Showing papers on "Microphysics published in 1999"

Advances in understanding clouds from ISCCP

Abstract: This progress report on the International Satellite Cloud Climatology Project (ISCCP) describes changes made to produce new cloud data products (D data), examines the evidence that these changes are improvements over the previous version (C data), summarizes some results, and discusses plans for the ISCCP through 2005. By late 1999 all datasets will be available for the period from July 1983 through December 1997. The most significant changes in the new D-series cloud datasets are 1) revised radiance calibrations to remove spurious changes in the long-term record, 2) increased cirrus detection sensitivity over land, 3) increased low-level cloud detection sensitivity in polar regions, 4) reduced biases in cirrus cloud properties using an ice crystal microphysics model in place of a liquid droplet microphysics model, and 5) increased detail about the variations of cloud properties. The ISCCP calibrations are now the most complete and self-consistent set of calibrations available for all the weather...

Validation and Sensitivities of Frontal Clouds Simulated by the ECMWF Model

Abstract: Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front. Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ic...

Spectral Models of Advection‐dominated Accretion Flows with Winds

Abstract: We calculate spectral models of advection-dominated accretion flows, taking into account the possibility that significant mass may be lost to an outflow/wind. We apply the models to the soft X-ray transient V404 Cyg in quiescence, the Galactic center source Sgr A*, and the nucleus of NGC 4649. We show that there are qualitative degeneracies between the mass-loss rate in the wind and parameters characterizing the microphysics of the accretion flow; of particular importance is δ, the fraction of the turbulent energy which heats the electrons. For small δ, current observations suggest that at most 90% of the mass originating at large radii is lost to a wind, so that at least ~10% reaches the central object. For large δ ~ 0.3, however, models with significantly more mass loss are in agreement with the observations. We conclude by highlighting future observations which may clarify the importance of mass loss in sub-Eddington accretion flows.

Nonhydrostatic, Mesobeta-Scale Model Simulations of Cloud Ceiling and Visibility for an East Coast Winter Precipitation Event

Abstract: Experiments are described that provide an example of the baseline skill level for the numerical prediction of cloud ceiling and visibility, where application to aviation-system safety and efficiency is emphasized. Model simulations of a light, mixed-phase, East Coast precipitation event are employed to assess ceiling and visibility predictive skill, and its sensitivity to the use of data assimilation and the use of simple versus complex microphysics schemes. To obtain ceiling and visibility from the model-simulated, state-of-the-atmosphere variables, a translation algorithm was developed based on empirical and theoretical relationships between hydrometeor characteristics and light extinction. The model-simulated ceilings were generally excessively high; however, the visibility simulations were reasonably accurate and comparable to the existing operational terminal forecasts. The benefit of data assimilation for such very short-range forecasts was demonstrated, as was the desirability of employing a reasonably sophisticated microphysics scheme.

Modeling of the melting layer. Part I : Dynamics and microphysics

Abstract: To obtain the full description of the dynamical and microphysical finescale structures required for the computation of the radar-derived brightband parameters, a numerical model has been developed. A bulk microphysics module was introduced into a nonhydrostatic, fully compressible dynamic framework. A microphysical parameterization scheme, with five water categories (vapor, cloud water, snow, melting snow, and rain), describes the interactions related to the evolution of the melting layer (melting and diffusional exchanges of mass of each hydrometeor category). Dynamic, thermodynamic, and microphysical processes are fully coupled. The main characteristics of the bulk parameterization scheme for melting of snow are the following: 1) wet snow is described by its water content and by an additional prognostic variable, namely, the diameter of the smallest snowflake not yet completely melted; 2) the fall velocity of the melting snowflakes is based on the laboratory observations; and 3) a size-dependen...

A Large Eddy Simulation Model with Explicit Microphysics: Validation against Aircraft Observations of a Stratocumulus-Topped Boundary Layer

Abstract: A new dynamical framework for the Cooperative Institute for Mesoscale Meteorological Studies large eddy simulation model (CIMMS LES) with an explicit microphysics scheme is developed. It is shown that simulation results are very sensitive to the drop spectrum remapping technique used in condensation calculations; however, the results are almost insensitive to doubling of the spectrum resolution used in the CIMMS LES model. It is also shown that the drop coagulation procedure conserves the liquid water content as long as the predominant radius of the drop size spectrum, defined as the cube root of the ratio of the drop radar reflectivity to the liquid water content, is below a threshold value of 250 μm. Finally, it is demonstrated that for typical maritime conditions this threshold radius is exceeded only in 0.1% of all cloudy points. Realism of the model is evaluated by a direct comparison of its predictions with the aircraft observations of a stratocumulus-topped boundary layer. The first simula...

A GCSS boundary-layer cloud model intercomparison study of the first ASTEX Lagrangian experiment

Abstract: Three single-column models (all with an explicit liquid water budget and compara- tively high vertical resolution) and three two-dimensional eddy-resolving models (including one with bin-resolved microphysics) are compared with observations from the first ASTEX Lagrangian experiment. This intercomparison was a part of the second GCSS boundary-layer cloud modelling workshop in August 1995. In the air column tracked during the first ASTEX Lagrangian experiment, a shallow subtropical drizzling stratocumulus-capped marine boundary layer deepens after two days into a cumulus capped boundary layer with patchy stratocumulus. The models are forced with time varying boundary con- ditions at the sea-surface and the capping inversion to simulate the changing environment of the air column. The models all predict the observed deepening and decoupling of the boundary layer quite well, with cumulus cloud evolution and thinning of the overlying stratocumulus. Thus these models all appear capable of predicting transitions between cloud and boundary-layer types with some skill. The models also produce realistic drizzle rates, but there are substantial quantitative differences in the cloud cover and liquid water path between models. The differences between the eddy-resolving model results are nearly as large as between the single column model results. The eddy resolving models give a more detailed picture of the boundary-layer evolution than the single-column models, but are still sensitive to the choice of microphysical and radiative parameterizations, sub-grid-scale turbulence models, and probably model resolution and dimensionality. One important example of the differences seen in these parameterizations is the absorption of solar radiation in a specified cloud layer, which varied by a factor of four between the model radiation parameterizations.

Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS). Part I: Design and Evaluation with GATE Phase III Data

Abstract: A prognostic cloud scheme named McRAS (Microphysics of Clouds with Relaxed Arakawa–Schubert Scheme) has been designed and developed with the aim of improving moist processes, microphysics of clouds, and cloud–radiation interactions in GCMs. McRAS distinguishes three types of clouds: convective, stratiform, and boundary layer. The convective clouds transform and merge into stratiform clouds on an hourly timescale, while the boundary layer clouds merge into the stratiform clouds instantly. The cloud condensate converts into precipitation following the autoconversion equations of Sundqvist that contain a parametric adaptation for the Bergeron–Findeisen process of ice crystal growth and collection of cloud condensate by precipitation. All clouds convect, advect, as well as diffuse both horizontally and vertically with a fully interactive cloud microphysics throughout the life cycle of the cloud, while the optical properties of clouds are derived from the statistical distribution of hydrometeors and i...

Long-Term Behavior of Cloud Systems in TOGA COARE and Their Interactions with Radiative and Surface Processes. Part II: Effects of Ice Microphysics on Cloud–Radiation Interaction

Abstract: A two-dimensional cloud-resolving model with a large domain is integrated for 39 days during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) to study the effects of ice phase processes on cloud properties and cloud radiative properties. The ice microphysical parameterization scheme is modified based on microphysical measurements from the Central Equatorial Pacific Experiment. A nonlocal boundary layer diffusion scheme is included to improve the simulation of the surface heat fluxes. The modified ice scheme produces fewer ice clouds during the 39-day simulation. The cloud radiative properties show significant improvement and compare well with various observations. Both the 39-day mean value (202 W m−2) and month-long evolution of outgoing longwave radiative flux from the model are comparable with satellite observations. The 39-day mean surface shortwave cloud forcing is −110 W m−2, consistent with other estimates obtained for TOGA COARE. The 39-day me...

The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model

Abstract: Summary Global Circulation Models (GCMs) have generally treated only the radiative impacts of vertically varying cloud fraction by use of a cloud overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a sub-grid scale precipitation model which resolves the vertical variation of cloud fraction. This sub-grid model subdivides the grid boxes into homogeneous columns which are either completely clear or cloudy. By comparing the columnaveraged microphysical quantities from the sub-grid scale precipitation model to the parametrization in the ECMWF model, the ability of the ECMWF model to account for the sub-grid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates precipitation evaporation in the tropical mid-troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box. In addition to assessing the ability of the ECMWF model to parametrize the sub-grid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrization in GCMs, more attention should be paid to the impact of vertical cloud fraction variations on the parametrized microphysics.

Cloud Resolving Modeling of Tropical Cloud Systems during Phase III of GATE. Part III: Effects of Cloud Microphysics

Abstract: Large-scale conditions during the 7-day period of Phase III of the Global Atmospheric Research Program Atlantic Tropical Experiment are used to study effects of cloud microphysics on the convecting tropical atmosphere. Two-dimensional numerical experiments evaluate the effects of extreme changes to the cloud microphysics in the cloud resolving model. The main conclusions are the following. (a) Extreme changes in cloud microphysics affect the temperature and moisture profiles in a way that approximately retains relative humidity profiles in all experiments. (b) With prescribed radiative tendencies, effects of cloud microphysics on surface processes are paramount. Extreme changes in warm rain microphysics indirectly affect the temperature and moisture profiles by modifying surface sensible and latent heat fluxes. For instance, smaller raindrops, and to a lesser degree slower conversion of cloud water into rain, result in enhanced updraft and downdraft cloud mass fluxes, a colder and drier boundary ...

Three-Dimensional Cloud-System Modeling of GATE Convection

Abstract: Deep convection and its associated mesoscale circulations are modeled using a three-dimensional elastic model with bulk microphysics and interactive radiation for a composite easterly wave from the Global Atmospheric Research Program Atlantic Tropical Experiment. The energy and moisture budgets, large-scale heat sources and moisture sinks, microphysics, and radiation are examined. The modeled cloud system undergoes a life cycle dominated by deep convection in its early stages, followed by an upper-tropospheric mesoscale circulation. The large-scale heat sources and moisture sinks associated with the convective system agree broadly with diagnoses from field observations. The modeled upper-tropospheric moisture exceeds observed values. Strong radiative cooling at the top of the mesoscale circulation can produce overturning there. Qualitative features of observed changes in large-scale convective available potential energy and convective inhibition are found in the model integrations, although quantitative magnitudes can differ, especially for convective inhibition. Radiation exerts a strong influence on the microphysical properties of the cloud system. The three-dimensional integrations exhibit considerably less sporadic temporal behavior than corresponding two-dimensional integrations. While the third dimension is less important over timescales longer than the duration of a phase of an easterly wave in the lower and middle troposphere, it enables stronger interactions between radiation and dynamics in the upper-tropospheric mesoscale circulation over a substantial fraction of the life cycle of the convective system.

Effect of environmental conditions on volcanic plume rise

Abstract: Sensitivity studies were performed with a complex nonhydrostatic volcano plume model that explicitly treats turbulence and microphysics. The impact of environmental conditions such as wind, temperature and humidity profiles was studied for standard observational data. To investigate the wind effects, a two-dimensional Cartesian formulation of the model was used, while for the temperature and humidity effects a cylindrical coordinate system had to be applied, since this treats the entrainment process more realistically. It was found that horizontal wind generally reduces the height of the ash plume. The gaseous part of the plume sometimes may rise higher than without wind owing to the more effective separation between gas and solid material. Besides reduced static stability, the absolute temperature and humidity also increase the plume height. All environmental impacts strongly depend on the strength of entrainment and thus on the quality of the prognostic turbulence.

On the maintenance of high tropical cirrus

Abstract: Observations reveal that cirrus clouds are ubiquitous in the tropics and therefore significantly impact the radiation budget in this region. Here numerical studies are conducted using a two-dimensional cloud-resolving model with explicit microphysics to study the processes that lead to the maintenance of tropical cirrus. It is found that despite significant differences in the cloud dynamics among the simulations performed, the model is unable to maintain a cloud against the processes of sedimentation and evaporation under the assumed environmental conditions. It is hypothesized that a source of large-scale upward motion must be present to maintain tropical cirrus.

Aviation-Produced Aerosols and Contrails

Abstract: Liquid and solid particles in the plumes of jet aircraft cruising in the upper troposphere and lower stratosphere lead to the formation of ice clouds (contrails), modify the microphysical properties of existing cirrus clouds, and provide sites for heterogeneous chemical reactions. Characterization of aviation-produced particles in terms of physico-chemical properties is an important step in assessing the global impact of aircraft emissions upon atmospheric chemistry and climate parameters. Chemistry and microphysics of the gas-aerosol system in aircraft plumes and its evolution in the atmosphere is a field of intense research. This paper reviews the current knowledge (mid-1998) and outlines possible atmospheric implications.

Microphysics of Clouds with the Relaxed Arakawa-Schubert Scheme (McRAS). Part II: Implementation and Performance in GEOS II GCM

Abstract: A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS) and the Simple Biosphere Model have been implemented in a version of the Goddard Earth Observing System (GEOS) II GCM at a 4° latitude × 5° longitude × 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialized with the European Centre for Medium-Range Weather Forecasts analysis of observations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vegetation properties (biomes and soils) were climatological, while the soil moisture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiative forcing (CRF) even though the cloudiness has some discernible systematic errors. The simulated intertropical convergence zone (ITCZ) has a realistic time mean structure and seasonal cycle. The sim...

Mesoscale temperature fluctuations induced by a spectrum of gravity waves: A comparison of parameterizations and their impact on stratospheric microphysics

Abstract: Power spectral densities (PSDs) of mesoscale fluctuations of temperature and rate of change of temperature (heating–cooling rate) due to a spectrum of stratospheric gravity waves are derived using canonical spectral forms based on observations and linear gravity wave theory. The parameterization developed here assumes a continuous distribution of horizontal wave phase speeds, as opposed to a previous spectral parameterization in which all waves were assigned stationary ground-based phase speeds. Significantly different heating–cooling rate PSDs result in each case. The differences are largest at small horizontal scales, where the continuous phase-speed parameterization yields heating–cooling rate PSDs that are several orders of magnitude smaller than in the stationary phase-speed parameterization. A simple Monte Carlo method is used to synthesize randomly phased temperature perturbation time series within tagged air parcels using either spectral parameterization. These time series are incorporate...

In situ observations of contrail microphysics and implications for their radiative impact

Abstract: In this study we present the microphysical characteristics of 21 jet contrail clouds sampled in situ and examine the possible effects of exhaust on natural cirrus and radiative effects of contrails. Microphysical samples were obtained with Particle Measuring Systems (PMS) 2D-C, 1D-C, and FSSP probes. About one half of the study contrails were generated by the sampling aircraft, a Cessna Citation, primarily at times of 3–15 min after generation; the source and age of the others is unknown. On average, the contrails contained particles of mean diameter of the order of 10 μm in concentrations exceeding 10,000 L−1. Contrails embedded in natural cirrus appeared to have little effect on the natural cloud microphysics. Anomalous diffraction theory was used to model radiative properties of sampled contrails. The contrail cirrus showed considerably more spectral variation in extinction and absorption efficiencies than natural cirrus because of the large numbers of small crystals in contrails. Embedded contrails also displayed greater emissivity and emission than natural cirrus and a greater spectral variation in transmission.

Microwave Properties of Frozen Precipitation around a North Atlantic Cyclone

Abstract: Microwave brightness temperatures emanating from a North Atlantic cyclone were measured by the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological Satellite Program satellite. As other investigators have found before, low 85.5-GHz brightness temperatures (215 ± 20 K) were observed from cumulonimbus clouds along the squall line; however, 85.5-GHz microwave brightness temperatures observed from the nimbostratus clouds north of the low center were significantly higher (255 ± 20 K). In situ measurements from aircraft during the Canadian Atlantic Storm Program II showed that heavy snowfall consisting of large tenuous aggregates existed in the nimbostratus clouds at the time of the SSM/I overpass. Distributions of snow, rain, liquid cloud water, and cloud ice mass were computed from a modified version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model. That model employed a mixed-phase ice microphysics (MPIM) scheme that only considered one type of frozen hydro...

Remote Sensing of Cirrus Radiative Parameters during EUCREX’94. Case Study of 17 April 1994. Part II: Microphysical Models

Abstract: In this paper, a quantitative analysis of in situ and radiative measurements concerning cirrus clouds is presented. These measurements were performed during the European Cloud and Radiative Experiment 1994 (EUCREX’94) as discussed in an earlier paper (Part I). The analyses are expressed in terms of cirrus microphysics structure. The complex microphysical structure of cirrus cloud is approximated by simple hexagonal monocrystalline particles (columns and plates) and by polycrystalline particles (randomized triadic Koch fractals of second generation) both arbitrarily oriented in space (3D). The authors have also considered hexagonal plates randomly oriented in horizontal planes with a tilted angle of 15° (2D). Radiative properties of cirrus cloud are analyzed, assuming that the cloud is composed of 3D ice crystals, by way of an adding–doubling code. For the hypothesis of 2D ice crystals, a modified successive order of scattering code has been used. The first order of scattering is calculated exactl...

Can a relaxation technique be used to validate clouds and sulphur species in a GCM

Abstract: The Hamburg version of the European Centre for Medium-Range Weather Forecasting (ECMWF) general circulation model, ECHAM4, developed for climate studies, is used to study the hydrological cycle and the atmospheric sulphur cycle. A technique, the Newtonian relaxation, is introduced to compare results from the model with measurements of cloud parameters and sulphur species concentrations. The model is forced by ECMWF analyses to simulate September 1993, and results are compared to observations on different spatial and temporal scales: monthly mean horizontal satellite data, daily mean averages at European and Canadian stations and aircraft soundings over the north Atlantic off the Canadian coast. The model is able to reproduce monthly mean horizontal distributions of liquid water path and precipitation. Daily mean precipitation fluxes and the temporal evolution of gaseous and particulate sulphur also agree quite well with observations. Based on this comparison with observations the performance of two different cloud physics parametrizations is discussed. These experiments indicate that variables like relative humidity, cloud cover and precipitation are controlled by the large-scale dynamics, whereas the cloud water content depends on the parametrization of the cloud microphysics. Coupling of the model-calculated sulphate mixing ratios to the cloud microphysics improves the model's sulphate distribution but slightly worsens the agreement between calculated and observed precipitation.

Atmospheric sulfur and deep convective clouds in tropical Pacific: A model study

Abstract: A high-resolution limited area nonhydrostatic model was used to simulate sulfate-cloud interactions during the convective activity in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, December 20–25, 1992. The model includes a new detailed sulfate-cloud microphysics scheme designed to estimate the effects of sulfate on cloud microphysics and radiative properties and the effects of deep convection on the transport and redistribution of aerosol. The data for SO2 and SO4(2−) species were taken from the Pacific Exploratory Mission West B observations during February-March 1994. Results show that a change in sulfate loading from the minimum to the maximum observed value scenarios (i.e., from about 0.01 to 1 μg m−3) causes a significant decrease of the effective radius of cloud droplets (changes up to 2 μm on average) and an increase of the diagnostic number concentration of cloud droplets (typical changes about 5–20 cm−3). The change in the average net shortwave (SW) radiation flux above the clouds was estimated to be on average −1.5 W m−2, with significant spatial and temporal variations. The horizontal average of the changes in the net SW radiation fluxes above clouds has a diurnal cycle, reaching typical values approximately −3 W m−2. The changes in the average net longwave radiation flux above the clouds were negligible, but they showed significant variations, typically between −10 W m−2 and 10 W m−2 near the surface. These variations were associated mainly with the changes in the distribution of cloud water, which showed typical relative changes of cloud water path of about 10–20%. Other notable changes induced by the increase of aerosol were the variations in air temperature of the order of 1°C. The case study presented here suggests that characteristics of convective clouds in tropical areas are sensitive to atmospheric sulfate loading, particularly during enhanced sulfate episodes.

Evolution of aircraft‐generated volatile particles in the far wake regime: Potential contributions to ambient CCN/IN

Abstract: We apply a detailed aerosol microphysics model to study the evolution of aerosols in the far wake of an aircraft plume. Constrained by in-situ measurements, our simulations reveal that the largest volatile particles—those most likely to contribute to the background abundance of condensation nuclei—are dominated by ion-mode aerosols formed on chemiions. By tracking the evolution of the aircraft-generated particles from the near wake to the far wake, we estimate the properties of the aircraft particles that are effectively injected into the upper troposphere. The entrainment of ambient vapors and aerosols into the aging plume is taken into account, which significantly affects the outcome. The model calculations reveal that a substantial number of the larger ion-mode volatile particles may survive long enough to act as potential cloud nuclei, thus perturbing the background CCN population. This perturbation is significant during the winter season, and in other locales with low background aerosol concentrations, but is likely to be negligible during the summer season or at locations with heavy aerosol loading. The implications and uncertainties of the results are discussed.