be widely used. However, despite its success and longevity, it has some inherent biases. Recent efforts to improve the scheme have significantly reduced those biases. Comparisons with both ground-based radar data and satellite scattering signatures are used to guide as well as validate the improvements based on two convective cases from TRMM LBA. Qualitative aircraft information is also used as a guide. Initial improvements include eliminating the dry growth of graupel and reducing the efficiency with which cloud particles were collected by snow. Corrections to the Bergerson process resulted in further gains. Finally, adjustments to the saturation scheme, the ice concentration formulation, and properly allowing the sublimation of graupel succeeded in reducing the excessive penetration of 40-dBZ echoes above 10 km. The simulated ppt ice structure after the modifications is far more realistic and in much better agreement with radar and satellite observations. The excessive amount of graupel produced in the base scheme is vastly reduced.
A44B-06
Stratiform and convective precipitation in 2-D cloud resolving simulations.
There is observational evidence that the partition of precipitation into stratiform and convective is correlated with the local sea surface temperature. At least two competing mechanisms can be proposed to explain these observations based on an increase in the efficiency of precipitation with higher SST. The first is that the increase in efficiency is a consequence of the increase in specific humidity in the air that participates in convection which consequently increases the efficiency of collection-coalescence processes inside the convective clouds. A second possibility is simply that precipitation is more efficient in regions of higher mid-tropospheric humidity (which coincide geographically with regions of higher precipitation and higher SST) due to less dilution of dry air into the cloud. We will investigate how these mechanisms operate in idealized 2-D simulations using the Weather Research and Forecasting Model (WRF). We will use simulations in which the growth of convection is forced by an initial thermal perturbation into conditionally unstable environments with different vertical distributions of specific humidity. We will focus on developing meaningful comparisons between the simulations and the observations provided by TRMM and Kwajalein ground based radar. We will also discuss the possible implications of the sensitivity of the precipitation efficiency in the problem of rainfall estimation from geostationary satellites.
A44B-07
Cloud-Precipitation Microphysical Characteristics of Tropical Storm Bilis (2006). Part I: Observations
Tropical Storm Bilis (2006) made a landfall on Fujian, China on 14 July 2006, and caused direct economic losses of 34.83B Chinese dollars, 654 deaths and 208 missing. In this paper, the cloud-precipitation microphysical properties associated with Bilis are constructed using various observation data including CloudSat and Doppler radar data. The cloud microphysical properties include vertical hydrometeor profiles retrieved from satellite and radar measurements and precipitation microphysical properties contain surface rainfall measurements from mesonet rain gauge observations. Different development stages of Bilis are identified with different cloud- precipitation microphysical properties. Cloud-precipitation microphysical properties associated with individual convective cells from different stages including the landfall are also studied.
A44B-08
A Link between the Pacific Storm Track and Asian Pollution Aerosols
Indirect radiative forcing of atmospheric aerosols by modification of cloud processes poses the largest uncertainty in climate prediction. We show here a trend of increasing deep convective clouds over the Pacific in winter from long-term satellite cloud measurements (1984-2005). Simulations using a cloud-resolving Weather Research and Forecast model reveal that the enhanced deep convective clouds are reproduced when accounting for the aerosol effect from the Asian pollution outflow, which leads to intensified storms. We suggest that the wintertime Pacific is highly vulnerable to the aerosol-cloud interaction because of favorable cloud dynamical and microphysical conditions from the coupling between the Pacific storm track and Asian pollution outflow. The intensified Pacific storm track is climatically significant and represents possibly the first detected climate signal of the aerosol-cloud interaction associated with anthropogenic pollution. In addition to radiative forcing on climate, intensification of the Pacific storm track likely impacts the global general circulation due to its fundamental role in meridional heat transport and forcing of stationary waves.