Showing papers on "Microphysics published in 1998"

Rainfall Microphysics and Radar Properties: Analysis Methods for Drop Size Spectra

Abstract: Analyses are performed of experimental drop size spectra to explore the relationships among integral parameters for rain. The data used in this work were acquired with an airborne optical 2D precipitation probe in TOGA COARE during a 4-month period in 1992–93. It is assumed that the experimental size spectra can be described by a gamma drop size distribution (DSD) of the form N(D) = N0Dμ exp(−ΛD) involving three parameters (N0, μ, Λ), which are determined using a new method of truncated moments. The method allows for truncation of the DSD at the large-diameter end of the spectrum due in part to instrumental effects and also in part to the trajectory of the aircraft through a rain streamer that has been sorted by wind shear. An effect analogous to truncation can occur at the small-diameter end of the size spectrum due to evaporation. However, truncation of the spectrum at the small-diameter end is not considered in this work. It is found that spectra with small space and timescales display conside...

Toward Cloud Resolving Modeling of Large-Scale Tropical Circulations: A Simple Cloud Microphysics Parameterization

Abstract: This paper discusses cloud microphysical processes essential for the large-scale tropical circulations and the tropical climate, as well as the strategy to include them in large-scale models that resolve cloud dynamics. The emphasis is on the ice microphysics, which traditional cloud models consider in a fairly complex manner and where a simplified approach is desirable. An extension of the classical warm rain bulk parameterization is presented. The proposed scheme retains simplicity of the warm rain parameterization (e.g., only two classes of condensed water are considered) but introduces two important modifications for temperatures well below freezing:1) the saturation conditions are prescribed based on saturation with respect to ice, not water; and 2) growth characteristics and terminal velocities of precipitation particles are representative for ice particles, not raindrops. Numerical tests suggest that, despite its simplicity, the parameterization is able to capture essential aspects of the ...

Particle microphysics and chemistry in remotely observed mountain polar stratospheric clouds

Abstract: Polar stratospheric clouds (PSCs) at 22–26 km were observed over the Norwegian mountains by airborne lidar on January 15, 1995. Simulations using a mesoscale model reveal that they were caused by mountain-induced gravity waves. The clouds had a highly detailed filamentary structure with bands as thin as 100 m in the vertical, and moved insignificantly over 4 hours, suggesting them to be quasi-stationary. The aircraft flight path was parallel or close to parallel with the wind at cloud level. Such a quasi-Lagrangian observation, together with the presence of distinct aerosol layers, allows an air parcel trajectory through the cloud to be constructed and enables the lidar images to be simulated using a microphysical box model and light scattering calculations. The results yield detailed information about particle evolution in PSCs and suggest that water ice nucleated directly from liquid HNO3/H2SO4/H2O droplets as much as 4 K below the ice frost point. The observation of solid nitric acid hydrate particles downwind of the mountains shows that such mesoscale events can generate solid PSC particles that can persist on the synoptic scale. We also draw attention to the possible role of mesoscale PSCs in chlorine activation and subsequent ozone destruction.

Two-dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning

Abstract: By direct hydrodynamic simulation, using the piecewise parabolic method code PROMETHEUS, we study the properties of a convective oxygen-burning shell in a SN 1987A progenitor star (20 M☉) prior to collapse. The convection is too heterogeneous and dynamic to be well approximated by one-dimensional diffusion-like algorithms that have previously been used for this epoch. Qualitatively new phenomena are seen. The simulations are two-dimensional, with good resolution in radius and angle, and used a large (90°) slice centered at the equator. The microphysics and the initial model were carefully treated. Many of the qualitative features of previous multidimensional simulations of convection are seen, including large kinetic and acoustic energy fluxes, which are not accounted for by mixing length theory. Small but significant amounts of 12C are mixed nonuniformly into the oxygen-burning convection zone, resulting in hot spots of nuclear energy production that are more than an order of magnitude more energetic than the oxygen flame itself. Density perturbations (up to 8%) occur at the edges of the convective zone and are the result of gravity waves generated by interaction of penetrating flows into the stable region. Perturbations of temperature and Ye (or neutron excess η) at the base of the convective zone are of sufficient magnitude to create angular inhomogeneities in explosive nucleosynthesis products and need to be included in quantitative estimates of yields. Combined with the plumelike velocity structure arising from convection, the perturbations will contribute to the mixing of 56Ni throughout supernovae envelopes. Runs of different resolution and angular extent were performed to test the robustness of these simulations.

Use of cloud model microphysics for passive microwave-based precipitation retrieval : Significance of consistency between model and measurement manifolds

Abstract: Precipitation estimation from passive microwave radiometry based on physically based profile retrieval algorithms must be aided by a microphysical generator providing structure information on the lower portions of the cloud, consistent with the upper-cloud structures that are sensed. One of the sources for this information is mesoscale model simulations involving explicit or parameterized microphysics. Such microphysical information can be then associated to brightness temperature signatures by using radiative transfer models, forming what are referred to as cloud‐radiation databases. In this study cloud‐radiation databases from three different storm simulations involving two different mesoscale models run at cloud scales are developed and analyzed. Each database relates a set of microphysical profile realizations describing the space‐time properties of a given precipitating storm to multifrequency brightness temperatures associated to a measuring radiometer. In calculating the multifrequency signatures associated with the individual microphysical profiles over model space‐time, the authors form what are called brightness temperature model manifolds. Their dimensionality is determined by the number of frequencies carried by the measuring radiometer. By then forming an analogous measurement manifold based on the actual radiometer observations, the radiative consistency between the model representation of a rain cloud and the measured representation are compared. In the analysis, the authors explore how various microphysical, macrophysical, and environmental factors affect the nature of the model manifolds, and how these factors produce or mitigate mismatch between the measurement and model manifolds. Various methods are examined that can be used to eliminate such mismatch. The various cloud‐radiation databases are also used with a simplified profile retrieval algorithm to examine the sensitivity of the retrieved hydrometeor profiles and surface rainrates to the different microphysical, macrophysical, and environmental factors of the simulated storms. The results emphasize the need for physical retrieval algorithms to account for a number of these factors, thus preventing biased interpretation of the rain properties of precipitating storms, and minimizing rms uncertainties in the retrieved quantities.

The role of heterogeneous freezing nucleation in upper tropospheric clouds: Inferences from SUCCESS

Abstract: A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions.

Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part II: Microphysics, Vapor and Ice Mass Budgets, and Optical and Radiative Properties

Abstract: The 2D/3D cloud model complex with explicit microphysics and radiation described in Part I is used to simulate the development of a midlatitude cirrus cloud, including interaction with radiation. To account for the effects of the interaction of various scales of motion on cloud development, a synoptic-scale vertical velocity field is superimposed on the mesoscale velocity field generated by the model, mimicking the effects of an upper-level shortwave trough. The main results under the conditions simulated here are the following. Cirrus cloud growth is much slower than assumed previously, because the process of vapor deposition to ice crystals is far from instantaneous: the crystal phase relaxation time (i.e., the characteristic time of vapor absorption by crystals) takes 0.5–2.0 h. Even after 1 h of cloud development, supersaturation with respect to ice can remain 5%–10%, while the condensed ice is only 40%–60% of the amount that would be realized assuming that all excess vapor is transformed int...

Finescale Structure and Microphysics of Coastal Stratus

Abstract: Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases. Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2‐1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

Parameterizations of the microphysical and shortwave radiative properties of boundary layer stratus from ground-based measurements

Abstract: In earlier work a δ2-stream radiative transfer model was used in conjunction with ground-based measurements to retrieve the rnicrophysical and shortwave radiative properties of boundary layer stratus. The effective radii output by the retrieval scheme for optically thick clouds have now been parameterized as a function of the cloud liquid water path, the transmission ratio, and the cosine of the solar zenith angle. The parameterization enables estimation of stratus cloud microphysical properties using ground-based measurements that are readily available at a number of locations. To validate the retrieved and parameterized cloud microphysics, ∼5 hours of data were analyzed and compared to collocated in situ measurements made by a Forward Scattering Spectrometer Probe. On average, the retrieved values of the cloud droplet effective radius and the cloud droplet number concentration differed from the corresponding aircraft measurements by 7% and 27%, respectively, while the parameterized values differed from the aircraft measurements by 15% and 32%, respectively. Averaging all of the data to 30-min resolution significantly reduced the differences between the aircraft data and the retrieved and parameterized results, suggesting that both the aircraft and the ground-based data are capable of characterizing the cloud microphysics over this temporal scale. The parameterizations of the stratus shortwave radiative properties were generally within 5% of Slingo's four-band, model-derived parameterization when absorption above cloud top was incorporated into the Slingo parameterization.

The formation and evolution of aerosols in stratospheric aircraft plumes: Numerical simulations and comparisons with observations

Abstract: The formation and evolution of aerosols in jet aircraft plumes at high altitude are simulated using a detailed aerosol microphysics model that explicitly resolves particle size and composition. Two approaches are used to simulate the microphysics: a standard, or “classical,” approach in which new particle formation initially occurs via homogeneous binary nucleation, followed by condensation and coagulation; and a “kinetic” approach in which the entire course of particle evolution is calculated as a unified collisional mechanism. In the latter approach chemiions generated in the engine combustors can affect molecular cluster growth and ultrafine particle aggregation. Simulations with both approaches reveal that large numbers of volatile ultrafine sulfuric acid particles are generated in the near field behind the engines. The concentrations and subsequent evolution of these “volatile” particles are sensitive to the initial sulfuric acid vapor concentration, the plume dilution rate, and microphysical factors, especially the chemiion abundance, which is brought out through a sensitivity analysis. By contrasting predictions against available field data, it is demonstrated that the kinetic model provides a more realistic representation of particle formation and growth and hence their observable properties than the classical model. Moreover, the effects of chemiions during the early evolution of the plume are found to be crucial to the evolution of the largest volatile aerosols, which may play a role as cloud condensation nuclei. Major sources of uncertainty in the plume aerosol models are noted.

Cirrus Cloud Simulation Using Explicit Microphysics and Radiation. Part I: Model Description

Abstract: A mesoscale 2D/3D cloud model complex with explicit account for the water and ice cloud microphysics and radiative processes is described. The model has several versions suitable for the simulation of various cloud types, including, of particular concern in the current version, cirrus clouds. Model computations are based on the two kinetic equations for droplet and crystal size distribution functions, with division of droplet and crystal size spectra into 30 bins from 1 μm to 3.5 mm, and with account for the various mechanisms of cloud condensation and ice nuclei activation, condensation–deposition, and coalescence–accretion growth. These equations are solved along with supersaturation and radiative transfer equations for longwave and solar radiation. Simple yet accurate analytical expressions are presented for the scattering and absorption coefficients, and the single-scattering albedo. This allows detailed calculations of the optical and radiative characteristics of clouds (i.e., fluxes, diverg...

Interaction of Aerosol Particles and Clouds

Abstract: With the spectral scavenging and microphysics model DESCAM coupled to a 2D dynamic framework the transport and scavenging of aerosol particles by a medium-sized convective cloud are investigated. For typical marine conditions the author has found a depletion of the marine boundary layer of about 70%. Also, about 70% of the vented up aerosol particle mass entered cloud drops due to nucleation. For these simulations the author found an increase in relative humidity, as well as number concentration of Aitken and large and giant particles near cloud top. The increase in relative humidity and large and giant particle concentration was coupled to cloud outflow regions, while the increase in Aitken particles can be attributed to lifting of free tropospheric air due to cloud-top rising. Sensitivity tests varying the number of free tropospheric sulfate particles changed the number of cloud drops but had little effect on precipitation formation and evolution.

Contrail formation and impacts on aerosol properties in aircraft plumes: Effects of fuel sulfur content

Abstract: The formation and evolution of fine particles and ice contrails in an aircraft exhaust plume containing varying amounts of fuel sulfur have been simulated using an advanced aerosol microphysics model. The “core” sulfate and soot particles are tracked during the contrail formation and dissipation phases. When ion electrostatic effects are incorporated into the microphysics, sulfuric acid vapor emitted by high-sulfur-content fuels generates water-soluble particles that are large enough to be activated into contrails, improving the agreement between simulations and measurements. Our results also suggest that ice crystals formed in contrails efficiently scavenge vapors and particles, creating a sulfate aerosol accumulation mode that may contribute to cloud CCN/IN. The size distributions of aerosols produced both in the presence and absence of contrails agree reasonable well with the two characteristic types observed in the plumes of commercial aircraft.

Statistical Geometry and Applications to Microphysics and Cosmology

Abstract: Preface 1 Introduction and Historical Survey 2 Statistical Behavior of Microparticles 3 Space-Time Fluctuation and Random Potential 4 Non-Local Fields 5 Astro-Particle Physics 6 Einstein Equation for Gravitational Field 7 Origin of Stochasticity 8 Possible Implications 9 Epilogue: New Dialogue of Man with Nature Bibliography Index

Statistical Characteristics of a Real-Time Precipitation Forecasting Model

Abstract: At Colorado State University the Regional Atmospheric Modeling System (RAMS) has been used to produce real-time forecasts of precipitation for the Colorado mountain region since 1991. Originally a so-called dumpbucket scheme was used to generate precipitation, but starting in the fall of 1995 real-time forecasts used the bulk microphysics scheme available with RAMS. For the month of April 1995, a series of 24-h accumulated precipitation forecasts for the month were generated with both the dump-bucket and microphysics versions of the forecast model. Both sets of output were compared to a set of 167 community-based station reports and another set of 32 snow telemetry (SNOTEL) automatic pillow-sensor stations. The addition of microphysics improved the forecasting of the areal extent and maximum amount of precipitation, especially when compared to the SNOTEL observational set, which is found at locations more representative of the model topography. Climatological station precipitation forecasts were improved on the average by correcting for the difference between a station’s actual elevation and the cell-averaged topography used by the model. The model had more problems with the precise timing and geographical location of the precipitation features, probably due in part to the influence of other model physics, the failure of the model to resolve adequately wintertime convection events, and inadequate initializations.

The effect of cloud-processing of aerosol particles on clouds and radiation

Abstract: A detailed spectral microphysics and scavenging model coupled to a dynamic framework describing a medium-sized convective cloud has been used to simulate the evolution of different clouds forming precipitation sized drops in a marine air mass. The resulting drop spectra entered a radiation code to yield the up- and downdwelling radiative fluxes, the cloud optical depth and the cloud albedo. If we start from the scenario that in a perturbed marine environment the number of small aerosol particles has doubled, this can increase the albedo of a cloud forming in this air mass of about 5% with respect to clouds forming in unperturbed conditions. This capacity to increase the cloud albedo, however, is not persistent. The cloud itself changes the particle spectrum. The smallest aerosol particles are reduced by impaction scavenging. The particles between 0.01 µm and 0.1 µm are depleted due to the fact that they serve as CCN and grow through in-cloud processes. Here, our studies have shown, however, that growth due to absorption and oxidation of gases (e.g., SO 2 ) plays a minor role and that collision and coalescence of drops is the dominant growth mechanism in a region with low gas concentrations like the remote oceans. As a result, an aerosol particle spectrum which has gained small particles due to an enhanced production of new particles seems to relax back to a spectrum similar to the previous undisturbed after cycling through some repeated cloud events. DOI: 10.1034/j.1600-0889.1998.t01-4-00006.x

Precision measurement of the lifetime of the 1s2s 3s1 metastable level in heliumlike o6

Abstract: The lifetime of the 1s2s S1 level of the He-like O 61 ion has been measured using the Electron Beam Ion Trap in the magnetic trapping mode. A value of 956 24 15 ms is found, which corresponds to a radiative transition rate of 104625 14 s for the magnetic dipole transition to the 1 s S0 ground state. This value is in excellent agreement with recent theoretical predictions and distinguishes among different treatments of negative energy states and correlation in multiconfiguration Dirac-Fock calculations. @S1050-2947 ~98!05307-4#

Model interpretation of Jovian lightning activity and the Galileo Probe results

Abstract: An axisymmetric numerical cloud model with detailed microphysics is used to evaluate the development of water clouds under various Jovian conditions. A formulation of the noninductive ice-ice charge separation mechanism is used to calculate the electrical structure of the clouds and to identify the charged regions at the altitudes of the Galileo entry probe trajectory. The results show that for midlatitude conditions, with reasonable assumptions, the water clouds in the deep Jovian troposphere have an electrical dipole structure, with a large positive charge center located at the 3 bar pressure level and a lower negative charge center at the 4.5 bar level. This structure is modified as the cloud matures and the breakdown electric field is exceeded and lightning occurs. Our model predicts that the weak convection, which occurs at the stable equatorial region, is capable of producing water clouds with 10 m s -1 updrafts and water and ice content of 5 g kg -1 . Charge separation and lightning generation in these clouds is less intensive and lightning frequency is lower than at midlatitudes. For a low oxygen abundance of 0.2 times solar, we show that ice clouds form at temperatures where charge separation is weak, and no lightning can occur. The results suggest a very low probability that there was lightning activity near the path of the descending probe.

Derivation and Applications of Near-Infrared Cloud Reflectances from GOES-8 and GOES-9

Abstract: Recent deployments of the National Oceanic and Atmospheric Administration (NOAA) Geostationary Operational Environmental Satellites (GOES-8 and -9) include full-time 3.9-μm imaging capabilities. This shortwave (near infrared) channel has been available at 3.7 μm on the Advanced Very High Resolution Radiometer (AVHRR) instrument aboard the NOAA polar-orbiting satellite systems. In this spectral region, daytime satellite-observed radiances include contributions from both the reflected solar radiation and the emitted thermal emission. In particular, typical stratus and fog clouds posess near-infrared emissivities less than unity, which requires special processing to account for the angular dependence of the solar reflection. In this paper, a side-by-side comparison of time-coincident GOES- and AVHRR-derived near-infrared cloud reflectance is carried out in order to demonstrate the capability of GOES-8 and -9 in both identifying and characterizing the microphysics of stratus and fog clouds during the...

The venusian Y-shaped cloud pattern based on an aerosol-transport model

Abstract: Venusian Y-shaped clouds have been observed by ultraviolet (UV) detectors. Recently, it has been demonstrated that the Y-shaped cloud pattern is maintained by the dynamical combination of an equatorial 4-day wave and a Rossby wave. In the model of the Y-shaped cloud, however, material transport was not considered. Present numerical simulations, including not only dynamical transport, but also the essential microphysics of aerosols, are conducted together with the chemical reactions of the aerosol precursor gas, SO2. Results indicate that aerosols are accumulated in the polar regions due to poleward transport. As a result, the scattering coefficient becomes higher with increasing latitude. At the low-latitude cloud-top heights, the dark (bright) region is formed by the small (large) aerosol concentration associated with planetary-scale waves since the zonal-mean aerosol concentration is relatively small. On the other hand, the longitudinal contrast of the scattering coefficient becomes very small ...

Retrieval of contrail microphysical properties during SUCCESS by the split-window method

Abstract: Estimates of cloud particle size and infrared optical depth were inferred from multi-spectral observations of contrails and cirrus during the SUbsonic aircraft: Contrails and Cloud Effects Special Study (SUCCESS) experiment on 20 April 1996. The particles in a newly formed contrail were generally smaller than those in an older contrail, while the microphysics of the older contrail resembled the surrounding cirrus. Split-window retrievals of contrails were sensitive to changes in surface temperature and surface emissivity.

Numerical Sensitivity Study of Rainband Precipitation and Evolution

Abstract: Using a deep, two-dimensional rainband model, the authors examined the effect of different microphysical treatments on rain evolution and precipitation processes in an otherwise fixed environment. Not only rain evolution but cloud cell activity and heat release patterns also varied greatly depending on the microphysical processes used to drive the model. When maritime microphysics were used, in the early stage, models produced intense rainfall peaks at both the front and the rear of the emerging system. Then, between these two first peaks, new rain peaks successively appeared, forming a broad rainband. Heat was released rather uniformly throughout the cloud depth above the freezing level. Strong winds that blew near freezing level were a common feature. Updraft was enhanced during the intermittent development of new cloud cells. In contrast, when continental microphysics were used, in the early stage, models produced heavy rainfall at the rear of the emerging system and only weak rainfall at the ...