Partial Cloudiness Research Articles

Retrieval of Cloud Properties for Partly Cloudy Imager Pixels

Abstract Retrievals of cloud properties from satellite imagery often invoke the assumption that the fields of view are overcast when cloud-contaminated, even though a significant fraction are only partially cloud-covered. The overcast assumption leads to biases in the retrieved cloud properties: cloud amounts and droplet effective radii are typically overestimated, while visible optical depths, cloud altitudes, cloud liquid water amounts, and column droplet number concentrations are typically underestimated. In order to estimate these biases, a retrieval scheme was developed to obtain the properties of clouds for partially covered imager fields of view. The partly cloudy pixel retrieval scheme is applicable to single-layered cloud systems and invokes the assumption that clouds that only partially cover a field of view are at the same altitude as nearby clouds from the same layer that completely cover imager pixels. The properties of the retrieval are illustrated through its application to 2-km Visible and Infrared Scanner (VIRS) data from the Tropical Rainfall Measuring Mission (TRMM) for a marine stratocumulus scene. The scene was chosen because the cloud properties are typical of such systems based on an analysis of VIRS data for February and March 1998. Comparisons of properties for clouds in partly cloudy pixels and those for clouds in nearby overcast pixels reveal that the optical depths and droplet effective radii are generally smaller for the clouds in the partly cloudy pixels. In addition, for pixel-scale cloud fractions between 0.2 and 0.8, optical depth, droplet effective radius, and column droplet number concentration decrease slowly with decreasing cloud cover fraction. The changes are only about 20%–30%, while cloud cover fraction changes by 80%. For comparison, changes in optical depth and column number concentration retrieved using a threshold method decrease by 80%–90%. As long as the cloud cover in partly cloudy pixels is greater than about 0.1, uncertainties in the estimates of the cloud altitudes and of the radiances for the cloud-free portions of the fields of view give rise to uncertainties in the retrieved cloud properties that are comparable to the uncertainties in the properties retrieved for overcast pixels.

A buoyancy‐based mixing‐length proposal for cloudy boundary layers

AbstractThe parametrization of boundary‐layer clouds is approached here using a turbulence–kinetic energy closure, in conservative variables for non‐precipitating processes and through a new proposal for the turbulence mixing length. A subgrid condensation scheme able to treat partially saturated conditions in a model grid cell is also necessary. An extension of the buoyancy‐based Bougeault–Lacarrère mixing length is proposed, to take into account moist processes, and specifically the effects of phase changes. The asymmetry between upward and downward mixing processes is also considered for partial cloudiness conditions. The new proposal is tested for stratocumulus‐capped marine boundary layers and for shallow cumulus convection, both over land and sea. The validation ismade against large‐eddy simulation results obtained from model intercomparison cases, for stationary regimes as a first step, and then for the diurnal evolution of marine stratocumulus and shallow convection over land. Copyright © 2004 Royal Meteorological Society

The Partly Cloudy Presidency

Written against the backdrop of jingoistic patriotism, the “War on Terror” marketing slogan, and the draconian neoconservative agenda currently dominating American cultural politics, the author seeks to police the crisis of these wayward times through a series of narrative collages that intersect with and comment on our current historical conjuncture. Deploying quotes from and engaging in dialogue with voices from popular culture as a means of critiquing the false hall of mirrors that has become the Bush administration’s prevailing model of “truth” in post-9/11 America, the author’s words stand as a resounding alternative to the unquestioning voices of appeasement currently dominating mainstream corporate media.

A Simple Cloud Parameterization Derived from Cloud Resolving Model Data: Diagnostic and Prognostic Applications

A simple statistical parameterization of cloud water‐related variables that has been originally developed for nonprecipitating boundary layer clouds is extended for all cloud types including deep precipitating convection. Based on three-dimensional cloud resolving model (CRM) simulations of observed tropical maritime and continental midlatitude convective periods, expressions for the partial cloudiness and the cloud water content are derived, which are a function of the normalized saturation deficitQ1. It turns out that these relations are equivalent to boundary layer cloud relations described earlier, therefore allowing for a general description of subgrid-scale clouds. The usefulness of the cloud relations is assessed by applying them diagnostically and prognostically in a mesoscale model for a midlatitude cyclone case and a subtropical case, and comparing the simulated cloud fields to satellite observations and to reference simulations with an explicit microphysical scheme. The comparison uses a model-to-satellite approach where synthetic radiances are computed from the meteorological fields and are compared to Meteosat satellite observations both in the visible and the thermal infrared spectral channels. The impact of the statistical cloud scheme is most pronounced for shallow and deep convective cloud fields (where Q1 , 0), provided that the host models convection parameterization is able to correctly represent the ensemble average water vapor profile in the troposphere. The scheme significantly reduces the biases in the infrared and especially shortwave spectral range with respect to the explicit microphysical scheme. Furthermore, it produces more realistic (smooth) horizontal and vertical condensate distributions in both diagnostic or prognostic applications showing the potential use of this simple parameterization in larger-scale models.

Test of the Volume-of-Fluid Method on Simulations of Marine Boundary Layer Clouds

Abstract The impact of using grid-averaged thermodynamic properties (i.e., neglecting their subgrid variability due to partial cloudiness) to represent forcings for condensation or evaporation has long been recognized. In particular, numerical difficulties in terms of spurious oscillations and/or diffusion in vicinity of a cloud–environment interface have been encountered in most of the conventional finite-difference Eulerian advection schemes. This problem is equivalent to the inability of models to accurately track the cloud boundary within a grid cell, which eventually leads to spurious production or destruction of cloud water at leading or trailing edges of clouds. This paper employs a specialized technique called the “volume-of-fluid” (VOF) method to better parameterize the subgrid-scale advection process that accounts for the transport of material interfaces. VOF also determines the actual location of the partial cloudiness within a grid box. Consequently, relevant microphysical parameterizations in...

Parameterizations for Cloud Overlapping and Shortwave Single-Scattering Properties for Use in General Circulation and Cloud Ensemble Models

Abstract Parameterizations for cloud single-scattering properties and the scaling of optical thickness in a partial cloudiness condition have been developed for use in atmospheric models. Cloud optical properties are parameterized for four broad bands in the solar (or shortwave) spectrum; one in the ultraviolet and visible region and three in the infrared region. The extinction coefficient, single-scattering albedo, and asymmetry factor are parameterized separately for ice and water clouds. Based on high spectral-resolution calculations, the effective single-scattering coalbedo of a broad band is determined such that errors in the fluxes at the top of the atmosphere and at the surface are minimized. This parameterization introduces errors of a few percent in the absorption of shortwave radiation in the atmosphere and at the surface. Scaling of the optical thickness is based on the maximum-random cloud-overlapping approximation. The atmosphere is divided into three height groups separated approximately by ...

Evaluation of model generated cloud cover by means of satellite data

An automated cloud retrieval algorithm has been developed and applied to determine cloud cover from NOAA9 AVHRR (Advanced Very High Resolution Radiometer) satellite data. This satellite derived cloud cover is used to evaluate the model generated cloud cover provided by two different cloud cover parameterization schemes established in a 3-D-chemical transport model. In the standard version of this model cloud cover depends on rain rate for raining clouds and on the relative humidity at cloud base for fair weather clouds. In the second cloud cover parameterization scheme predictions of liquid water content and ice content in combination with values of water and ice content derived from several observations are used to generate the cloud cover by the model. Partial cloudiness is allowed to form when mesoscale relative humidity is less than 100%. The comparison of the model generated with the satellite derived cloud cover shows that the second cloud cover parameterization scheme substantially improves the determination of cloud cover by the model.

A Simple Parameterization of Cloud Water Related Variables for Use in Boundary Layer Models

Abstract A simple parameterization of cloud water related variables has been developed which can be used in meteorological models that use a prognostic equation for the turbulent kinetic energy. Based on the results of large-eddy simulations (LES), expressions are derived for the liquid water flux, the partial cloudiness, and the cloud water content as a function of the normalized saturation deficit. With these relations, which are based on LESs ranging from situations with small to moderate amount of cumulus clouds to a stratocumulus case, no further knowledge about distribution functions or skewness is needed.

A Parameterization for Land–Atmosphere-Cloud Exchange (PLACE): Documentation and Testing of a Detailed Process Model of the Partly Cloudy Boundary Layer over Heterogeneous Land

This paper presents a general description of, and demonstrates the capabilities of, the Parameterization for Land–Atmosphere–Cloud Exchange (PLACE). The PLACE model is a detailed process model of the partly cloudy atmospheric boundary layer and underlying heterogeneous land surfaces. In its development, particular attention has been given to three of the model's subprocesses: the prediction of boundary layer cloud amount, the treatment of surface and soil subgrid heterogeneity, and the liquid water budget. The model includes a three-parameter nonprecipitating cumulus model that feeds back to the surface and boundary layer through radiative effects. Surface heterogeneity in the PLACE model is treated both statistically and by resolving explicit subgrid patches. The model maintains a vertical column of liquid water that is divided into seven reservoirs, from the surface interception store down to bedrock. Five single-day demonstration cases are presented, in which the PLACE model was initialized, run, and compared to field observations from four diverse sites. The model is shown to predict cloud amount well in these while predicting the surface fluxes with similar accuracy. A slight tendency to underpredict boundary layer depth is noted in all cases. Sensitivity tests were also run using anemometer-level forcing provided by the Project for Inter-comparison of Land-surface Parameterization Schemes (PILPS). The purpose is to demonstrate the relative impact of heterogeneity of surface parameters on the predicted annual mean surface fluxes. Significant sensitivity to subgrid variability of certain parameters is demonstrated, particularly to parameters related to soil moisture. A major result is that the PLACE-computed impact of total (homogeneous) deforestation of a rain forest is comparable in magnitude to the effect of imposing heterogeneity of certain surface variables, and is similarly comparable to the overall variance among the other PILPS participant models. Were this result to be bourne out by further analysis, it would suggest that today's average land surface parameterization has little credibility when applied to discriminating the local impacts of any plausible future climate change.

Modeling of Trade Wind Cumuli with a Low-Order Turbulence Model: Toward a Unified Description of Cu and Se Clouds in Meteorological Models

Abstract A simple method is proposed to extend a low-order turbulence scheme including a subgrid-scale cloudiness scheme to represent not only nonconvective (stratiform) cloudiness and turbulence but also shallow, nonprecipitating cumulus convection by utilizing an appropriate subgrid-scale distribution function. A simple approach is chosen that avoids the knowledge of the skewness parameter. All cloud water-related variables (cloud water content, partial cloudiness, liquid water flux) are computed by interpolating linearly as a function of the saturation deficit between two limit cases: the stratocumulus case, which can be well represented by a Gaussian distribution function, and the trade wind cumulus case, characterized by a positively skewed distribution function with a skewness of 2. Comparisons of the scheme with a third-order turbulence scheme and large-eddy simulations (LES) for the Puerto Rico Field Experiment show satisfying results. It is shown that the liquid water flux term, which is strongly...

The Use of Partial Cloudiness in a Warm-Rain Parameterization: A Subgrid-Scale Precipitation Scheme

Abstract A method is proposed on how to handle the effects of partial cloudiness in a warm-rain microphysical scheme and how to generate subgrid-scale precipitation. The method is simple and concerns essentially two ideas: the use of the vertical distribution of the partial cloudiness and the use of environmental and cloud-scale values for the thermodynamic variables instead of their grid-mean values. It applies to any microphysical scheme. Here, the method has been applied to a warm-rain parameterization scheme that has been implemented in a mesoscale model using a statistical partial cloudiness scheme. Numerical tests have been done for two one-dimensional cases of boundary-layer cloudiness: a cumulus case and a case of a decoupled stratocumulus layer. The results show that the correct coupling of a partial cloudiness scheme and a microphysical scheme allows for a better description of the actual cloudiness and precipitation fields by ensuring a consistent computation of partial cloudiness, cloud water,...

Comparison of an Experimental NOAA AVHRR Cloud Dataset with Other Observed and Forecast Cloud Datasets

CLAVR [cloud from AVHRR (Advanced Very High Resolution Radiometer)] is a global cloud dataset under development at NOAA/NESDIS (National Environmental Satellite, Data, and Information Service). Total cloud amount from two experimental cases, 9 July 1986 and 9 February 1990, are intercompared with two independent products, the Air Force Real-Time Nephanalysis (RTNEPH), and the International Satellite Cloud Climatology Project (ISCCP). The ISCCP cloud database is a climate product processed retrospectively some years after the data are collected. Thus, only CLAVR and RTNEPH can satisfy the real-time requirements for numerical weather prediction (NWP) models. Compared with RTNEPH and ISCCP, which only use two channels in daytime retrievals and one at night, CLAVR utilizes all five channels in daytime and three at night from AVHRR data. That gives CLAVR a greater ability to detect certain cloud types, such as thin cirrus and low stratus. Designed to be an operational product, CLAVR is also compared with total cloud forecasts from the National Meteorological Center (NMC) Medium Range Forecast (MRF) Model. The datasets are mapped to the orbits of NOAA polar satellites, such that errors from temporal sampling are minimized. A set of statistical scores, histograms, and maps are used to display the characteristics of the datasets. The results show that the CLAVR data can realistically resolve global cloud distributions. The spatial variation is, however, less than that of RTNEPH and ISCCP, due to current constraints in the CLAVR treatment of partial cloudiness. Results suggest that if the satellite cloud data is available in real time, it can be used to improve the cloud parameterization in numerical forecast models and data assimilation systems.

Binormal Model of Ensemble Partial Cloudiness

Abstract A new formulation of partial cloudiness parameterization has been introduced that agrees with that for a random model in one limit and approaches the simple updraft/downdraft model of larger-scale models in the limit of very highly skewed flow. Each of the conserved variables, liquid potential temperature and total humidity, along with the vertical velocity, are assumed to have probability distributions that may be parameterized as combinations of two multivariate normal distributions. This allows the skewness of the variables to be controlled by the bias between the means of the two normals and their relative fractions. It also provides a smooth transition between the normal distribution and the two limiting delta function distribution of the updraft/downdraft model. Comparisons with large-eddy-simulation data show this new model to be valid over a much wider range of conditions than the single normal distribution. When a simple cloud-top entrainment instability (CTEI) analysis is made using the...

Contrasting microclimates among clearcut, edge, and interior of old-growth Douglas-fir forest

Clearcut, remnant old-growth forest patch, and edge are the three primary landscape elements in northwestern North America. Microclimatic information on this forest landscape is needed for both research and resources management purposes. In this paper, seasonal summaries and diurnal changes in air temperature and moisture, soil temperature and moisture, short-wave radiation, and wind velocity are quantified for recent clearcut (10–15 years old), edge, and adjacent interior old-growth Douglas-fir forest environments in southern Washington state, USA, over two growing seasons. Influences of local weather condition and edge orientation (relationship of edge to the azimuth) are also assessed. Over the growing season, daily averages of air and soil temperatures, wind velocity, and short-wave radiation are consistently lower, and soil and air moisture are higher, inside the forest than in the clearcut or at the edge. Daily differences (i.e. maximums minus minimums) of all variables are consistently lower in the forest. The microclimates at the edge and the clearcut show a variable relationship with regard to averages and differences. Between the edge and the forest, greater differences occur under clear sky conditions for air temperature, but under partial cloudy conditions for relative humidity and soil temperature. Edge orientation is critical in assessing solar radiation, soil moisture, and relative humidity. The highest variability in microclimate exists at the edge, rather than in either clearcut or interior forest, primarily because of the influences related to edge orientation. The supposition that edge microclimates are intermediate between clearcut and interior forest is consistently true only for wind velocity and solar radiation, not for temperature and moisture.

A Model of Marine Boundary-Layer Cloudiness for Mesoscale Applications

Abstract A one-dimensional version of a multilevel mesoscale model is used to represent the cloud-topped boundary layer (CTBL). Turbulent exchanges are parameterized with a prognostic equation for the turbulent kinetic energy and an improved length-scale formulation. Furthermore, the scheme is extended to give a statistical description of the subgrid-scale condensation with a one-and-a-half-order closure.Several observed reference cases are simulated in order to test the model against observational data and results obtained with a higher-order turbulence model. The latter one is used as a powerful approach for testing the closure of the second-order moments involved in the partial cloudiness scheme. Two of the reference cases are extracted from stratocumulus (Sc) observations off the coast of the United Kingdom with a purely buoyancy-driven and a purely shear-driven CTBL, respectively. The third experiment tries to reproduce a case of Californian Sc clouds where both turbulent effects are important. Final...

A study of a two-dimensional cloudiness transition during a cold air outbreak event

A two-dimensional version of a hydrostatic mesoscale model with a partial cloudiness scheme is used to study a cold air outbreak event during MASEX (Mesoscale Air-Sea Exchange experiment). The model produces a weak mesoscale circulation which is slightly advected offshore and simulates well an observed cloudiness transition zone of 80 km where the cloud cover ranges between 0 and 100%. It is shown that the cloud top entrainment instability criterion can explain the observed cloudiness phenomenon i.e., the transition from a cumulus field to a solid stratocumulus cloud layer. The usefulness of the partial cloudiness scheme is demonstrated by comparing the results to those obtained with a simple “all or nothing” condensation scheme. A model sensitivity study shows that the sea surface temperature and the horizontal advection speed control the surface heat and moisture fluxes and so explain the structure and horizontal extent of the cloudiness transition zone.

A three-dimensional large-scale cloud model: testing the role of radiative heating and ice phase processes

A time-dependent, three-dimensional, large-scale cloud model has been developed for the prediction of cloud cover, cloud liquid/ice water content (LWC/IWC), precipitation, specific humidity and temperature. Partial cloudiness is allowed to form when large-scale relative.humidity is less than 100%. Both liquid and ice phases are included in the model. The liquid phase processes consist of evaporation, condensation, autoconversion and precipitation. The ice phase processes include heterogeneous nucleation to generate ice crystals, depositional growth to simulate Bergeron-Findeisen's process, sublimation to deplete ice crystals, and gravitational settling of ice crystals. The radiative transfer parameterization scheme is based on a broadband method and involves the transfer of infrared and solar radiation in clear and cloudy regions. The broadband infrared emissivity, reflectivity, and transmissivity for cirrus clouds, as well as the broadband solar absorption, reflection, and transmission values for low, middle and high clouds are computed based on the cloud LWC and IWC interactively generated by the cloud model. Large amounts of satellite data, including cloud cover climatology derived from the US Air Force three-dimensional nephanalysis (3DNEPH) and earth radiation budget (ERB), have been processed into formats appropriate for verification. The 96-h model simulations of cloud cover, outgoing long-wave radiation (OLR), and cloud LWC have been verified against data analyzed from 3DNEPH, ERB and the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) satellite observations, respectively. The predicted cloud IWC is compared to in situ observations as well as to results from other studies. The predicted cloud and radiation results compare well with those analyzed from satellite data. Numerical experiments are carried out with and without radiative heating and ice phase processes in the cloud formation scheme. The inclusion of radiative heating produces a significant change in temperature, cloud cover and total cloud water content, while the inclusion of ice phase processes generates a substantial change only in total cloud water content. If the cloud LWC is sufficiently large to initiate effective Bergeron-Findeisen's processes, the total cloud water content decreases, indicating that gravitational settling is an efficient mechanism in reducing the cloud IWC. DOI: 10.1034/j.1600-0870.1992.t01-1-00001.x

Aspects of the determination of ocean wave parameters by means of an optoelectronic satellite sensor

In this paper, the task and mode of operation of an optoelectronic spaceborne sensor system with CCD lines and a digital 2-D Fourier transform device for the determination of such parameters of ocean waves, like wavelength and propagation direction, are outlined. Starting from both required and estimated parameters of the sensor system, the signal-to-noise ratio of spatial frequencies of wave representation is calculated outside the glitter range. Presupposed for the simulation was a sinusoidal wave pattern with different propagation directions in relation to the sensor viewing angle and the sun elevation angle. It turns out that the optimization of the sounding direction plays an important role for the determination of wave parameters at given sun angles. Furthermore, for the interesting range of wave parameters it is sufficient to analyse an area of about 5 × 5 km. Therefore it is possible to work under partial cloudy conditions too. Obtaining good estimates for the height of ocean waves seems to be difficult with that type of sensor. As a result from comparing some types of transforms, their implementation requirements into a real-time on-board processor, and their data compression estimates, the “bottlenecks” of a very sophisticated optoelectronic sensor system are characterized.

Radiative Transfer through Arbitrarily Shaped Optical Media. Part II. Group Theory and Simple Closures

Abstract This paper presents a formulation of the radiative transfer equation which allows for the distinction between various groups of spatial scales of variation that comprise the radiance field. Such a formulation provides a convenient means for studying the effects of spatial inhomogeneity and scale interaction on the radiative transfer. Notions of scale hierarchy and closure are introduced into the radiative transfer equation, and it is demonstrated how the customary treatment of partial cloudiness based on cloud amount as a weighting parameter is a special form of closure. Discussion of this particular closure and other assumptions relevant to this partial cloud treatment are presented. Another simple example of closure is described which allows for the treatment of spatial inhomogeneities as a new form of optical property. This concept is introduced into a two-stream model to demonstrate, in a gross way, the effects of inhomogeneities on radiative transfer. Comparisons with the more formal calcula...

Cloud screening for determination of land surface characteristics in a reduced resolution satellite data set

Abstract A spatial coherence method is developed for cloud screening of a reduced resolution data set over the U.S. Great Plains, The method is based upon a comparison of the spatial standard deviation of scene radiance for a particular observation time with that for clear sky conditions (the background variability). The procedure relies on the assumption that the presence of partial cloudiness increases spatial variability. The standard deviation thresholds are tabulated functions of space and time and are presented as the background variability maps. Complete cloud covers are screened by testing the computed skin temperatures and albedos against constant thresholds, whereas night-time cirrus and low levels of cloudiness are delected using the 3.7-11 μm brightness temperature differences. The cloud-screening scheme is tested on daytime and night-time data of July 1986.