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Cloud Fraction (cloud + fraction)
Selected AbstractsThe surface radiation budget over North America: gridded data assessment and evaluation of regional climate modelsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 15 2009Marko Markovic Abstract While surface station observations of downwelling radiation offer accuracy at high temporal resolution, they do not easily allow an evaluation of model surface radiation budgets (SRB) over a wide geographical area. We evaluate three gridded SRB data sets against detailed observations from six surface radiation sites from the US surface radiation (SURFRAD) network. We subsequently use the most accurate surrogate observational data set for evaluation of model-simulated SRB. The data sets assessed are: ERA40,reanalysis of European Centre for Medium-Range Weather Forecasts (ECMWF), North American Regional Reanalysis (NARR),regional reanalysis of National Centres for Environmental Prediction (NCEP) and the surface radiative budget (SRB) from the International Satellite Cloud Climatology Project (ISCCP). Due to varying constraints with respect to temporal coverage of each data set, the evaluation period used in this study is 1996,2001, inclusive. The ERA40 downwelling longwave radiation (DLR) appears the most accurate surrogate observation, while both ERA40 and ISCCP show accurate results when the incoming shortwave radiation (ISR) is considered across the annual cycle. Winter DLR is less accurate in ISCCP with a positive bias and lack of very low (<200 Wm,2) flux values. The NARR SRB shows a large positive bias in the ISR throughout the annual cycle, linked to a significant underestimate of cloud cover. The ERA40 data are subsequently used to evaluate the simulated SRB in three regional climate models across North America. With respect to solar radiation, cloud cover biases are seen to be crucial, while for longwave fluxes both cloud fraction and in-cloud water content are important to simulate correctly. Inclusion of trace gases beyond H2O, CO2 and O3 appears necessary for an accurate calculation of clear-sky longwave radiation. Error compensation frequently occurs between the various components contributing to a model total-sky SRB. This is important to consider when trying to identify the underlying causes of errors in the simulated total SRB. Copyright © 2009 Royal Meteorological Society [source] The accuracy of downward short- and long-wave radiation at the earth's surface calculated using simple modelsMETEOROLOGICAL APPLICATIONS, Issue 1 2004J. W. Finch Estimates of the downward global solar and long-wave radiations are commonly made using simple models. We have tested the estimates produced by a number of these simple models against the values predicted by the radiative transfer model used in a climate model in order to determine their suitability for global applications. For clear sky, two simple models were comparable, but under cloudy conditions a combination of a clear-sky model based on the Angstrom-Prescott equation (which deals with the downwelling solar radiation) with a cloud transmissivity utilising total cloud fraction proved best. The lowest root mean square errors were 27 W m,2 for clear-sky global solar radiation and 90 W m,2 for cloudy conditions. For downward long-wave radiation in clear-sky conditions, the model of Garratt (1992) performed best with a root mean square error of 24 W m,2. However, in cloudy conditions the model of Idso & Jackson (1969) performed best with a root mean square error of 22 W m,2, and, as it performs nearly as well as that of Garratt (1992) in clear-sky conditions, it is probably the best choice. Copyright © 2004 Royal Meteorological Society. [source] Intercomparison of model simulations of mixed-phase clouds observed during the ARM Mixed-Phase Arctic Cloud Experiment.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 641 2009II: Multilayer cloud Abstract Results are presented from an intercomparison of single-column and cloud-resolving model simulations of a deep, multilayered, mixed-phase cloud system observed during the Atmospheric Radiation Measurement (ARM) Mixed-Phase Arctic Cloud Experiment. This cloud system was associated with strong surface turbulent sensible and latent heat fluxes as cold air flowed over the open Arctic Ocean, combined with a low pressure system that supplied moisture at mid-levels. The simulations, performed by 13 single-column and 4 cloud-resolving models, generally overestimate liquid water path and strongly underestimate ice water path, although there is a large spread among models. This finding is in contrast with results for the single-layer, low-level mixed-phase stratocumulus case in Part I, as well as previous studies of shallow mixed-phase Arctic clouds, that showed an underprediction of liquid water path. These results suggest important differences in the ability of models to simulate deeper Arctic mixed-phase clouds versus the shallow, single-layered mixed-phase clouds in Part I. The observed liquid-ice mass ratios were much smaller than in Part I, despite the similarity of cloud temperatures. Thus, models employing microphysics schemes with temperature-based partitioning of cloud liquid and ice masses are not able to produce results consistent with observations for both cases. Models with more sophisticated, two-moment treatment of cloud microphysics produce a somewhat smaller liquid water path closer to observations. Cloud-resolving models tend to produce a larger cloud fraction than single-column models. The liquid water path and cloud fraction have a large impact on the cloud radiative forcing at the surface, which is dominated by long-wave flux. Copyright © 2009 Royal Meteorological Society [source] Representation of 3D heterogeneous cloud fields using copulas: Theory for water cloudsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 636 2008Peter M. Norris Abstract It is shown that a general representation of GCM column cloud fraction within probability density function (PDF)-based statistical cloud parametrizations can be obtained using statistical functions called copulas that encapsulate the dependence structure of rank statistics in a multivariate system. Using this theory, a new Gaussian copula formulation of GCM cloud overlap is obtained. The copula approach provides complete flexibility in the choice of the marginal PDF of each layer's moisture and temperature, and, compared with earlier approaches, including the ,generalized overlap' approach, allows a far more general specification of the correlation between any pair of layers. It also allows easy addition of new layer variables, such as temperature, into the modelled grid-column statistics. As a preliminary test of this formulation, its ability to statistically describe a cloud-resolving model simulation of a complex multi-layer case-study, including both large-scale and convective clouds, is examined. The Gaussian copula cloud fraction is found to be significantly less biased than other common cloud overlap methods for this case-study. Estimates of several nonlinear quantities are also improved with the Gaussian copula model: the variance of condensed water path and the fluxes of solar and thermal radiation at atmospheric column boundaries. This first paper, though limited to the simpler case of water clouds, addresses subgrid-scale variability in both moisture and temperature. This work is envisaged as a first step towards developing a generalized statistical framework for GCM cloud parametrization and for assimilating statistical information from high-resolution satellite observations into GCMs and global analyses. Copyright © 2008 Royal Meteorological Society [source] Sensitivity of one-dimensional radiative biases to vertical cloud-structure assumptions: Validation with aircraft dataTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 608 2005F. Di Giuseppe Abstract Three representations of an observed stratocumulus system are generated by combining aircraft observations with a simple statistical model. The realizations differ in their representation of the vertical cloud structure while the horizontal variability is identical. In the control case (A) both the adiabatic liquid-water profile and the effect of wind-shear induced vertical decorrelation are represented. The second simulation (B) removes the wind-shear effect by assuming maximum overlap between adjacent layers. The third case (C) instead removes vertical variability by averaging the in-cloud liquid water for each column. For each of these scenes Monte Carlo simulated solar fluxes are compared against observed flux measurements. Cases A and B agree with observed (horizontal) flux variations within statistical uncertainty, while case C, which neglects vertical variability, is not able to reproduce the observed fluxes. The comparison between the radiative fields produced by these three representations of the stratocumulus system, calculated using a three-dimensional radiative-transfer solution, an independent pixel approximation (IPA) and a plane-parallel (PP) approach, shows substantial differences. Not accounting for the adiabatic liquid-water profile generates a systematic increase in the optical depth, , when the effective radius is computed from mean liquid-water content and droplet-number concentration, that can be responsible for a 5% increase in the reflection for shallow boundary-layer cloud systems (,,1). A much stronger effect in the radiative properties is produced by varying the cloud-overlap rule applied. While changing from maximum to random overlap does not introduce any variation in the optical depth of the cloud scene, it does introduce an increase in the reflection that is proportional to the relative change in total cloud fraction. The magnitude of these latter biases is comparable to that produced by unresolved horizontal variability. Moreover, it is shown that, when the vertical cloud structure is properly resolved, the effect of horizontal fluctuations is also reduced. Copyright © 2005 Royal Meteorological Society [source] A study of probability distributions of boundary-layer humidity and associated errors in parametrized cloud-fractionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 573 2001Jeremy D. Price Abstract Observations of boundary-layer-humidity probability-distributions made with the Meteorological Office tethered-balloon facility have been derived from nine representative days between 1996 and 1999. Measurements include those made at several heights simultaneously. Four types of distribution have been defined, including skewed and multimodal types. These have been analysed on scales relevant to sub-grid parametrizations. Four distribution functions have been fitted to the observations and their accuracy in predicting cloud fraction assessed. It was found that a standard beta-type distribution gave the best representation of the observed humidity-distribution. In addition, it was found that for grid scales greater than about 5 km, advection plays a significant role in determining the form of the humidity distribution, and that this must be accounted for in any parametrizations. The cloud-fraction error arising from errors in the mean total specific-humidity and saturation specific-humidity is also discussed. That from the former is found to be relatively large. [source] | ||||||||||