Use In General Circulation Models Research Articles

A simple diagnostic calculation of marine stratocumulus cloud cover for use in general circulation models

An improved diagnostic calculation for determining the marine stratocumulus (SCu) cloud cover has been implemented for atmospheric and coupled general circulation models (GCMs). The approach aims to improve the simulated climatological features of the lower troposphere and to reduce warm sea surface temperature (SST) biases that develop along western continental coasts in the subtropical regions of coupled GCMs. In the new diagnostic calculation introduced in the present study, the SCu cloud cover was linearly regressed to atmospheric stability, and the temporal and spatial distributions of the regression coefficients were estimated beforehand using observational data sets. The upward transport of heat, moisture and momentum in subcloud layers accompanying the SCu was also simulated by controlling the vertical diffusion coefficients. Using the new calculation, the SCu cloud cover in an atmospheric GCM become more representative in both spatial and seasonal variations. In addition, the simulation of related atmospheric structures in the lower troposphere is considerably improved. Furthermore, when the new calculation is applied to a coupled GCM, SST values to the west of continents in subtropical areas display weakened warm biases and more realistic seasonal cycles resulting from the modification of the downward shortwave radiation flux at the sea surface. The new calculation introduces both local and large‐scale improvements in model climatology over low‐latitude and midlatitude regions. In the eastern Pacific, for example, a shallow meridional circulation across the equator is excited, and the strength and location of the intertropical convergence zone and the seasonal cycle of equatorial SST become more realistic.

A new parametrization for the radiative properties of ice crystals: Comparison with existing schemes and impact in a GCM

A new parametrization of the single scattering properties of ice crystals suitable for use in general circulation models of the atmosphere is presented. The parametrization is developed from a recent treatment of the scattering properties of an ensemble of ice crystals, which has been validated against observational data and is based on a more rigorous treatment of ice optics than has been possible until recently. The new parametrization is compared with existing schemes in idealized test cases and its impact in a climate model is assessed.

A nonparametric stochastic downscaling framework for daily rainfall at multiple locations

Use of General Circulation Models (GCMs) for climate change impact assessment is often limited by their incapability at representing local features and dynamics at spatial scales finer than the in‐built GCM grid scale. This has led to the development of downscaling techniques for transfer of coarse GCM simulated weather output to finer spatial resolutions. This paper presents a nonparametric stochastic spatial downscaling framework for multisite daily rainfall occurrence and amount. At site rainfall occurrences are downscaled using a nonparametric nonhomogeneous hidden Markov model (NNHMM) that represents spatial dependence across the rainfall occurrence field using a dynamic weather state indicative of the centroid and average wetness fraction of the rainfall occurrence field. The rainfall amounts on the wet days are downscaled using a nonparametric kernel density approach that accommodates variations in the rainfall downscaling model at individual locations. Spatial dependence in the rainfall amounts is simulated by driving each of the single‐site amounts model with spatially correlated random numbers. The proposed framework is applied for downscaling of rainfall at a network of 30 rain gauge stations around Sydney in Australia, and its performance is evaluated. The analyses of the results show that the logic of providing separate treatments for rainfall occurrence and amounts at individual locations imparts considerable accuracy in the representation of characteristics of interest in hydrologic studies. These characteristics include representation of rainfall spell patterns, spatial distribution of the rainfall occurrence and amount fields, representation of low and high rainfall extremes at individual stations and across the field, as well as common indicators of water balance and variability that are of importance in a catchment scale water balance simulation.

Modelling hydrological consequences of climate change—Progress and challenges

The simulation of hydrological consequences of climate change has received increasing attention from the hydrology and land-surface modelling communities. There have been many studies of climate-change effects on hydrology and water resources which usually consist of three steps: (1) use of general circulation models (GCMs) to provide future global climate scenarios under the effect of increasing greenhouse gases, (2) use of downscaling techniques (both nested regional climate models, RCMs, and statistical methods) for “downscaling” the GCM output to the scales compatible with hydrological models, and (3) use of hydrologic models to simulate the effects of climate change on hydrological regimes at various scales. Great progress has been achieved in all three steps during the past few years, however, large uncertainties still exist in every stage of such study. This paper first reviews the present achievements in this field and then discusses the challenges for future studies of the hydrological impacts of climate change.

Evaluating the efficacy of planktonic foraminifer calcite δ 18O data for sea surface temperature reconstruction for the Late Miocene

This study examines the efficacy of published δ 18O data from the calcite of Late Miocene surface dwelling planktonic foraminifer shells, for sea surface temperature estimates for the pre-Quaternary. The data are from 33 Late Miocene (Messinian) marine sites from a modern latitudinal gradient of 64°N to 48°S. They give estimates of SSTs in the tropics/subtropics (to 30°N and S) that are mostly cooler than present. Possible causes of this temperature discrepancy are ecological factors (e.g. calcification of shells at levels below the ocean mixed layer), taphonomic effects (e.g. diagenesis or dissolution), inaccurate estimation of Late Miocene seawater oxygen isotope composition, or a real Late Miocene cool climate. The scale of apparent cooling in the tropics suggests that the SST signal of the foraminifer calcite has been reset, at least in part, by early diagenetic calcite with higher δ 18O, formed in the foraminifer shells in cool sea bottom pore waters, probably coupled with the effects of calcite formed below the mixed layer during the life of the foraminifera. This hypothesis is supported by the markedly cooler SST estimates from low latitudes—in some cases more than 9 °C cooler than present—where the gradients of temperature and the δ 18O composition of seawater between sea surface and sea bottom are most marked, and where ocean surface stratification is high. At higher latitudes, particularly N and S of 30°, the temperature signal is still cooler, though maximum temperature estimates overlap with modern SSTs N and S of 40°. Comparison of SST estimates for the Late Miocene from alkenone unsaturation analysis from the eastern tropical Atlantic at Ocean Drilling Program (ODP) Site 958—which suggest a warmer sea surface by 2–4 °C, with estimates from oxygen isotopes at Deep Sea Drilling Project (DSDP) Site 366 and ODP Site 959, indicating cooler than present SSTs, also suggest a significant impact on the δ 18O signal. Nevertheless, much of the original SST variation is clearly preserved in the primary calcite formed in the mixed layer, and records secular and temporal oceanographic changes at the sea surface, such as movement of the Antarctic Polar Front in the Southern Ocean. Cooler SSTs in the tropics and sub-tropics are also consistent with the Late Miocene latitude reduction in the coral reef belt and with interrupted reef growth on the Queensland Plateau of eastern Australia, though it is not possible to quantify absolute SSTs with the existing oxygen isotope data. Reconstruction of an accurate global SST dataset for Neogene time-slices from the existing published DSDP/ODP isotope data, for use in general circulation models, may require a detailed re-assessment of taphonomy at many sites.

The Interannual Variability of the Onset of the Maize Growing Season over South Africa and Zimbabwe

Abstract Subsistence farmers within southern Africa have identified the onset of the maize growing season as an important seasonal characteristic, advance knowledge of which would aid preparations for the planting of rain-fed maize. Onset over South Africa and Zimbabwe is calculated using rainfall data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Computing Center for Water Research (CCWR). The two datasets present similar estimates of the mean, standard deviation, and trend of onset for the common period (1979–97) over South Africa. During this period, onset has been tending to occur later in the season, in particular over the coastal regions and the Limpopo valley. However, the CCWR data (1950–97) indicate that this is part of long-term (decadal) variability. Characteristic rainfall patterns associated with late and early onset are estimated using a self-organizing map (SOM). Late onset is associated with heavier rainfall over the subcontinent. When onset is early over Zimbabwe, there is an increased frequency of more intense rainfall over northeast Madagascar during the preceding August. Accompanying these intense events is an increased frequency of positive 500-hPa geopotential height anomalies to the southeast of the continent. Similar positive height anomalies are also frequently present during early onset. The study indicates that onset variability is partly forced by synoptic conditions, and the successful use of general circulation models to estimate onset will depend on their simulation of the zonally asymmetric component of the westerly circulation.

Thermal effects of saturating gravity waves in the atmosphere

Breaking/saturating gravity waves (GWs) not only exert drag on the mean flow due to their momentum deposition but also affect the background thermally because of the associated energy flux divergence. We present a rigorous derivation of terms describing the thermal effects of GWs on the mean flow, based on the corresponding energy cycle for wave/mean flow interactions. The combined effect of saturating GWs is to produce both differential heating and cooling by inducing a downward wave heat flux, and an irreversible conversion of wave energy into heat. The former effect can also be represented as thermal diffusion acting on the mean potential temperature gradient. This rigorous theory for the thermal exchange between waves and the mean flow can be closed once the mechanism for GW dissipation is parameterized. To illustrate the procedure, we employ our recent nonlinear theory of GW spectra to derive expressions for the wave‐induced heating rates. This yields a parameterization of the thermal effects of GWs, which is suitable for use in general circulation models and requires the source GW spectrum as the only tunable parameter. We present results of numerical calculations of wave heating terms for typical wind and temperature profiles as well as simulations with the full‐scale Canadian Middle Atmosphere Model (CMAM).

Near-surface turbulent fluxes in stable stratification: Calculation techniques for use in general-circulation models

Practically oriented flux-calculation techniques based on correction functions to the neutral drag and heat/mass transfer coefficients are further developed. In the traditional formulation, the correction functions depend only on the bulk Richardson number. However, data from measurements of turbulent fluxes and mean profiles in stable stratification over different sites exhibit too strong variability in this type of dependencies. Indirect evidence from climate and weather prediction modelling also shows that the traditional flux-calculation technique is not sufficiently advanced. It is conceivable that other mechanisms besides the surface-layer stratification and, therefore, other arguments besides the bulk Richardson number must be considered. The proposed technique includes a newly discovered effect of the static stability in the free atmosphere on the surface-layer scaling and accounts for the general essential difference between the roughness lengths for momentum and scalars. Besides bulk Richardson number, recommended correction functions depend on one more stability parameter, involving the Brunt–Väisälä frequency in the free atmosphere, and on the roughness lengths. Copyright © 2002 Royal Meteorological Society.

On advection and diffusion of plankton in coarse resolution ocean models

There is a widespread use of general circulation models (GCMs) to study the importance of various physical and biological processes for the primary production, and to understand the structure of the oceanic food web. However, it is shown that low-resolution GCMs may produce too high advection speeds for a plankton bloom that invade an area with potentially high plankton growth rates. Under the prescribed situation, the advection and diffusion act to initiate an exponential growth of plankton in a large grid cell implying that the plankton signal is quickly carried over the entire grid cell. When a certain concentration is reached, it may seed the surrounding grid cells with phytoplankton, and the procedure is repeated. Accordingly, the phytoplankton bloom can travel faster in the low-resolution model than the ocean currents that carry them. Some numerical experiments show that the resolution of the model needs to be at least on the order of Δ x∼ U/ γ P or D/γ P —whichever is largest—to describe the plankton dynamics in an adequate way. Here, U is the typical horizontal advection velocity, D is the horizontal diffusivity for plankton, and γ P represents the growth rate of phytoplankton. With U=0.1 m/s and γ P=1 day −1, it follows that the resolution needs to be at least 8 km to describe the phytoplankton dynamics in a reasonably correct way. Most GCM experiments use coarser resolution; accordingly, the results regarding the advection of plankton in these models should thus be viewed with some caution.

Comments on “Atmospheric Disturbance Caused by Human Modification of the Landscape”

In a recent article published in the February 2001 issue of the Bulletin of American Meteorological Society, Weaver and Avissar (hereafter WA) investigated the response of the atmosphere to landscape heterogeneity to assess the importance of mesoscale fluxes of sensible and latent heat arising from differences in land use in the southern Great Plains. To do so they used the RAMS mesoscale model and a surface flux data set derived from observations and the Simple Biosphere Model. They selected six days for analysis based on the criterion that the study region located at the Department of Energy's Atmospheric Radiation Measurement Program's Cloud and Radiation Testbed (CART) was not obscured by clouds with obvious non-local origins. WA noted that mesoscale fluxes of latent heat on those six days were large, especially near and above the top of the boundary layer. They argued that regions of strong vertical motion that they associated with mesoscale fluxes were closely correlated with regions of precipitation on one of the six days they studied and with cloudy regions on two other days. From these findings, WA concluded that landscape heterogeneity arising from human influences can''significantly affect local weather and climate.'' They further concluded that the inabilitymore » to predict mesoscale fluxes is a serious failing of general circulation models (GCMs) used to predict climate change and that further studies in parameterizing such fluxes for use in GCMs is therefore necessary. In this paper we offer a somewhat different interpretation and perspective on the findings of WA.« less

General Circulation Model Development: Past, Present, and Future. International Geophysics Series, Vol 70

9R53. General Circulation Model Development: Past, Present, and Future. International Geophysics Series, Vol 70. - Edited by DA Randall (Dept of Atmos Sci, Colorado State Univ, Ft Collins CO). Academic Press, San Diego CA. 2000. 803 pp. ISBN 0-12-578010-9. $99.00.Reviewed by J Zehnder (Dept of Geography, Arizona State Univ, PO Box 870104, Tempe AZ 85287).This book is a collection of papers prepared for a symposium honoring Prof Akio Arakawa upon his retirement. The symposium was held in January 1998 on the campus of the University of California-Los Angeles, where Arakawa spent the majority of his career on the faculty of the Department of Atmospheric Sciences. Arakawa received a BSc in physics from Tokyo University in 1950 and a DSc from the same in 1961. He gained practical experience as a research meteorologist with the Japan Meteorological Agency and at UCLA working with Prof Yale Mintz. Arakawa’s research has led to landmark contributions in the development of general circulation models (GCMs) and to the understanding of the atmosphere in general. He provided early and significant contributions to numerical methods which allow for finite difference solutions of the Navier-Stokes equations on a rotating sphere (the so-called primitive equations) and to the parameterization of moist processes and cloud formation. Each of these seminal contributions helped shaped areas of research that have led to the current state of the art GCMs. The book consists of 23 papers that span a wide range of topics from historical perspectives, discussions of the current state of global scale atmospheric modeling, and theoretical aspects of the subject. Arakawa himself contributed three of the articles: his personal recollections of early model development at UCLA, a discussion of the origin of cumulus parameterizations, and his views on the future development of GCMs. The remainder of the contributions are provided by recognized researchers, many of whom are former students and collaborators. Arakawa’s contributions to the field, through his ideas, research, and mentoring of future scientists are well illustrated in this volume. The historical perspectives are provided in the first three chapters in an article by Arakawa, one on the history of GCMs by Paul Lewis, and a discussion of Phillip’s early work by John Lewis. Discussions of early work on data assimilation by Halem et al. (Ch 5) and on the development of cumulus paramterization by Wayne Schubert (Ch 6) round out the historical perspectives. A fair amount of the text is devoted to technical details of GCMs. Mesinger provides a review of finite difference techniques, which Arakawa helped pioneer, in Chapter 13. Randall et al. describes an alternative to the finite difference and spectral models, the geodesic grid, in Chapter 17. A wide range of physical processes such as boundary layer (Moeng and Stevens, Ch 19), clouds and moisture (Moorthi, Ch 9; Kruger, Ch 20) radiative transfer (Sommerville, Ch 21) are discussed. An overview of ocean models is provided by McWilliams (Ch 14) and the formidable task of coupling oceanic and atmospheric GCMs is described by Mechoso et al. in Chapter 18. The more theoretical perspectives are provided by Kerry Emanuel (Ch 8) in a discussion of Quasi-Equilibrium Thinking, which includes the statistical equilibrium representation of cumulus convection developed by Schubert and Arakawa. Problem solving with GCMs and aspects of chaotic dynamics are described by Ghil and Robertson (Ch 10), and Donald Johnson reviews the energetics of the equilibrium climate state in Chapter 22. Aspects of operational long-term weather and climate forecasts at the European Center for Medium Range Weather Forecasts (Hollingsworth, Ch 11) and the Japan Meteorological Agency (Tokioka, Ch 12) are also presented. The final chapter of the book is devoted to Arakawa’s discussion of the future development of GCMs, focusing on the vertical and horizontal discretization problems and issues related to the parameterization of the boundary layer, stratiform, and cumulus clouds. General Circulation Model Development: Past, Present, and Future is of great value to researchers active in the development and use of GCMs and to anyone interested in the history of GCM development. There is also discussion of the use of GMCs to study global warming, in which both sides of the heated debate are elucidated. The reference list at the end of each chapter is quite comprehensive and serves as a starting point for anyone interested further detail. The book would also serve as a text for a graduate course on modeling of the earth’s general circulation and is a valuable addition to any individual’s or library’s collection.

A new snow cover fraction parametrization for the ECHAM4 GCM

Snow cover fraction (SCF) has a significant influence on the surface albedo and thus on the radiation balance and surface climate. Long-term three dimensional simulations with general circulation models (GCMs) show that the SCF greatly affects the climate in the Northern Hemisphere. By means of both ground observations and remotely sensed data, several deficiencies in the SCF simulated by the current ECHAM4 GCM were identified: over mountainous areas a substantial overestimation in the SCF was found whereas flat areas showed a distinctly underestimated SCF. This work proposes a new parametrization of the SCF for use in GCMs. Evaluations illustrate that it is beneficial to distinguish between the following three terrains: (1) flat, non-forested areas, (2) mountainous regions and (3) forests. The modified SCF parametrization for flat, non-forested areas was derived by using global datasets of ground-based snow depth and remote sensing observations of snow cover data. A 3-dimensional ECHAM4 simulation showed that this modification raises the SCF by up to approximately 20%, mainly in areas with a relatively thin snow cover. The comparison between remotely sensed and simulated mean monthly surface albedo revealed a significant overestimation of the surface albedo in snow-covered mountainous areas. An extension of the current SCF parametrization in ECHAM4 to take into account mountain effects, based on the French climate model Arpege, yielded a close agreement with satellite-derived surface albedo. The adoption of the submodel for snow albedo, as used in the Canadian Land Surface Scheme (CLASS), combined with a newly developed simple snow interception model, demonstrated the ability to capture the main physical processes of snow-covered canopies, including the albedo. The validation of the new parametrization with Boreal Ecosystem-Atmosphere Study (BOREAS) field data showed that the modification is appropriate to capture the main features of the albedo over snow-covered forests during and after heavy snowfall events. Furthermore, the proposed modification has a beneficial impact on the delayed snow melt in spring, a well-known problem in many current GCMs: The simulated surface albedo over the boreal forests decreases by approximately 0.1 during winter and spring, which is in better agreement with ground-based observations. This induces a significant rise in the surface temperature over extended parts of Eurasia and North America in late spring, which subsequently yields a faster snowmelt and an accelerated retreat of the snow line.

Modeling climatic effects of anthropogenic carbon dioxide emissions: unknowns and uncertainties

A likelihood of disastrous global environmental consequences has been surmised as a result of projected increases in anthropogenic greenhouse gas emissions. These estimates are based on computer climate modeling, a branch of science still in its infancy despite recent substantial strides in knowledge. Because the expected anthropogenic climate forcings are relatively small compared to other background and forcing factors (internal and external), the credibility of the modeled global and regional responses rests on the validity of the models. We focus on this important question of climate model validation. Specifically, we review common deficiencies in general circulation model (GCM) calculations of atmospheric temperature, surface temperature, precipitation and their spatial and tem- poral variability. These deficiencies arise from complex problems associated with parameterization of multiply interacting climate components, forcings and feedbacks, involving especially clouds and oceans. We also review examples of expected climatic impacts from anthropogenic CO2 forcing. Given the host of uncertainties and unknowns in the difficult but important task of climate modeling, the unique attribution of observed current climate change to increased atmospheric CO2 concentration, including the relatively well-observed latest 20 yr, is not possible. We further conclude that the incau- tious use of GCMs to make future climate projections from incomplete or unknown forcing scenarios is antithetical to the intrinsically heuristic value of models. Such uncritical application of climate models has led to the commonly held but erroneous impression that modeling has proven or substantiated the hypothesis that CO2 added to the air has caused or will cause significant global warming. An assess- ment of the merits of GCMs and their use in suggesting a discernible human influence on global cli- mate can be found in the joint World Meteorological Organisation and United Nations Environmental Programme's Intergovernmental Panel on Climate Change (IPCC) reports (1990, 1995 and the up- coming 2001 report). Our review highlights only the enormous scientific difficulties facing the calcu- lation of climatic effects of added atmospheric CO2 in a GCM. The purpose of such a limited review of the deficiencies of climate model physics and the use of GCMs is to illuminate areas for improvement. Our review does not disprove a significant anthropogenic influence on global climate.

Efficient and Accurate Bulk Parameterizations of Air–Sea Fluxes for Use in General Circulation Models

Abstract Efficient and computationally inexpensive simple bulk formulas that include the effects of dynamic stability are developed to provide wind stress, and latent and sensible heat fluxes at the air–sea interface in general circulation models (GCMs). In these formulas the exchange coefficients for momentum and heat (i.e., wind stress drag coefficient, and latent and sensible heat flux coefficients, respectively) have a simple polynomial dependence on wind speed and a linear dependence on the air–sea temperature difference that are derived from a statistical analysis of global monthly climatologies according to wind speed and air–sea temperature difference intervals. Using surface meteorological observations from a central Arabian Sea mooring, these formulas are shown to yield air–sea fluxes on daily timescales that are highly accurate relative to those obtained with the standard algorithm used by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), where the ...

A parametrization of the effects of cloud and precipitation overlap for use in general-circulation models

The necessity for treating the effects of vertically varying cloud fraction when parametrizing microphysical processes in general-circulation models (GCMs) was recently highlighted by Jakob and Klein. In this study a parametrization to include such effects in a GCM is developed, and the new scheme is applied in the ECMWF global model. The basic idea of the new scheme is to separate the model's rain and snow fluxes into a cloudy and a clear-sky part. The scheme is tested using the subgrid-scale precipitation model of Jakob and Klein as a benchmark. The impact of the new scheme on the model climate is also investigated. It is shown that the new parametrization leads to a better representation of the effects of cloud and precipitation overlap, and that it alleviates most of the problems connected with their treatment in the current scheme. Due to the better treatment of cloud and precipitation overlap the new parametrization leads to a reduction in precipitation evaporation and an increase in accretion rates. When tested in seasonal model integrations the new scheme produces a drier tropical mid-troposphere with consequences for the hydrological cycle.

Parameterization of gravity wave momentum deposition based on nonlinear wave interactions: basic formulation and sensitivity tests

We present a practical method for parameterization of gravity wave drag based on the Medvedev and Klaassen (1995. Journal of Geophysical Research 100, 25,841–25,853) theory of gravity wave spectral evolution and saturation. The only tuning necessary for the scheme involves assumptions about the nature of the source spectrum of subgrid-scale gravity waves, i.e. the wavenumbers, launch heights and amplitudes of the spectral components. In this paper we employ a column model with representative distributions of mean wind and temperature to examine the sensitivity of the parameterized wave drag to the source spectra. For the range of anticipated variability of source spectra in the troposphere the scheme produces plausible results consistent with observations and with theoretical estimates. Computationally, the scheme is as efficient as a multiple-wave Lindzen scheme, and suitable for use in general circulation models (GCMs) of the atmosphere.

The relative importance of non‐sea‐salt sulphate and sea‐salt aerosol to the marine cloud condensation nuclei population: An improved multi‐component aerosol‐cloud droplet parametrization

AbstractThe effect of sub‐cloud aerosol on cloud droplet concentration was explored over the north Atlantic and east Pacific under a variety of low and high wind speed conditions. A relationship of the form of D = 197{1 ‐ exp(‐6.13 × 103 * A)} was found to fit best the relationship between cloud droplet concentration (D; cm‐3) and sub‐cloud aerosol concentration (A; cm‐3) under low to moderate wind conditions. A few noticeable deviations from this relationship were observed which occurred under moderate to high wind speed condition. Under these high wind conditions, sea‐salt aerosol provided the primary source of cloud nuclei due to their higher nucleation activity and larger sizes, even under sulphate‐rich conditions. Simple model simulations reveal that the activation of sea‐salt nuclei suppresses the peak supersaturation reached in cloud, and thus inhibits the activation of smaller sulphate nuclei into cloud droplets. A multi‐component aerosol‐droplet parametrization for use in general circulation models is developed to allow prediction of cloud droplet concentration as a function of sea‐salt and non‐sea‐salt‐(nss) sulphate nuclei. The effects of enhancing an existing nss‐sulphate cloud condensation nuclei (CCN) population with sea‐salt nuclei are to reduce the number of cloud droplets activated under high (polluted) sulphate conditions and to increase the cloud droplet concentration under low (clean) sulphate conditions. The presence of sea‐salt CCN reduces the influence of nss‐sulphate CCN on cloud droplet concentrations, and thus is likely to reduce the predicted effect of nss‐sulphate indirect radiative forcing.

A modelling study of aerosol processing by stratocumulus clouds and its impact on general circulation model parameterisations of cloud and aerosol

A model of the aqueous phase processing of an aerosol population undergoing multiple cycling through a stratocumulus (Sc) cloud layer is presented. Results indicate that a significant modification of the aerosol properties is achieved following the first cycle through cloud. In a polluted atmosphere, further modification in subsequent cycles is seen to be hydrogen peroxide limited unless there is a flux of ammonia entering the system through cloud base (CB). The modification of the aerosol population is seen to have little effect on the microphysics (specifically the cloud droplet concentration and effective radius) of the processing cloud. However, it enables processed aerosols to subsequently act as efficient cloud condensation nuclei (CCN) in less vigorous clouds (as a result of reducing the critical supersaturation required to activate them). The effects of variations in the internal mixture of soluble components of aerosols on the microphysics of clouds forming on them are also investigated using the cloud model. A (K2) parameterisation of the effects of variations in internally mixed nitrate loadings on the cloud droplet number concentration is presented. The effects of applying this K2 correction to the droplet number (derived from a parameterisation based on sulphate) for the presence of nitrate in aerosol have been investigated using the HadAM3 version of the Hadley Centre General Circulation Model (GCM). The effect on global annual mean simulations of the indirect forcing and effective radius is small, but more pronounced regionally. Suggestions (based on model results and observations) for parameterising the size distribution and in-cloud growth of aerosols for use in GCMs are presented.

A new parameterization scheme for the optical properties of ice crystals for use in general circulation models of the atmosphere

Abstract We present a new parameterization scheme for the optical properties of ice crystals, for use in general circulation models of the atmosphere (GCMs). The scheme consists of separate parameterizations for five different ice crystals shapes: hexagonal columns, hexagonal plates, bullet rosettes, spheres and planar polycrystals. The optical properties are specified as functions of the mean maximum dimension of the ice crystal size distribution using polynomial fits to a more elaborate scheme developed by the last author. The present scheme, based on 10th order fits, gives excellent accuracy and is efficient enough for use in GCMs. First, the scheme is tested in 1-dimensional models, showing a large sensitivity of albedo and emissivity to both ice crystal sizes and shapes. Also, long- and shortwave cloud radiative forcings at the top of the atmosphere are quite sensitive to those two parameters, in such a manner that the two effects do not cancel. It is shown that this may have a significant impact on climate sensitivity, through variations in net cloud forcing.

Two‐ and four‐stream optical properties for water clouds and solar wavelengths

Presented is a parameterization of the liquid water droplet optical properties for the solar spectrum. Two models are provided: a 4‐band model for use in general circulation models and a 31‐band model for use in higher spectral resolution investigations. The form of the parameterization is a simple extension of Slingo [1989], and the subdivision of wavelengths into bands is almost identical. The parameterization scheme is for all of the optical properties needed to perform 2‐ and 4‐stream calculations. The 4‐band model has the same spectral divisions as Slingo [1989], whereas the 31‐band model has finer resolution of the absorbing bands in the near infrared. The parameterized single‐scattering optical properties are accurate to within 0.5% compared to exact Mie calculations for both band models over the reff range from 5 to 40 μm. The layer radiative properties, from both two‐ and four‐stream methods, are almost always within 1% accuracy compared to calculations using exact optical properties as inputs. Also, it is shown that using the Henyey‐Greenstein phase function to obtain the higher‐order moments, used in four‐stream calculations, results in substantial error in the layer radiative properties compared to using exact Mie inputs.