Broken Cloud Fields Research Articles

Actinic fluxes in broken cloud fields

Photochemical processes in the atmosphere are driven by solar ultraviolet radiation. The photodissociation rate coefficients of atmospheric species are determined by the actinic flux, which is defined as 4π times the mean ultraviolet intensity. Because of the presence of clouds the actinic flux can change drastically throughout the atmosphere. Therefore clouds have large effects on photodissociation rate coefficients. At cloud top, photodissociation rate coefficients can be 300% higher than in clear sky conditions. We use Monte Carlo simulations to investigate the reflectance, the transmittance, and the actinic flux for cloud fields at various degrees of cloudiness. Scattering processes in the clouds are due to cloud particles only. We do not take absorption of radiation into account. The atmosphere outside the clouds is assumed to be completely transparent. The simulated reflectance and transmittance of plane‐parallel cloud fields and in broken cloud field conditions reproduce the results of previous model studies within statistical uncertainties. The results of actinic flux calculations for plane‐parallel cloud fields agree with the results obtained with a doubling‐adding algorithm. Horizontal and vertical actinic flux profiles in broken cloud fields are studied for various solar zenith angles and for different cloud optical thicknesses. The aim of the present model study is to obtain insight into the effect of broken cloud fields on the actinic flux.

Scattering Effects on Longwave Fluxes in Broken Cloud Fields

Abstract General circulation models use the flat black plate approximation to calculate longwave radiative transfer through broken cloud fields. This neglects both cloud geometry and longwave optical properties. It is known that cloud geometry is important in longwave transmission. Since the longwave single scattering albedo is as high as 0.75, it is also necessary to determine the importance of scattering effects. A Monte Carlo simulation was used to compute the upward and downward fluxes for simplified cloud fields with a range of cloud geometries and optical properties. Based on these fluxes, the effective cloud fractions were found. The results show that scattering can have a significant effect on fluxes and effective cloud fractions. The effects are largest for low cloud upward fluxes and high cloud downward fluxes. To attain high percentage accuracy, it is necessary to model both cloud geometry and scattering.

Solar Radiative Transfer in Clouds with Vertical Internal Inhomogeneity

Abstract To investigate the photon transport in inhomogeneous clouds, a Monte Carlo cloud model with internal variation of optical properties is developed. The data for cloud vertical internal inhomogeneity are chosen from published observations. Parameterization of the solar radiative properties of clouds is used in the form of the liquid water content and the effective radius of cloud droplet. The Monte Carlo simulations show that for overcast stratocumulus clouds, the differences in reflectance between the vertical inhomogeneous clouds and their planeparallel counterpart are very small (only about 1%). These differences can be enhanced up to 10% for large solar zenith angles, when the overcast clouds are separated into broken cloud fields. If the cloud coverage is large, the vertical inhomogeneity of clouds can cause about 7% increase in cloud absorption, which may help to explain the cloud absorption anomaly. Also, the parameterization of effective cloud amount for cloud absorption is discussed.

On the sampling of airborne measurements of radiative properties of cloud fields: Results from International Cirrus Experiment (ICE), 1987

During the first pilot phase of the International Cirrus Experiment (ICE) in September/October 1987 radiation measurements were carried out above and below broken cloud fields over the German Bight. Analyses of the measurements of 25.9.1987 (broken cumulus cloud field) showed an unexpected high “absorption” of solar radiation but not those of 28.9.1987 (stratocumulus/stratus cloud field). This might be explained by a partial negligence of cloud side factor under the viewing geometry of the pyranometer and other reasons. The light guide effect (“leakage”) within holes can account for the depletion of cloud reflectance and the enhancement of cloud transmittance at large cloud cover, as observed during the measurements on 28.9.1987. The fluctuation of the irradiances, which can be characterized by the intermittency parameter, has been related to the corresponding cloud cover by a regression equation. On the condition of similarity, this scaling parameter derived from sample measurements could be extrapolated to that of the whole cloud field. Thus, the estimation of the irradiance spectrum for the whole cloud field may be possible, from samples taken with aircraft and imaging data.

Effects Due to Cloud Geometry on Biases in the Albedo Derived from Radiance Measurements

Abstract The usual procedure in determining the earth's radiation budget is that observed radiances are first used to derive cloud properties. The properties are then used in a one-dimensional model to deduct cloud albedos. This conventional procedure yields more accurate estimates than a simple scheme that uses a bidirectional function. The procedure is, however, known to give rise to biases in the inferred albedos. In order to assess the effects due to cloud geometry on the biases, the reflected radiances are calculated for broken cloud fields, making use of a Monte Carlo model. The results of the calculations indicate that the anisotropy of the upwelling radiance from broken clouds differs markedly from that of plane-parallel clouds, which gives rise to the biases in the inferred albedos. Potential errors in the inferred albedos for broken clouds are, however, significantly reduced by the choice of appropriate viewing directions at which there are less effects due to the cloud geometry. In general, the...

Reflectance of Broken Cloud Fields: Simulation and Parameterization

The transfer of solar irradiance in plane parallel and broken cloud fields is simulated using a Monte Carlo method. The angular distribution pattern of radiances exiting the cloud layer is studied with varying cloud geometries, optical thicknesses, cloudiness, and solar zenith angles. A rather large anisotropy of the reflected flux is found, usually increasing with solar zenith angle and with patterns that strongly depend on cloud geometry. The main features are (1) a local maximum of reflected intensity in the forward direction for all cases, (2) a limb darkening for the plane parallel case, and (3) a limb brightening and a local maximum of reflected intensity in the backward direction for broken clouds. A parameterization for the azimuth-average reflectance function is developed. It reproduces the Monte Carlo simulation with a reasonable accuracy and illustrates that, when azimuthally averaged, the reflectance function is dominated by side viewing and intercloud shadowing effects.

Solar Radiative Fluxes for Broken Cloud Fields above Reflecting Surfaces

Abstract A statistical bidirectional method for including the effects of underlying reflecting surfaces in Monte Carlo simulations of atmospheric photon transport is presented. It is illustrated for the idealized Lambertian surface and a general particulate surface. It is shown that the simple Lambertian surface should be an adequate approximation (to mildly anisotropic surfaces) for calculation of overall fluxes for broken cloud fields. The problem of incorporating multiple internal reflections of solar radiation between cloud and surface is reviewed, and cloud field reflectances to upwelling radiation rk as a function of the number of internal reflections k are calculated. Unlike plane-parallel and regular arrays of cloud, all rk are generally not equal for more realistic scaling cloud fields. Strictly speaking, this prohibits use of the familiar geometric sum formulas for flux calculation in a multiple reflecting system. Fortunately, though fortuitously, the geometric sum formulas can be used in such c...

Aircraft measurements of the solar absorption by broken cloud fields: A case study

Aircraft measurements of the solar absorption by broken cloud fields: A case study

Aircraft measurements of the solar absorption by broken cloud fields: A case study

AbstractMeasurements of broad‐band radiation at solar (0.3–3.0 μm) and near‐infrared (0.7–3.0 μm) wavelengths are presented for three flights of the Meteorological Research Flight C130 aircraft in broken cloud fields. The solar absorption is determined from observations made above cloud top and below cloud base, using data in the two spectral bands to minimize errors of cloud sampling between different runs and to reduce the scatter due to cloud edge effects. The method assumes that cloud layer absorption in the visible region is negligible compared with that in the near‐infrared. This allows average absorption to be calculated for small sections of the aircraft track, which will generally include both fully clear and totally overcast cases. The smallest absorption measured is about 2%, in agreement with model results for the clear atmosphere. The largest absorption was found to be 8–12% (for a maximum cloud depth of 1 km), which is similar to measurements reported for layer cloud. An additional finding is that absorption by the broken cloud field varies uniformly with albedo, indicating that the parametrization of absorption in numerical models can be adequately described as a simple function of the albedo in partially cloudy areas.

Parameterization of Reflectivity for Broken Cloud Fields

To parameterize the reflected fluxes from broken cloud fields, Monte Carlo calculations are performed for several cumulus cloud fields. The results with the effects of cloud-cloud interaction and enhanced illuminated area are analyzed. A general relationship between the reflectivity of an isolated cloud and that of finite cloud fields is developed using the above two effects. From this relationship, the effective cloud fraction is parameterized. The result shows that the effective cloud fraction is a nonlinear function of the square of that value and the enhanced illuminated area. This parameterization is examined for cuboidal cylindrical, capped cylindrical and hemispherical cloud geometries arranged in regular, random and checkerboard patterns. The results show that the parameterization gives reflected fluxes that are considerably more accurate than these obtained with the plane-parallel approximation.

The albedo of broken cloud fields

AbstractA Monte Carlo model of short‐wave radiative transfer in a broken cloud field is described. Output from the model is presented and the effect of cloud shape and depth, droplet absorption and water vapour absorption on the effective fractional cloud cover, Ne, is investigated. Inhomogeneities are introduced into the cloud field in the form of a variable cloud width and a cloud optical depth which is dependent upon cloud cover. It is shown that these inhomogeneities lead to much better agreement between the modelled values of Ne and experimental values derived from data collected with the Meteorological Research Flight Hercules. the conclusion is that Ne is a nonlinear function of the fractional cloud cover, N, which may be approximated by the relationship Ne = N2.

On the sampling of airborne reflectance measurements above broken cloud fields

The geometry of airborne reflectance measurements above broken cloud fields is simulated for various flight levels, artificial and natural cloud distributions with a simple ray-tracing technique. A similarity relation, which uses the cloud structure within the field of view of the pyranometer below the aircraft, could be derived to extrapolate from these airborne samples to averages over larger areas, which for instance are covered by several pixels of satellite measurements. The data probes were obtained during the first flight of the metric camera (MOMS-O1) in the “Shuttle Pallet Satellite”. The application of this method in field experiments requires areal overviews (photographs from aircraft or satellites) of the experiment site to be sampled from aircraft.

The Effect of Cloud Sides on Reflected Solar Radiation as Deduced from Satellite Observations

We report the observation of a feature that is characteristic of the reflection of solar radiation from absorbing, finite clouds. When absorption takes place, more radiation can be reflected by broken cloud fields than by extensive unbroken cloud fields. We observe this feature in solar radiation at 3.7 μm reflected by low-level, single-layered systems of water clouds over the Pacific Ocean. Interpreting the effect as due to geometrical factors, we note that absorption causes the reflected radiances to be highly anisotropic, so that they are generally greater from the sun-facing cloud sides than from the cloud tops. Diffusive leakage of radiation through the cloud sides is also reduced, and as a result maximum reflectivities occur in situations that maximize the contributions to the reflected radiation from the sides relative to that from the tops. Interpreting the effect as due to changes in liquid water content and cloud droplet sizes, we note that the observations at 0.63, 3.7 and 11 μm are consistent with a cloud model in which the liquid water content and droplet sizes are greater in the cloud centers and smaller at their edges. We present theoretical calculations that qualitatively support both interpretations.

Anisotropy of Longwave Radiation Emergent from a Broken Cloud Field and its Effect on Satellite Estimates of Flux

Abstract Simple geometric considerations suggest that in the same way as broken cloud cover appears practically total near a ground observer's horizon, the anisotropy of the longwave radiation emitted to space from an area of broken cloud should be substantially enhanced. Using a simple model consisting of parallel bands of “black” clouds of rectangular cross section, imbedded in an otherwise horizontally homogeneous vertically stratified atmosphere, we evaluate the longwave radiance emergent in different directions, averaged over the cloud pattern, and find this expectation to be confirmed in a wide range of cases. At large zenith angles, the emergent radiance can exhibit considerable dependence on azimuth, relative to the direction of the band structure. Although such azimuthal dependence practically disappears for many 3-dimensional broken cloud fields (e.g., hexagonal cells), the enhanced zenith angle dependence remains. This can be a source of bias in cloud extractions or Earth radiation budget determinations based on geostationary satellite data, since zenith angle is directly related to geographical location of the point under observation. With our model cloud arrays, and considering radiative transfer in the atmosphere, we have evaluated the bias in the broad-band longwave (3–100 μm) radiant exitance values derived, assuming horizontal homogeneity, from either broad-band or narrow-band (10.5–12.5 μm) radiance data. We find that the bias for near-nadir observations can be as large as 20% (40 W m−2) in extreme cases involving bands of opaque high-level cloud over warm ground. It may remain substantial in situations which are encountered in nature. We examine the dependence of the broken cloud effects on the geometrical parameters of the field. We discuss how such effects might be detected and how they can influence determinations of Earth radiation budget components and of cloud cover. We note that “brokenness” will be a factor in cloud-radiation feedback.

The angular distribution of infrared radiances emerging from broken fields of cumulus clouds

Infrared radiances were simultaneously measured from broken cloud fields over the eastern Pacific Ocean by means of the eastern and western geostationary satellites. The measurements were compared with the results of models that characterized the clouds as black circular cylinders disposed randomly on a plane and as black cuboids disposed in regular and in shifted periodic arrays. The data were also compared with the results obtained from a radiative transfer model that considered emission and scattering by a regular array of periodic cuboidal clouds. It was found that the radiances did not depend significantly on the azimuth angle; this suggested that the observed cloud fields were not regular periodic arrays. However, the dependence on zenith angle suggested that the clouds were not disposed randomly either. The implication of these measurements on the understanding of the transfer of infrared radiances through broken cloud fields is considered.