Showing papers on "Microphysics published in 2014"

Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds

Abstract: Water ice clouds play a key role in the radiative transfer of the Martian atmosphere, impacting its thermal structure, its circulation, and, in turn, the water cycle. Recent studies including the radiative effects of clouds in global climate models (GCMs) have found that the corresponding feedbacks amplify the model defaults. In particular, it prevents models with simple microphysics from reproducing even the basic characteristics of the water cycle. Within that context, we propose a new implementation of the water cycle in GCMs, including a detailed cloud microphysics taking into account nucleation on dust particles, ice particle growth, and scavenging of dust particles due to the condensation of ice. We implement these new methods in the Laboratoire de Meteorologie Dynamique GCM and find satisfying agreement with the Thermal Emission Spectrometer observations of both water vapor and cloud opacities, with a significant improvement when compared to GCMs taking into account radiative effects of water ice clouds without this implementation. However, a lack of water vapor in the tropics after Ls = 180° is persistent in simulations compared to observations, as a consequence of aphelion cloud radiative effects strengthening the Hadley cell. Our improvements also allow us to explore questions raised by recent observations of the Martian atmosphere. Supersaturation above the hygropause is predicted in line with Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars observations. The model also suggests for the first time that the scavenging of dust by water ice clouds alone fails to fully account for the detached dust layers observed by the Mars Climate Sounder.

Intercomparison and evaluation of global aerosol microphysical properties among AeroCom models of a range of complexity

Abstract: Many of the next generation of global climate models will include aerosol schemes which explicitly simulate the microphysical processes that determine the particle size distribution. These models enable aerosol optical properties and cloud condensation nuclei (CCN) concentrations to be determined by fundamental aerosol processes, which should lead to a more physically based simulation of aerosol direct and indirect radiative forcings. This study examines the global variation in particle size distribution simulated by 12 global aerosol microphysics models to quantify model diversity and to identify any common biases against observations. Evaluation against size distribution measurements from a new European network of aerosol supersites shows that the mean model agrees quite well with the observations at many sites on the annual mean, but there are some seasonal biases common to many sites. In particular, at many of these European sites, the accumulation mode number concentration is biased low during winter and Aitken mode concentrations tend to be overestimated in winter and underestimated in summer. At high northern latitudes, the models strongly underpredict Aitken and accumulation particle concentrations compared to the measurements, consistent with previous studies that have highlighted the poor performance of global aerosol models in the Arctic. In the marine boundary layer, the models capture the observed meridional variation in the size distribution, which is dominated by the Aitken mode at high latitudes, with an increasing concentration of accumulation particles with decreasing latitude. Considering vertical profiles, the models reproduce the observed peak in total particle concentrations in the upper troposphere due to new particle formation, although modelled peak concentrations tend to be biased high over Europe. Overall, the multi-model-mean data set simulates the global variation of the particle size distribution with a good degree of skill, suggesting that most of the individual global aerosol microphysics models are performing well, although the large model diversity indicates that some models are in poor agreement with the observations. Further work is required to better constrain size-resolved primary and secondary particle number sources, and an improved understanding of nucleation and growth (e.g. the role of nitrate and secondary organics) will improve the fidelity of simulated particle size distributions. © 2014 Author(s).

Evaluation of cloud-resolving and limited area model intercomparison simulations using TWP-ICE observations: 1. Deep convective updraft properties

Abstract: Ten 3-D cloud-resolving model simulations and four 3-D limited area model simulations of an intense mesoscale convective system observed on 23–24 January 2006 during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are compared with each other and with observed radar reflectivity fields and dual-Doppler retrievals of vertical wind speeds in an attempt to explain published results showing a high bias in simulated convective radar reflectivity aloft. This high-bias results from ice water content being large, which is a product of large, strong convective updrafts, although hydrometeor size distribution assumptions modulate the size of this bias. Making snow mass more realistically proportional to D2 rather than D3 eliminates unrealistically large snow reflectivities over 40 dBZ in some simulations. Graupel, unlike snow, produces high biased reflectivity in all simulations, which is partly a result of parameterized microphysics but also partly a result of overly intense simulated updrafts. Peak vertical velocities in deep convective updrafts are greater than dual-Doppler-retrieved values, especially in the upper troposphere. Freezing of liquid condensate, often rain, lofted above the freezing level in simulated updraft cores greatly contributes to these excessive upper tropospheric vertical velocities. The strongest simulated updraft cores are nearly undiluted, with some of the strongest showing supercell characteristics during the multicellular (presquall) stage of the event. Decreasing horizontal grid spacing from 900 to 100 m slightly weakens deep updraft vertical velocity and moderately decreases the amount of condensate aloft but not enough to match observational retrievals. Therefore, overly intense simulated updrafts may additionally be a product of unrealistic interactions between convective dynamics, parameterized microphysics, and large-scale model forcing that promote different convective strengths than observed.

On the Representation of High-Latitude Boundary Layer Mixed-Phase Cloud in the ECMWF Global Model

Abstract: Supercooled liquid water (SLW) layers in boundary layer clouds are abundantly observed in the atmosphere at high latitudes, but remain a challenge to represent in numerical weather prediction (NWP) and climate models. Unresolved processes such as small-scale turbulence and mixed-phase microphysics act to maintain the liquid layer at cloud top, directly affecting cloud radiative properties and prolonging cloud lifetimes. This paper describes the representation of supercooled liquid water in boundary layer clouds in the European Centre for Medium-Range Weather Forecasts (ECMWF) global NWP model and in particular the change from a diagnostic temperature-dependent mixed phase to a prognostic representation with separate cloud liquid and ice variables. Data from the Atmospheric Radiation Measurement site in Alaska and from the CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite missions are used to evaluate the model parameterizations. The prognostic scheme s...

Microphysics of clouds and rain over the Western Ghat

Abstract: In an attempt to unravel the interactions between cloud microphysics and dynamics that make shallow clouds precipitate heavily in this region, some unique observations of rain and cloud microphysical parameters are presented here from two stations, Pune and Mahabaleshwar, one each on the lee and windward sides, respectively, of the Western Ghat (WG) mountains in peninsular India. To elucidate rain microphysics, we used the raindrop size distribution (DSD) by fitting three parameter Gamma functions to the observed raindrop spectra. Over Pune, during stratiform rain with bright band (BB) at 0°C isotherm; concave upward DSD shapes are observed below the BB which becomes concave downward at lower altitudes. It is due to breakup process of large raindrops which increases drop concentration at midsizes suggesting coalescence, collision, and breakup processes. Both slope and intercept parameters of Gamma DSD decrease during no BB condition as altitudes decrease, signifying collision and coalescence processes. Over Mahabaleshwar, bimodal and monomodal DSD are observed during light and heavy rainfall, respectively. With shallow storm heights, small raindrops mainly contribute to both types of rainfall. The DSDs are parameterized, and their radar reflectivity factor-rainfall intensity relationships are evaluated suggesting the dominance of collision-coalescence processes. Aircraft measurements of orographic clouds over the WG suggest interaction of cloud mass with the ambient updraft speed. The orographically forced updrafts foster rapid condensational growth of cloud droplets triggering coalescence process within few hundred meters of cloud depth. Hence, these clouds are dynamically forced to produce precipitation over the WG.

Evaluating the Performance of Planetary Boundary Layer and Cloud Microphysical Parameterization Schemes in Convection-Permitting Ensemble Forecasts Using Synthetic GOES-13 Satellite Observations

Abstract: In this study, the ability of several cloud microphysical and planetary boundary layer parameterization schemes to accurately simulate cloud characteristics within 4-km grid-spacing ensemble forecasts over the contiguous United States was evaluated through comparison of synthetic Geostationary Operational Environmental Satellite (GOES) infrared brightness temperatures with observations. Four double-moment microphysics schemes and five planetary boundary layer (PBL) schemes were evaluated. Large differences were found in the simulated cloud cover, especially in the upper troposphere, when using different microphysics schemes. Overall, the results revealed that the Milbrandt–Yau and Morrison microphysics schemes tended to produce too much upper-level cloud cover, whereas the Thompson and the Weather Research and Forecasting Model (WRF) double-moment 6-class (WDM6) microphysics schemes did not contain enough high clouds. Smaller differences occurred in the cloud fields when using different PBL scheme...

Influence of microphysics on the scaling of precipitation extremes with temperature

Abstract: Simulations of radiative-convective equilibrium with a cloud-system resolving model are used to investigate the scaling of high percentiles of the precipitation distribution (precipitation extremes) over a wide range of surface temperatures. At surface temperatures above roughly 295 K, precipitation extremes increase with warming in proportion to the increase in surface moisture, following what is termed Clausius-Clapeyron (CC) scaling. At lower temperatures, the rate of increase of precipitation extremes depends on the choice of cloud and precipitation microphysics scheme and the accumulation period, and it differs markedly from CC scaling in some cases. Precipitation extremes are found to be sensitive to the fall speeds of hydrometeors, and this partly explains the different scaling results obtained with different microphysics schemes. The results suggest that microphysics play an important role in determining the response of convective precipitation extremes to warming, particularly when ice- and mixed-phase processes are important.

Development of two-moment cloud microphysics for liquid and ice within the NASA Goddard Earth Observing System Model (GEOS-5)

Abstract: . This work presents the development of a two-moment cloud microphysics scheme within version 5 of the NASA Goddard Earth Observing System (GEOS-5). The scheme includes the implementation of a comprehensive stratiform microphysics module, a new cloud coverage scheme that allows ice supersaturation, and a new microphysics module embedded within the moist convection parameterization of GEOS-5. Comprehensive physically based descriptions of ice nucleation, including homogeneous and heterogeneous freezing, and liquid droplet activation are implemented to describe the formation of cloud particles in stratiform clouds and convective cumulus. The effect of preexisting ice crystals on the formation of cirrus clouds is also accounted for. A new parameterization of the subgrid-scale vertical velocity distribution accounting for turbulence and gravity wave motion is also implemented. The new microphysics significantly improves the representation of liquid water and ice in GEOS-5. Evaluation of the model against satellite retrievals and in situ observations shows agreement of the simulated droplet and ice crystal effective radius, the ice mass mixing ratio and number concentration, and the relative humidity with respect to ice. When using the new microphysics, the fraction of condensate that remains as liquid follows a sigmoidal dependency with temperature, which is in agreement with observations and which fundamentally differs from the linear increase assumed in most models. The performance of the new microphysics in reproducing the observed total cloud fraction, longwave and shortwave cloud forcing, and total precipitation is similar to the operational version of GEOS-5 and in agreement with satellite retrievals. The new microphysics tends to underestimate the coverage of persistent low-level stratocumulus. Sensitivity studies showed that the simulated cloud properties are robust to moderate variation in cloud microphysical parameters. Significant sensitivity remains to variation in the dispersion of the ice crystal size distribution and the critical size for ice autoconversion. Despite these issues, the implementation of the new microphysics leads to a considerably improved and more realistic representation of cloud processes in GEOS-5, and allows the linkage of cloud properties to aerosol emissions.

Application of Object-Based Time-Domain Diagnostics for Tracking Precipitation Systems in Convection-Allowing Models

Abstract: Meaningful verification and evaluation of convection-allowing models requires approaches that do not rely on point-to-point matches of forecast and observed fields. In this study, one such approach—a beta version of the Method for Object-Based Diagnostic Evaluation (MODE) that incorporates the time dimension [known as MODE time-domain (MODE-TD)]—was applied to 30-h precipitation forecasts from four 4-km grid-spacing members of the 2010 Storm-Scale Ensemble Forecast system with different microphysics parameterizations. Including time in MODE-TD provides information on rainfall system evolution like lifetime, timing of initiation and dissipation, and translation.The simulations depicted the spatial distribution of time-domain precipitation objects across the United States quite well. However, all simulations overpredicted the number of objects, with the Thompson microphysics scheme overpredicting the most and the Morrison method the least. For the smallest smoothing radius and rainfall threshold use...

Influence of Cloud–Radiative Forcing on Tropical Cyclone Structure

Abstract: The authors demonstrate how and why cloud–radiative forcing (CRF), the interaction of hydrometeors with longwave and shortwave radiation, can influence tropical cyclone structure through “semi idealized” integrations of the Hurricane Weather Research and Forecasting model (HWRF) and an axisymmetric cloud model. Averaged through a diurnal cycle, CRF consists of pronounced cooling along the anvil top and weak warming through the cloudy air, which locally reverses the large net cooling that occurs in the troposphere under clear-sky conditions. CRF itself depends on the microphysics parameterization and represents one of the major reasons why simulations can be sensitive to microphysical assumptions.By itself, CRF enhances convective activity in the tropical cyclone’s outer core, leading to a wider eye, a broader tangential wind field, and a stronger secondary circulation. This forcing also functions as a positive feedback, assisting in the development of a thicker and more radially extensive anvil th...

Benefits of a Fourth Ice Class in the Simulated Radar Reflectivities of Convective Systems Using a Bulk Microphysics Scheme

Abstract: Numerous cloud microphysical schemes designed for cloud and mesoscale models are currently in use, ranging from simple bulk to multi-moment, multi-class to explicit bin schemes. This study details the benefits of adding a 4th ice class (hail) to an already improved 3-class ice bulk microphysics scheme developed for the Goddard Cumulus Ensemble model based on Rutledge and Hobbs (1983,1984). Besides the addition and modification of several hail processes from Lin et al. (1983), further modifications were made to the 3-ice processes, including allowing greater ice super saturation and mitigating spurious evaporationsublimation in the saturation adjustment scheme, allowing graupelhail to become snow via vapor growth and hail to become graupel via riming, and the inclusion of a rain evaporation correction and vapor diffusivity factor. The improved 3-ice snowgraupel size-mapping schemes were adjusted to be more stable at higher mixing rations and to increase the aggregation effect for snow. A snow density mapping was also added. The new scheme was applied to an intense continental squall line and a weaker, loosely-organized continental case using three different hail intercepts. Peak simulated reflectivities agree well with radar for both the intense and weaker case and were better than earlier 3-ice versions when using a moderate and large intercept for hail, respectively. Simulated reflectivity distributions versus height were also improved versus radar in both cases compared to earlier 3-ice versions. The bin-based rain evaporation correction affected the squall line case more but did not change the overall agreement in reflectivity distributions.

Aerosol microphysics simulations of the Mt.~Pinatubo eruption with the UM-UKCA composition-climate model

Abstract: . We use a stratosphere–troposphere composition–climate model with interactive sulfur chemistry and aerosol microphysics, to investigate the effect of the 1991 Mount Pinatubo eruption on stratospheric aerosol properties. Satellite measurements indicate that shortly after the eruption, between 14 and 23 Tg of SO2 (7 to 11.5 Tg of sulfur) was present in the tropical stratosphere. Best estimates of the peak global stratospheric aerosol burden are in the range 19 to 26 Tg, or 3.7 to 6.7 Tg of sulfur assuming a composition of between 59 and 77 % H2SO4. In light of this large uncertainty range, we performed two main simulations with 10 and 20 Tg of SO2 injected into the tropical lower stratosphere. Simulated stratospheric aerosol properties through the 1991 to 1995 period are compared against a range of available satellite and in situ measurements. Stratospheric aerosol optical depth (sAOD) and effective radius from both simulations show good qualitative agreement with the observations, with the timing of peak sAOD and decay timescale matching well with the observations in the tropics and mid-latitudes. However, injecting 20 Tg gives a factor of 2 too high stratospheric aerosol mass burden compared to the satellite data, with consequent strong high biases in simulated sAOD and surface area density, with the 10 Tg injection in much better agreement. Our model cannot explain the large fraction of the injected sulfur that the satellite-derived SO2 and aerosol burdens indicate was removed within the first few months after the eruption. We suggest that either there is an additional alternative loss pathway for the SO2 not included in our model (e.g. via accommodation into ash or ice in the volcanic cloud) or that a larger proportion of the injected sulfur was removed via cross-tropopause transport than in our simulations. We also critically evaluate the simulated evolution of the particle size distribution, comparing in detail to balloon-borne optical particle counter (OPC) measurements from Laramie, Wyoming, USA (41° N). Overall, the model captures remarkably well the complex variations in particle concentration profiles across the different OPC size channels. However, for the 19 to 27 km injection height-range used here, both runs have a modest high bias in the lowermost stratosphere for the finest particles (radii less than 250 nm), and the decay timescale is longer in the model for these particles, with a much later return to background conditions. Also, whereas the 10 Tg run compared best to the satellite measurements, a significant low bias is apparent in the coarser size channels in the volcanically perturbed lower stratosphere. Overall, our results suggest that, with appropriate calibration, aerosol microphysics models are capable of capturing the observed variation in particle size distribution in the stratosphere across both volcanically perturbed and quiescent conditions. Furthermore, additional sensitivity simulations suggest that predictions with the models are robust to uncertainties in sub-grid particle formation and nucleation rates in the stratosphere.

Influence of microphysics on the scaling of precipitation extremes with temperature

Abstract: Simulations of radiative-convective equilibrium with a cloud-system resolving model are used to investigate the scaling of high percentiles of the precipitation distribution (precipitation extremes) over a wide range of surface temperatures. At surface temperatures above roughly 295 K, precipitation extremes increase with warming in proportion to the increase in surface moisture, following what is termed Clausius-Clapeyron (CC) scaling. At lower temperatures, the rate of increase of precipitation extremes depends on the choice of cloud and precipitation microphysics scheme and the accumulation period, and it differs markedly from CC scaling in some cases. Precipitation extremes are found to be sensitive to the fall speeds of hydrometeors, and this partly explains the different scaling results obtained with different microphysics schemes. The results suggest that microphysics play an important role in determining the response of convective precipitation extremes to warming, particularly when ice- and mixed-phase processes are important.

Gravitational wave emission and spin-down of young pulsars

Abstract: The rotation frequencies of young pulsars are systematically below their theoretical Kepler limit. r-modes have been suggested as a possible explanation for this observation. With the help of semi-analytic expressions that make it possible to assess the uncertainties of the r-mode scenario due to the impact of uncertainties in underlying microphysics, we perform a quantitative analysis of the spin-down and the emitted gravitational waves of young pulsars. We find that the frequency to which r-modes spin-down a young neutron star (NS) is surprisingly insensitive to both the microscopic details and the saturation amplitude. Comparing our result to astrophysical data, we show that for a range of sufficiently large saturation amplitudes r-modes provide a viable spin-down scenario and that all observed young pulsars are very likely already outside the r-mode instability region. Therefore, the most promising sources for gravitational wave detection are unobserved NSs associated with recent supernovae, and we find that advanced LIGO should be able to see several of them. Our analysis shows that despite the coupling of the spin-down and thermal evolution, a power-law spin-down with an effective braking index n rm ≤ 7 is realized. Because of this, the gravitational wave strain amplitude is completely independent of both the r-mode saturation amplitude and the microphysics and depends on the saturation mechanism only within some tens of percent. However, the gravitational wave frequency depends on the amplitude, and we provide the required expected timing parameter ranges to look for promising sources in future searches.

Evaluation of Precipitating Hydrometeor Parameterizations in a Single-Moment Bulk Microphysics Scheme for Deep Convective Systems over the Tropical Central Pacific

Abstract: Cloud microphysics of deep convective systems over the tropical central Pacific simulated by a cloud system–resolving model using satellite simulators are evaluated in terms of the joint histogram of cloud-top temperature and precipitation echo-top heights. A control experiment shows an underestimation of stratiform precipitation and a higher frequency of precipitating deep clouds with top heights higher than 12 km when compared with data from the Tropical Rainfall Measuring Mission. The comparison shows good agreement for horizontal distribution and statistical cloud size distributions of deep convective systems. Biases in the joint histogram are improved by changing cloud microphysics parameters of a single-moment bulk microphysics scheme. The effects of size distribution of precipitating hydrometeors are examined. Modification of the particle size distributions of rain, snow, and graupel size distributions based on observed relationships improves cloud precipitation statistics. This study impli...

The Analysis and Prediction of Microphysical States and Polarimetric Radar Variables in a Mesoscale Convective System Using Double-Moment Microphysics, Multinetwork Radar Data, and the Ensemble Kalman Filter

Abstract: Doppler radar data are assimilated with an ensemble Kalman Filter (EnKF) in combination with a double-moment (DM) microphysics scheme in order to improve the analysis and forecast of microphysical states and precipitation structures within a mesoscale convective system (MCS) that passed over western Oklahoma on 8–9 May 2007. Reflectivity and radial velocity data from five operational Weather Surveillance Radar-1988 Doppler (WSR-88D) S-band radars as well as four experimental Collaborative and Adaptive Sensing of the Atmosphere (CASA) X-band radars are assimilated over a 1-h period using either single-moment (SM) or DM microphysics schemes within the forecast ensemble. Three-hour deterministic forecasts are initialized from the final ensemble mean analyses using a SM or DM scheme, respectively. Polarimetric radar variables are simulated from the analyses and compared with polarimetric WSR-88D observations for verification. EnKF assimilation of radar data using a multimoment microphysics scheme for ...

Downscaling with a nested regional climate model in near-surface fields over the contiguous United States

Abstract: The Weather Research and Forecasting (WRF) model is used for dynamic downscaling of 2.5-degree National Centers for Environmental Prediction-U.S. Department of Energy Reanalysis II (NCEP-R2) data for 1980–2010 at 12 km resolution over most of North America. The model's performance for surface air temperature and precipitation is evaluated by comparison with high-resolution observational data sets. The model's ability to add value is investigated by comparison with NCEP-R2 data and a 50 km regional climate simulation. The causes for major model bias are studied through additional sensitivity experiments with various model setup/integration approaches and physics representations. The WRF captures the main features of the spatial patterns and annual cycles of air temperature and precipitation over most of the contiguous United States. However, simulated air temperatures over the south central region and precipitation over the Great Plains and the Southwest have significant biases. Allowing longer spin-up time, reducing the nudging strength, or replacing the WRF Single-Moment six-class microphysics with Morrison microphysics reduces the bias over some subregions. However, replacing the Grell-Devenyi cumulus parameterization with Kain-Fritsch shows no improvement. The 12 km simulation does add value above the NCEP-R2 data and the 50 km simulation over mountainous and coastal zones.

A self-consistent scattering model for cirrus. II: The high and low frequencies

Abstract: The predictive quality of an ensemble model of cirrus ice crystals to model passive and active measurements of ice cloud, from the ultraviolet (UV) to the microwave, is tested. The ensemble model predicts m ∝ D2, where D is the maximum dimension of the ice crystal, and m is its mass. This predicted m-D relationship is applied to a moment estimation parametrization of the particle size distribution (PSD), to estimate the PSD shape, given ice water content (IWC) and in-cloud temperature. The same microphysics is applied across the electromagnetic spectrum to model UV, infrared, microwave and radar observations. The short-wave measurements consist of airborne UV backscatter lidar (light detection and ranging) estimates of the volume extinction coefficient, total solar optical depth, and space-based multi-directional spherical albedo retrievals, at 0.865 µm, between the scattering angles 85° and 125°. The airborne long-wave measurements consist of high-resolution interferometer upwelling brightness temperatures, obtained between the wavelengths of about 3.45 µm and 4.1 µm, and 8.0 µm to 12.0 µm. The low-frequency measurements consist of ground-based Chilbolton 35 GHz radar reflectivity measurements and space-based upwelling 190 GHz brightness temperature measurements. The predictive quality of the ensemble model is demonstrated to be generally within the experimental uncertainty of the lidar backscatter estimates of the volume extinction coefficient and total solar optical depth. The ensemble model prediction of the high-resolution brightness temperature measurements is generally within ±2 K and ±1 K at solar and infrared wavelengths, respectively. The 35 GHz radar reflectivity and 190 GHz brightness temperatures are generally simulated to within ±2 dBZe, and ±2 K, respectively. The directional spherical albedo observations suggest that the scattering phase function of the most randomized ensemble model gives the best fit to the measurements (generally within ±3%). This article demonstrates that the ensemble model, assuming the same microphysics, is physically consistent across the electromagnetic spectrum.

Post-Asymptotic Giant Branch Evolution of Low- and Intermediate-Mass Stars. Preliminary Results

Abstract: Preliminary results from an ongoing project to compute a grid of post-AGB models is presented. Our preliminary results show that stellar evolution computations that include an updated treatment of the microphysics predict post-AGB timescales that are several times shorter that predicted by older models. Also the mass-luminosity relation of post-AGB models deviates from that of older grids. In addition, our results suggest only a slight metallicity dependence of the post-AGB timescales. We expect these results to have significant consequences for models of the formation of planetary nebulae and their luminosity function.

Comparison of WRF Model Physics Parameterizations over the MENA-CORDEX Domain

Abstract: We investigated the performance of 12 different physics configurations of the climate version of the Weather, Research and Forecasting (WRF) Model over the Middle East and North Africa (MENA) domain. Possible combinations among two Planetary Boundary Layer (PBL), three Cumulus (CUM) and two Microphysics (MIC) schemes were tested. The 2-year simulations (December 1988-November 1990) have been compared with gridded observational data and station measurements for several variables, including total precipitation and maximum and minimum 2-meter air temperature. An objective ranking method of the 12 different simulations and the selection procedure of the best performing configuration for the MENA domain are based on several statistical metrics and carried out for relevant sub-domains and individual stations. The setup for cloud microphysics is found to have the strongest impact on temperature biases while precipitation is most sensitive to the cumulus parameterization scheme and mainly in the tropics.

Implementation of an aerosol–cloud-microphysics–radiation coupling into the NASA unified WRF: Simulation results for the 6–7 August 2006 AMMA special observing period

Abstract: Aerosols affect the Earth's radiation balance directly and cloud microphysical processes indirectly via the activation of cloud condensation and ice nuclei. These two effects have often been considered separately and independently, hence the need to assess their combined impact given the differing nature of their effects on convective clouds. To study both effects, an aerosol–microphysics–radiation coupling, including Goddard microphysics and radiation schemes, was implemented into the NASA Unified Weather Research and Forecasting model (NU-WRF). Fully coupled NU-WRF simulations were conducted for a mesoscale convective system (MCS) that passed through the Niamey, Niger area on 6–7 August 2006 during an AMMA special observing period. The results suggest that rainfall is reduced when aerosol indirect effects are included, regardless of the aerosol direct effect. Daily mean radiation heating profiles in the area traversed by the MCS showed the aerosol (mainly mineral dust) direct effect had the largest impact near cloud tops just above 200 hPa where short-wave heating increased by about 0.8 K day−1; the weakest long-wave cooling was at around 250 hPa. It was also found that more condensation and ice nuclei as a result of higher aerosol/dust concentrations led to increased amounts of all cloud hydrometeors because of the microphysical indirect effect, and the radiation direct effect acts to reduce precipitating cloud particles (rain, snow and graupel) in the middle and lower cloud layers while increasing the non-precipitating particles (ice) in the cirrus anvil. However, when the aerosol direct effect was activated, regardless of the indirect effect, the onset of MCS precipitation was delayed about 2 h, in conjunction with the delay in the activation of cloud condensation and ice nuclei. Overall, for this particular environment, model set-up and physics configuration, the effect of aerosol radiative heating due to mineral dust overwhelmed the effect of the aerosols on microphysics.

Spectral evolution in gamma-ray bursts: predictions of the internal shock model and comparison to observations

Abstract: Several trends have been identified in the prompt gamma-ray burst (GRB) emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time lags, hardness-intensity/-fluence correlations. Recently Fermi has significantly extended the spectral coverage of GRB observations and improved the characterization of this spectral evolution. We study how internal shocks can reproduce these observations. In this model the emission comes from the synchrotron radiation of shock accelerated electrons, and the spectral evolution is governed by the evolution of the physical conditions in the shocked regions. We present a comprehensive set of simulations of a single pulse and investigate the impact of the model parameters, related to the shock microphysics and to the initial conditions in the ejecta. We find a general qualitative agreement between the model and the various observations used for the comparison. All these properties or relations are governed by the evolution of the peak energy and photon indices of the spectrum. In addition, we identify the conditions for a quantitative agreement. We find that the best agreement is obtained for (i) steep electron slopes (p>~2.7), (ii) microphysics parameters varying with shock conditions so that more electrons are accelerated in stronger shocks, (iii) steep variations of the initial Lorentz factor in the ejecta. When simulating short GRBs by contracting all timescales, all other parameters being unchanged, we show that the hardness-duration correlation is reproduced, as well as the evolution with duration of the pulse properties. Finally, we investigate the signature at high energy of these different scenarios and find distinct properties - delayed onset, longer emission, and flat spectrum in some cases - suggesting that internal shocks could have a significant contribution to the prompt LAT emission. [abridged]

Impact of large-scale dynamics on the microphysical properties of midlatitude cirrus

Abstract: In situ microphysical observations of midlatitude cirrus collected during the Department of Energy Small Particles in Cirrus (SPARTICUS) field campaign are combined with an atmospheric state classification for the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site to understand statistical relationships between cirrus microphysics and the large-scale meteorology. The atmospheric state classification is informed about the large-scale meteorology and state of cloudiness at the ARM SGP site by combining ECMWF ERA-Interim reanalysis data with 14 years of continuous observations from the millimeter-wavelength cloud radar. Almost half of the cirrus cloud occurrences in the vicinity of the ARM SGP site during SPARTICUS can be explained by three distinct synoptic conditions, namely, upper level ridges, midlatitude cyclones with frontal systems, and subtropical flows. Probability density functions (PDFs) of cirrus microphysical properties such as particle size distributions (PSDs), ice number concentrations, and ice water content (IWC) are examined and exhibit striking differences among the different synoptic regimes. Generally, narrower PSDs with lower IWC but higher ice number concentrations are found in cirrus sampled in upper level ridges, whereas cirrus sampled in subtropical flows, fronts, and aged anvils show broader PSDs with considerably lower ice number concentrations but higher IWC. Despite striking contrasts in the cirrus microphysics for different large-scale environments, the PDFs of vertical velocity are not different, suggesting that vertical velocity PDFs are a poor predictor for explaining the microphysical variability in cirrus. Instead, cirrus microphysical contrasts may be driven by differences in ice supersaturations or aerosols.

Model sensitivity analysis study for western disturbances over the Himalayas

Abstract: Western disturbances (WDs) are extratropical synoptic scale weather systems which cause significant precipitation over the Himalayas and surrounding areas during winter (December, January and February, DJF). Three intense WDs, 13–17 January 2002, 05–08 February 2002, and 11–13 February 2002, are chosen as two of the WDs are extensively studied by Hatwar et al. (Curr Sci 88:913–920, 2005) and one independent WD (Indian Meteorological Department, Delhi, Mausam 54(1):346–347, 2003) is considered. Firstly, it is planned to study model sensitivity with these WD cases, which are simulated with different combinations of cloud microphysics, planetary boundary layer and cumulus parameterization schemes in weather research and forecasting model to assess a better suite for the WD simulations. Sensitivity and error analyses carried out with different observations, show that the combination of Eta Ferrier or Eta Grid-scale cloud and precipitation microphysics scheme, Yonsei University scheme and Kain-Fritsch scheme has shown consistently lower error values. Further, the results suggest, that the model simulations of a WD capture the spatial distribution of precipitation, locations of low pressure region and the circulation patterns very well. It is observed that the WD system comprises of low pressure region in the vertical atmospheric column in form of a stationary surface low and a depression in the subtropical westerly jet moving eastwards. Further, the growth of convective cyclonic systems over the steep topographical region of the Himalayas is depicted by the increased positive vorticity and high values of CAPE, alluding to the propensity of WDs to cause orographically forced precipitation. WDs and associated precipitation show varied but significant impacts on the Indian winter climate such as snow cover variation and cold wave or fog conditions along with impact on winter crop production.