Microphysical Processes in Cloud-Resolving Models II Posters
Presiding: B S Ferrier, NCEP Environmental Modeling Center and GSO/SAIC; E Malek, Utah State University
A53A-01 1330h
Effects of Cold Microphysical Processes on the Surface Precipitation Variability of Non-squall Tropical Ocean Convection
The influence of cold microphysical processes on surface precipitation variability is investigated for a non-squall cluster and a scattered convective event that occurred over the tropical Pacific Ocean during the KWAJEX Experiment period. The MM5 model high resolution simulations of surface rainfall and ice concentrations are validated with available data from the experiment in terms of the ability to reproduce the character of variability. The validated model is then used to perform a number of sensitivity analyses pertaining to the dependence of simulated surface precipitation on various microphysical factors associated with cold microphysical processes. It is found that the graupel-related processes in the model microphysics scheme modify both the magnitude and the spatial variability of surface precipitation. The maximum precipitation simulated by the warm rain scheme is double the one simulated by including ice microphysics. The sizes of mean convective precipitation cells in the warm rain simulation are 48% and 41% larger than those in the ice microphysics simulation for the non-squall and scattered convective events, respectively. Further investigation points to the different treatment of supercooled raindrops in these two microphysics schemes being responsible for significant differences in the simulated maximum precipitation and spatial variability for the two rain events. Dividing the simulated precipitation into convective and stratiform portions based on simulated radar reflectivity shows that the non-squall cluster produces more convective rainfall (68%) than the scattering convection event (46%). A microphysical diagnosis of the causes is performed for each event. The findings have significant implications for the vertical profiles of atmospheric heating in the tropics.
A53A-02 1330h
The Diagnosis and application of a convective vorticity vector associated with convective systems
Although dry/moist potential vorticity is a very useful and powerful physical quantity in the large scale dynamics, it is not a quite ideal dynamical tool for the study of convective systems or severe storms. A new convective vorticity vector (CVV) is introduced in this study to identify the development of convective systems or severe storms. The daily Aviation (AVN) Model Data is used to diagnose the distribution of the CVV associated with rain storms occurred in the period of Meiyu in 1998. The results have clearly demonstrated that the CVV is an effective vector for indicating the convective actions along the Meiyu front. The CVV also is used to diagnose a 2-D cloud-resolving simulation data associated with 2-D tropical convection. The cloud model is forced by the vertical velocity, zonal wind, horizontal advection, and sea surface temperature obtained from the Tropical cean-Global tmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) and is integrated for a selected 10-day period. The CVV has zonal and vertical components in the 2-D x-z frame. Analysis of zonally averaged and mass-integrated quantities shows that the correlation coefficient between the vertical component of the CVV and the sum of the cloud hydrometeor mixing ratios is 0.81, whereas the correlation coefficient between the zonal component and the sum of the mixing ratios is only 0.18. This indicates that the vertical component of the CVV is closely associated with tropical convection. The tendency equation for the vertical component of the CVV is derived and the zonally averaged and mass-integrated tendency budgets are analyzed. The tendency of the vertical component of the CVV is determined by the interaction between the vorticity and the zonal gradient of cloud heating. The results demonstrate that the vertical component of the CVV is a cloud-linked parameter and can be used to study tropical convection.
A53A-03 1330h
The Enhancement of Condensational Growth of Cloud Droplets in Melting Layer Simulated by a Non-hydrostatic Model
We have conducted numerical simulations with the non-hydrostatic regional climate model which has been developed on the basis of Japan Meteorological Agency Non-Hydrostatic Model to study the Baiu front in global warming climate as well as that in present climate. In those simulations, the condensational growth of cloud droplets was found to be enhanced in melting layer so that the heating by condensation exceeded the cooling due to the melting of snow particles in the layer. This talk focuses on how this enhancement of condensation occurred. As a result of sensitivity test, the effect of sub-grid eddy diffusion as well as the cooling by the snow melting is found to be important. Turbulent kinetic energy (TKE) has large value where corresponds to the region of the enhancement of condensation. TKE induces the sub-grid eddy diffusion which produces super-saturation so as to enhance the condensational growth of cloud droplets in the melting layer in the experiments.
A53A-04 1330h
Microphysical sensitivities of cloud-resolving model simulations of KWAJEX
Cloud-resolving model simulations of the conditions around Kwajalein Island during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX), July 24--September 15, 1999, are performed to understand the sensitivities of the results to changes in the model's microphysics and radiation parameterizations. An extensive set of observations were collected during KWAJEX, including high quality estimates of precipitation by an S-band ground validation radar. Large-scale forcings for the simulations --- which use cyclic boundary conditions --- have been derived from the observations by Minghua Zhang. The cloud-resolving model used here is the System for Atmospheric Modeling (SAM), developed by Marat Khairoutdinov at Colorado State University, to which the authors have added a second microphysical package (Fu et al 1995) and radiation scheme (from CAM3.0). While the alternate microphysics represents all hydrometeor interactions and has prognostic equations for water vapor, rain, snow, graupel, cloud water and cloud ice, SAM's default microphysics uses temperature to partition the condensate and precipitate among the phases and has prognostic equations only for total water (vapor+cloud) and precipitating water. The simulations are able to track the observed conditions over the full 52 day period without nudging. The different versions of the model generally reproduce the observed precipitation rate, temperature and relative humidity profiles, with mean temperature biases of less than 2K below the tropopause. However, detailed comparisons of simulated ISCCP cloud amounts and radar reflectivities with observations from ISCCP and the ground validation radar reveal important differences that are also reflected in the top-of-atmosphere radiative fluxes. Such discrepancies are strongest in the suppressed periods during KWAJEX, and these are explored in detail to reveal factors that contribute to model biases. The different microphysical and radiation parameterizations can induce substantial changes in the structure of the condensate and precipitate fields but do not qualitatively change the model's biases in the radiative fluxes, ISCCP cloud amounts or radar reflectivities.
A53A-05 1330h
Marine Stratus Cloud Objects Simulated by a Cloud-Resolving Model and Observed by Earth Observing System Satellite
Using a new approach proposed by Xu et al. (2005), 2162 stratus cloud objects are diagnosed using the Clouds and the Earth's Radiant Energy System (CERES) Single Scanner Footprint (SSF) product on board the Tropical Rainfall Measuring Mission (TRMM) satellite during January-August 1998 at three regions of east Pacific: north subtropical, east equatorial, and south subtropical. A cloud object is identified as a contiguous region of the Earth composed of individual satellite footprints within a single dominant cloud-system type. To be identified as a stratus cloud object, the cloud top height must be less than 3 km and cloud fraction is 0.99-1.00. The probability density functions (PDFs) of several observed and retrieved fields from the CERES SSF product are found to be different among the three regions. Two versions of a cloud-resolving model (CRM), which includes either a two-moment or a one-moment microphysics scheme, are used to simulate a number of the observed marine stratus cloud objects. Number concentrations of cloud droplets and rain drops, in addition to their mixing ratios, are predicted by the CRM version with the two-moment microphysics scheme while they are not predicted by the CRM version with the one-moment microphysics scheme. ECMWF meteorological fields matched with the instantaneous satellite cloud-object data are used to drive the CRM, i.e. to provide initial atmospheric state, large-scale total advective tendencies of temperature and moisture, large-scale horizontal wind speed, and sea surface temperature for the CRM simulations. Detailed analyses of the satellite cloud object data and the CRM simulations will be performed (1) to evaluate the CRM, with focus on how the microphysics schemes influence the simulated cloud properties and cloud radiative forcing, and (2) to understand the physical mechanisms for the differences among the three regions of east Pacific. A bootstrapping technique will be used to identify the statistical differences between the simulated and observed histograms.
A53A-06 1330h
A Cloud Resolving Simulation of a Polar Low Over the Labrador Sea
Polar lows, high latitude mesoscale marine cyclones, remain one of the most enigmatic of meteorological phenomena. They can be often observed on satellite imagery as spiral cloud systems whose striking organization belies the significant threat they represent to maritime activity as a result of the hurricane force associated with them. Their small horizontal scale, often less than 500km, short life time, typically less than 24 hours, and their tendency to form in data sparse regions make them a challenge to forecast. Polar lows are often associated with significant fluxes of heat and moisture between the atmosphere and ocean that act to modify both fluids. In the atmosphere, the fluxes act to warm and moisten the boundary layer resulting in the formation of 2D roll and 3D cellular convection. In the ocean, fluxes act to densify the surface waters and may contribute to the preconditioning phase of deep ocean convection. The presence of sea ice often introduces a spatial heterogeneity into the air-sea flux fields, In this talk, we will present a numerical simulation of a polar low that formed over the Labrador Sea on February 8 1997 during the Labrador Sea Deep Ocean Convection Experiment. A flight on that date with an instrumented aircraft collected data on the cloud-scale structure of the polar low and its associated air-sea interaction. The simulation was performed with a cloud resolving mesoscale forecast model in a domain with a horizontal scale of 500 km by 400 km at a horizontal resolution of 500m. The high spatial resolution and large domain allowed for an explicit representation of both the cloud-scale and larger-scale circulations associated with the polar low. In addition, the model included an explicit representation of the heterogeneity associated with the Labrador Sea's marginal ice zone. A comparison with satellite and in-situ observations indicate that the simulation is able to capture many of the cloud-scale and large-scale features of this polar low. In particular, the simulated cloud field bears a close resemblance to that associated with the low.
A53A-07 1330h
The Influence of the Electric Field on Thunderstorm Microphysical Development Simulated with an Explicit Microphysics Model
Electric fields influence the microphysics of aerosol-cloud interactions. Hence, nucleation of ice is sensitive to the charge on nuclei. Furthermore, there is an increase in the collision efficiency when charged aerosol particles collide with droplets ('electroscavenging'), and rates of contact ice nucleation are enhanced by the charge on aerosol particles (Tinsley et al. 2000, Tripathi and Harrison, 2002). In addition, electric fields (EF) affect the collisional growth rate of hydrometeors and their fall velocity. The aim here is to assess how the collection efficiency for the coagulation of hydrometeors may be modified by a typical EF in a thunderstorm. Particular focus is given to effects on the generation of anvil ice particles. This is done by imposing a realistic EF in the control simulation with an Explicit Microphysics Model (EMM) of the storm, observed on 18th July 2002 near Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE), as described by Phillips et al. (2005). An additional aim is to analyze how updraft speed (w) and environmental CCN concentration may affect the charge separation process. The warm rain process is intensified and there is a 30-40% reduction in the anvil ice concentration when an evolving height-dependent EF, typical of continental electrified thunderstorms, is prescribed and applied to the collection efficiencies for coagulation processes in the model. The electric dependence of the collision efficiency for drop-drop collisions is the cause. There is a 150% increase in the broad peak of average mixing ratio of rain near the freezing level (see Figure 1). This boosts the mixing ratio of precipitation-sized ice in the lower half of the mixed phase region, changing the number of charging collisions and depleting the supercooled cloudwater. Primarily because of the high sensitivity of the Hallett-Mossop (H-M) process of ice particle multiplication with respect to changes in the warm rain process, the total charge separated is reduced by about an order of magnitude when these electric fields are applied to coagulation processes. This suggests that an electrical-microphysical feedback may exist between the electrification, warm rain and H-M processes in continental storms that are similar to this particular model cloud. Furthermore, the total charge separated is reduced by an order of magnitude, when w is reduced by 40%, at each EF strength assumed for coagulation. This is because graupel particles are smaller and H-M splinters are fewer. Some aspects of the land-ocean contrast in lightning occurrence are also discussed. Phillips, V.T.J., S. Sherwood, C. Andronache et al. (2005). Q. J. R. Met. Soc. In press. Tinsley, B. A., R. P. Rohrbaugh, M. Hei, and K. V. Beard, (2000). J. Atmos. Sci., 57, 2118-2134. Tripathi S.N. and Harrison R.G., (2002). Atmos. Res., 62, 57-70.
A53A-08 1330h
Evaluating Microphysics in Cloud-Resolving Models using TRMM and Ground-based Precipitation Radar Observations
Global satellite datasets such as those produced by ISCCP, ERBE, and CERES provide strong observational constraints on cloud radiative properties. Such observations have been widely used for model evaluation, tuning, and improvement. Cloud radiative properties depend primarily on small, non-precipitating cloud droplets and ice crystals, yet the dynamical, microphysical and radiative processes which produce these small particles often involve large, precipitating hydrometeors. There now exists a global dataset of tropical cloud system precipitation feature (PF) properties, collected by TRMM and produced by Steve Nesbitt, that provides additional observational constraints on cloud system properties. We are using the TRMM PF dataset to evaluate the precipitation microphysics of two simulations of deep, precipitating, convective cloud systems: one is a 29-day summertime, continental case (ARM Summer 1997 SCM IOP, at the Southern Great Plains site); the second is a tropical maritime case: the Kwajalein MCS of 11-12 August 1999 (part of a 52-day simulation). Both simulations employed the same bulk, three-ice category microphysical parameterization (Krueger et al. 1995). The ARM simulation was executed using the UCLA/Utah 2D CRM, while the KWAJEX simulation was produced using the 3D CSU CRM (SAM). The KWAJEX simulation described above is compared with both the actual radar data and the TRMM statistics. For the Kwajalein MCS of 11 to 12 August 1999, there are research radar data available for the lifetime of the system. This particular MCS was large in size and rained heavily, but it was weak to average in measures of convective intensity, against the 5-year TRMM sample of 108. For the Kwajalein MCS simulation, the 20 dBZ contour is at 15.7 km and the 40 dBZ contour at 14.5 km! Of all 108 MCSs observed by TRMM, the highest value for the 40 dBZ contour is 8 km. Clearly, the high reflectivity cores are off scale compared with observed cloud systems in this area. A similar conclusion can be reached by comparing the simulated microwave brightness temperatures with observed brightness temperatures at 85 GHz and 37 GHz. In each case, the simulations are more extreme than all observed MCSs in the region over the 5 year period. The situation is similar but less egregious for the southern Great Plains simulation. Inspection of the cloud microphysics output files reveals the source of the discrepancy between simulation and observations in the upper troposphere. The simulations have very large graupel concentrations between about 5-10 km, as high as 10 g/kg graupel mixing ratio. This guarantees that there are very high radar reflectivities extending into the upper troposphere, and unrealistically low microwave brightness temperatures. We also performed a set of short (6-h) numerical simulations of the life cycle of a single convection cell to examine the sensitivity of the simulated graupel fields to the intercept parameter and the density of the graupel. The control case used the same values as the ARM and KWAJEX simulations. Reducing the intercept parameter by a factor of 100 reduced the maximum graupel mixing ratios but increased the maximum dBZ values. This suggests that the discrepencies between the simulations and the observations must involve the graupel growth rates.
A53A-09 1330h
The Simulation of a Hailstorm Using a Multi-Moment Bulk Microphysics Scheme
A multi-moment bulk microphysics scheme has been designed and interfaced with the Canadian MC2 mesoscale model and applied to conduct high-resolution simulations of a hailstorm. Comparisons are made between the control simulation, which used the full three-moment version of the new scheme, and sensitivity runs with various one-moment and two-moment versions. We examine how the different approaches affected the simulated storm structure, hail sizes, and quantity and phase of the precipitation at the surface. It will be shown that there is a fundamental improvement in the overall simulation skill for multi-moment over one-moment schemes. Further, the versions which allow the relative dispersion parameter to vary, either prognostically or diagnostically, are able to control the problems associated with excessive size-sorting due to differential sedimentation that are inherent in standard fixed-dispersion two-moment schemes.
A53A-10 1330h
Toward the Parameterization of Inhomogeneous Mixing in Cloud Resolving Models: Results From a PDF Study
Currently, the accurate prediction of cloud droplet number concentration in cloud resolving, numerical weather prediction and climate models is a formidable challenge. The process of inhomogeneous mixing, in which droplets evaporate completely in centimeter-scale filaments of sub-saturated air during turbulent entrainment [Baker et al., QJRMS, 1980], is unresolved at even cloud-resolving scales. Despite the large body of observational evidence in support of the inhomogeneous mixing process affecting cloud droplet number [most recently, Brenguier et al., JAS, 2000], it is poorly understood and has yet to be parameterized and incorporated into a numerical model. In this talk, we investigate the inhomogeneous mixing process using a new approach based on simulations of the probability density function (PDF) of relative humidity during turbulent mixing. PDF methods offer a key advantage over Eulerian (spatial) models of cloud mixing and evaporation: the low probability (cm-scale) filaments of entrained air are explicitly resolved (in probability space) during the mixing event even though their spatial shape, size and location remain unknown. Our PDF approach reveals the following features of the inhomogeneous mixing process during the isobaric turbulent mixing of two parcels of clear and cloudy air: (1) The degree of total droplet evaporation depends linearly on the mixing fractions of clear and cloudy air and logarithmically on Damköhler number (Da)---the ratio of turbulent to evaporative time-scales. (2) Our simulations predict that the PDF of Lagrangian (time-integrated) supersaturation (S) goes as S-1 at high Da. This behavior results from a Gaussian mixing closure and requires observational validation. (3) Our PDF approach can be used to parameterize inhomogeneous mixing in cloud resolving models (via look-up tables) if an additional model that predicts subgrid cloud fraction is devised.