Shortwave Radiative Transfer Research Articles

Absorption of solar radiation by clouds and aerosols in the visible wavelength region

In connection with a number of recent publications on “anomalous” (“excessive”) absorption of shortwave radiation (SWR) by clouds, the data of aircraft observations of spectral and total SWR vertical profiles in the troposphere obtained within the CAENEX, GAAREX and FGGE programmes have been discussed. These data demonstrated persuasively the existence of substantial SWR absorption by clouds in the visible wavelength region. In this context, the results of simulation numerical medelling of the SWR transfer in clouds have been considered. These show a significant enhancement of the SWR absorption by clouds due to an increase of the mean free path of photons under an impact of multiple scattering in clouds. Another important consideration may be the impact of SWR-absorbing pollutants in clouds, especially soot aerosols.

The Second-Generation Global Forecast System at the Central Weather Bureau in Taiwan

The global forecast system (GFS), which started its operation in 1988 at the Central Weather Bureau in Taiwan, has been upgraded to incorporate better numerical methods and more complete parameterization schemes. The second-generation GFS uses multivariate optimum interpolation analysis and incremental nonlinear normal-mode initialization to initialize the forecast model. The forecast model is a global primitive equation model with a resolution of 18 sigma levels in the vertical and 79 waves of triangular truncation in the horizontal. The forecast model includes a 1.5-order eddy mixing parameterization, a gravity wave drag parameterization, a shallow convection parameterization, a relaxed version of Arakawa–Schubert cumulus parameterization, grid-scale condensation calculation, and longwave and shortwave radiative transfer calculations with consideration of fractional clouds. The performance of the second-generation GFS is significantly better than the first-generation GFS. For two 3-month periods in winter 1995/96 and summer 1996, the second-generation GFS provided forecasters with 5-day forecasts where the averaged 500-mb height anomaly correlation coefficients for the Northern Hemisphere were greater than 0.6. Observational data available to the GFS are much less than those at other numerical weather prediction centers, especially in the Tropics and Southern Hemisphere. The GRID messages of 5° resolution, ECMWF 24-h forecast 500-mb height and 850- and 200-mb wind fields available once a day on the Global Telecommunications System are used as supplemental observations to increase the data coverage for the GFS data assimilation. The supplemental data improve the GFS performance both in the analysis and forecast.

The Second–Generation Global Forecast System at the Central Weather Bureau in Taiwan

Abstract The global forecast system (GFS), which started its operation in 1988 at the Central Weather Bureau in Taiwan, has been upgraded to incorporate better numerical methods and more complete parameterization schemes. The second-generation GFS uses multivariate optimum interpolation analysis and incremental nonlinear normal-mode initialization to initialize the forecast model. The forecast model is a global primitive equation model with a resolution of 18 sigma levels in the vertical and 79 waves of triangular truncation in the horizontal. The forecast model includes a 1.5-order eddy mixing parameterization, a gravity wave drag parameterization, a shallow convection parameterization, a relaxed version of Arakawa–Schubert cumulus parameterization, grid-scale condensation calculation, and longwave and shortwave radiative transfer calculations with consideration of fractional clouds. The performance of the second-generation GFS is significantly better than the first-generation GFS. For two 3-month periods in winter 1995/96 and summer 1996, the second-generation GFS provided forecasters with 5-day forecasts where the averaged 500-mb height anomaly correlation coefficients for the Northern Hemisphere were greater than 0.6. Observational data available to the GFS are much less than those at other numerical weather prediction centers, especially in the Tropics and Southern Hemisphere. The GRID messages of 5° resolution, ECMWF 24-h forecast 500-mb height and 850- and 200-mb wind fields available once a day on the Global Telecommunications System are used as supplemental observations to increase the data coverage for the GFS data assimilation. The supplemental data improve the GFS performance both in the analysis and forecast.

Radiative forcing and temperature trends from stratospheric ozone changes

Detailed shortwave and longwave radiative transfer models are used to calculate the radiative forcing and temperature trends due to stratospheric ozone depletion. These were calculated using the fixed dynamical heating approximation to adjust the stratospheric temperatures. Recent estimates of stratospheric ozone loss between 1979 and 1991 (from solar backscattered ultraviolet (SBUV) and stratospheric aerosol and gas experiment (SAGE) instruments) and updated radiative transfer schemes are used to obtain improved estimates of the radiative forcings. An annually and globally averaged radiative forcing of −0.13±0.02 W m−2 decade−1 (−0.22±0.03 W m−2 for the 1979–1996 period) was found from SBUV total column ozone trends, applying a constant percentage ozone depletion to a 7 km thick layer directly above the tropopause. SAGE ozone trends gave forcing estimates of −0.10±0.02 W m−2 decade−1 (−0.17±0.03 W m−2 for the 1979–1996 period), although assumptions needed to be made about the choice of the vertical profile of the ozone depletion below 17 km. Using Dobson instrument trends from 1964 to 1996, the total ozone forcing could be as negative as −0.26±0.05 W m−2. The quoted error bars derive from uncertainties in the total ozone trends. Using these values, the stratospheric ozone change may have offset about 30% of the forcing due to increases in well‐mixed greenhouse gases since 1979, and about 15% of the forcing since 1964, at least on a global and annual mean. The ozone forcings are shown to be nearly a linear function of the ozone amount depleted from the atmospheric column, provided the vertical profile of the depletion remains constant. As in previous studies, it was found that stratospheric adjustment altered the sign of the ozone forcing by strongly cooling the lower stratosphere. It is shown that (depending on the vertical structure of ozone depletion) coolings of up to 0.4 K decade−1 can be found at altitudes of 35 km; this provides a mechanism for cooling the stratosphere nearly as large as that from well‐mixed greenhouse gas increases, at altitudes where ozone changes were previously thought not to strongly affect stratospheric temperature trends. Previous studies have examined the effect of ozone trends by looking at the response of the surface temperature to a fixed absolute ozone change at different heights in the atmosphere. We argue that using absolute ozone perturbations places an unrealistically large emphasis on ozone changes near the tropopause. As the vertical profile of ozone change is more often reported as a percentage change, we present the sensitivity of the surface temperature to constant percentage, rather than absolute, changes in ozone.

Changing trends in tropospheric methane and carbon monoxide: A sensitivity analysis of the OH-radical

A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.

A tractable physical model of shortwave radiation interception by vegetative canopies

Characterizing and understanding shortwave radiative transfer inside a vegetation canopy is essential because of the climatologically significant interactions between the Earth's atmosphere and the land surface. Particularly relevant is the radiation intercepted by the canopy elements which reveals the structural and optical vegetation properties. In this study, a tractable physical model of radiation interception is developed which accounts for multiple scattering to be applicable for near infrared. This model performs rather well whenever compared to the exact computation of the radiative transfer equation or a detailed code of radiative transfer. It seems to be, however, less relevant for thin vegetation canopies having large single‐scattering albedo. Its simple expression makes it useful to derive quantitative estimates of radiation intercepted by the canopy which will lead to improved retrievals of main vegetation parameters such as leaf area index.

Improved Simulation of Clear-Sky Shortwave Radiative Transfer in the CCC-GCM

Abstract The disposition of mean July clear-sky solar radiation in the Canadian Climate Centre second-generation general circulation model (CCC-GCMII) was analyzed by comparing top of the atmosphere (TOA) net fluxes with earth radiation budget experiment (ERBE) data and atmospheric and surface net fluxes with values inferred from Li's algorithm using ERBE data and European Centre for Medium-Range Weather Forecasts precipitable water data. GCMII tended to reflect ˜5 W m−2 too much to space. Corresponding atmospheric and surface absorption, however, tended to be too low and high, respectively, by ˜30 W m−2 over much of the Northern Hemisphere. These results were echoed when GCMII atmospheric absorption was compared to estimated results from Li's algorithm driven by GCMII TOA albedo and precipitable water. The latest version of the CCC-GCM (GCMIII) has numerous upgrades to its clear-sky solar radiative transfer algorithm, the most important of which involve water vapor transmittances and aerosols that tend t...

Cloud radiative forcing on surface shortwave fluxes: A case study based on Cloud Lidar and Radar Exploratory Test

Shortwave downward fluxes for selected stratus, cirrus, and mixed phase cloud cases are analyzed based on cloud and surface radiation measurements from the Cloud Lidar and Radar Exploratory Test conducted in the Denver‐Boulder area of Colorado during September–October, 1989. A medium resolution, discrete‐ordinate shortwave radiative transfer model is used to provide clear‐sky conditions and to examine the cloud shortwave radiative forcing. The model simulation indicates that for stratus clouds the effective radius increases with increasing liquid water path. For cirrus cloud simulation, the model results are within 10% agreement with the surface flux measurements. However, using the one‐dimensional plane‐parallel model, the model results are in poor agreement for the inhomogeneous mixed phase cloud case. Over the elevated observation site, the reduction in shortwave downward flux by clouds can be as large as 40% for a small cloud water path value of 20 g m−2. The variation in observed cloud shortwave forcing is highly correlated with the integrated cloud water path. The normalized (by the clear‐sky value) cloud shortwave forcing increases rapidly when the cloud water path is small. The rate of increase decreases, and the normalized cloud forcing approaches saturation when cloud water path becomes large. The magnitude of the saturation value depends on cloud optical properties. The variation in observed cloud forcing is consistent with the theoretical curve for cloudy atmospheric albedo variation. At a constant value of cloud water path, the normalized cloud forcing increases with solar zenith angle. The solar zenith angle effect is less significant for larger value of cloud water path.

Radiative transfer in stratified atmospheres: Development and verification of a unified model

A reliable and efficient discrete method is verified for multiple scattering, radiative-transfer calculations in vertically inhomogeneous, non-isothermal atmospheres in local thermodynamic equilibrium. The linear-in-optical-depth approximation to the Planck function is used to obtain accurate solutions for thermal radiation. We show that this approximation significantly improves computing efficiency while still maintaining adequate accuracy. Using this multiple-scattering scheme, we have constructed and validated a radiation model for stratified atmospheres, taking into account molecular (Rayleigh) scattering and absorption/emission. The exponential sum fitting of transmissions technique is utilized to parameterize gaseous absorption, thereby achieving a unified treatment of shortwave and longwave radiative transfer. To validate the model, we present computed fluxes and heating/cooling rate profiles for the five McClatchey atmospheres. The results are compared with other models having different spectral resolution (Δ s) and gaseous scattering and absorption/ emission structure. Specifically, we compare broad-band (Δ s > 100 cm -1), narrow-band (Δ s < 100 cm -1) and line-by-line computations and find that at the expense of accuracy by a few W-m -2 for flux or a few tenth °C/day for heating/cooling rate computations, the broad-band models are very fast and may be suitable for many applications. We also find that in spite of good agreement between fluxes (at the top and bottom boundaries) computed by different methods, the heating/cooling rate profiles may differ substantially due to compensating errors.

Methods of determination of atmospheric effects for interpretation of brightness‐field images of the “earth‐atmosphere” system

A mathematical model of shortwave radiation transfer in the atmosphere is being developed both in scalar (neglecting polarization) and vector forms. The equation is derived relating a space photo with a near‐surface photo via atmospheric transfer operators. These operators in scalar form are calculated by Monte‐Carlo method and used for Fourier filtration of image. Some analytical approximations also are given. Finally, method for synchronous evaluation measurements of atmospheric parameters is discussed.

An efficient scheme to calculate radiative transfer in mesoscale models

Mesoscale meteorological models require the parameterization of radiative transfer. In this paper a radiation scheme is presented which is found to be reasonably accurate for inclusion in a three-dimensional mesoscale model. The scheme provides atmospheric radiative heating/cooling and radiative fluxes at ground level for both clear and polluted or cloudy atmospheres. The parameterization of longwave radiation is based on the emissivity method and takes into account the spectral overlap in the 15 μm band. Shortwave radiative transfer is described by means of a new approach allowing the consideration of multiple reflections; this is found to be of particular importance in the case of a cloudy atmosphere. Approximative methods are applied to parameterize both extinction due to pollution and cloud transmission. By a proper numerical treatment the conservation of radiative fluxes is ensured and computational efficiency is achieved. Selected simulation results are used to illustrate the features of the proposed scheme and to point out its advantages compared to previous methods.

Thermische Energieflüsse an der Wasseroberfläche: Beispiel Sempachersee

Data collected every 20 minutes for 18 months by a meteorological buoy moored on Lake Sempach in Switzerland (maximum depth 86 m, surface area 14.1 km2) are used to calculate different processes contributing to the net heat flux between water and atmosphere. The processes considered are shortwave and longwave radiation, evaporation/condensation and sensible heat transfer. The temporal resolution of the measurements allows the evaluation of the processes occurring on three different time scales: diurnal variations, weather events of a few days and yearly cycles. The heat content of the lake is calculated from quasi-continuous measurements of water temperature at different depths. The yearly amplitude of the heat content is 1100·106 J/m2. Short-time variations of the heat flux determined from water temperature analysis agree well with the flux variations modeled using meteorological data. However, the latter generally underestimates the measured net heat flux in the long term. Wind measurements, together with the net heat flux, are used to calculate the Flux Richardson Number and the Monin-Obukov Length. Examples are given to show the predominant influence of the wind on the stratification of the upper water column and thus on the surface water temperature.

Simulation of heat and moisture transfer through a surface residue—soil system

A simulation model suitable for describing the dynamic aspects of mass and energy transfer in a soil—residue—atmosphere system has been developed and used to determine soil heat and water budgets. The model has been programmed in BASIC and uses numerical methods suitable for microcomputer applications. It displays a fundamental coupling of the surface residue to the soil—atmosphere system, and uses network analysis to describe heat and moisture transfer. Short-wave and long-wave radiative transfer, changes in energy status, rainfall interception by the residue, infiltration, redistribution, evaporation, and drainage are all accounted for. Daily input requirements include global short-wave radiation, maximum and minimum air temperatures, average wind speed, precipitation, and temperature and water content deep within the soil. General site, residue, and soil characteristics are also needed. Surface residues are treated as a thin layer with uniform horizontal distribution and residue loading rate is 3000 kg ha −1. Application of the model to energy and water budget studies using environmental data recorded at Pullman, WA during 1981/82 is discussed. Simulation results indicate that for the Pullman conditions surface residues decreased evaporation by roughly 36% when compared with simulations of bare soil evaporation. Monthly trends, however, indicate that the effectiveness of residues in conserving soil moisture decreases as the length of rain-free periods increases.

The Saudi Arabian heat low: Aerosol distributions and thermodynamic structure

Radiation and microphysical data are employed to deduce the bulk radiative and microphysical characteristics of the aerosol layer over the Saudi Arabian Peninsula and the adjacent Arabian Sea. Observed particle size distributions, total mass loadings, and vertical distributions of the dust are presented for five analysis periods. There is a large daily variability of the dust mass loading over the desert. The broadband radiative characteristics of the dust are inferred by comparing calculations for a dust free atmosphere with in situ broadband radiative measurements. The dust had no detectable effect on the longwave radiative fluxes, while it approximately doubled the clear sky shortwave absorption. The total shortwave absorption by the dust is dependent on the underlying surface albedo. The solar absorptance of the dust is expressed in terms of the dust mass loading and is incorporated into a broadband shortwave radiative transfer model to assess the surface and tropospheric energy budgets. For the desert cases the dust is largely responsible for a net 24 hour radiative energy convergence between, 530 and 800 mbar. A 24‐hour radiative energy convergence is also found over the ocean; however, the layer depth is shallower than the desert case, being limited to the top of the dust layer. The thermodynamic structure of the heat low is presented. Calculations of temperature advection from the Saudi Arabian Peninsula to the Arabian Sea, suggest that horizontal advection aids in offsetting a net 24 hour tropospheric energy convergence over the desert. This warm air advection, along with the transport of dust over the Arabian Sea, may assist in maintaining the low level inversion and shallow moist layer observed over the western and central Arabian Sea.

Radiative Heating Rates for Saharan Dust

Abstract A combined longwave and shortwave radiative transfer model was used to determine effects of Saharan dust on the radiative fluxes and heating/cooling rates in the atmosphere. Cases are treated for cloud-free and overcast conditions over the ocean and for cloud-free sky over desert. A benchmark comparison, made for the cloud-free ocean case between our calculations and those from Wiscombe’s detailed model, yielded results which were in close agreement. For moderately heavy dust amounts commonly measured over the Sahara and the eastern tropical Atlantic Ocean, typical calculated aerosol heating rates for the combined longwave and shortwave spectrum were in excess of 1 K day−1 for all three cases for most of the atmosphere beneath the top of the dust layer (500 mb). For the ocean case, maximum heating rates are found near the level of maximum concentration (700 mb), and also near the surface beneath the Saharan air layer (below 900 mb). Net fluxes determined at the top of the atmosphere for the ocean...

On the Use of Pyranometers in the Study of Spectral Solar Radiation and Atmospheric Aerosols

Abstract The zero-point deviation of Eppley precision spectral pyranometers is considered for different types of Schott filter glass hemispheres. It is found that the zero-point deviation may significantly affect the measurements of spectral global and diffuse fluxes, and may also lead to considerable errors in the estimates from pyranometer data of the influence of atmospheric aerosols on shortwave radiative transfer. When the zero-point deviation is properly corrected for, however, precision spectral pyranometers may yield precise measurements of even very low spectral irradiances.

The sea surface temperature deviation and the heat flow at the sea-air interface

The deviation of the sea surface temperature from the water temperature below is calculated as a function of the heat flow through the air-sea interface, using wind tunnel determinations of the effective thermal diffusivity in a boundary layer. The influence ofQ, shortwave radiation, andH, latent and sensible heat transfer plus effective back radiation, and U, wind speed, can be described by:T0 −Tw =C1 ·H/U +C2 ·Q/U. The calculated coefficients vary slightly with reference depth, Tables II and III. They are in good agreement with independent observations.