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Jean-Jacques Morcrette

Abstract

A new radiation package, shown to correct most of the systematic errors of the operational ECMWF radiation scheme, has been extensively tested in the ECMWF forecast model. Improvements in the clear-sky fluxes and radiative heating/cooling rate profiles mainly stem from a better representation of the transmission functions. New cloud optical properties have been set from detailed narrowband calculations for a more realistic model cloud.

Results indicate a greater overall sensitivity of the model to cloud-radiation interactions. Radiative cooling in the subtropics is increased. A decreased radiative cooling in the higher layers of the tropics is caused by larger longwave impact of the high level clouds. An increase in the radiative energy available at the surface and an overall cooling of the troposphere generate larger turbulent heal fluxes. All these changes contribute to a higher level of convective activity, resulting in a more energetic hydrological cycle and a more active model with higher levels of zonal and eddy available potential energy and of eddy kinetic energy at all wavenumbers. The increased contrast in the deposit of radiative energy between land and ocean, as well as between clear-sky and cloudy areas, improves the distribution of diabatic heating. The warm bias in stratospheric temperature is greatly reduced. The divergence in the tropics is larger and does not weaken as much after a few days of integration as with the old operational radiation scheme, thus improving the Hadley circulation. The radiation budget at the top of the atmosphere is now in good agreement with satellite observations. Several other systematic errors of the ECMWF model are also partially corrected.

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Jean-Jacques Morcrette

Abstract

The surface downward longwave radiation, computed by the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast system used for the ECMWF 40-yr reanalysis, is compared with surface radiation measurements for the April–May 1999 period, available as part of the Baseline Surface Radiation Network (BSRN), Surface Radiation (SURFRAD), and Atmospheric Radiation Measurement (ARM) programs. Emphasis is put on comparisons on a 1-h basis, as this allows discrepancies to be more easily linked to differences between model description and observations of temperature, humidity, and clouds. It is also possible to compare the model and observed temporal variability in the surface radiation fluxes.

Comparisons are first carried out at locations for which the spectral model orography differs from the actual station height. Sensitivity of the model fluxes to various algorithms to correct for this discrepancy is explored. A simple interpolation–extrapolation scheme for pressure, temperature, and specific humidity allows the improvement of model calculations of the longwave surface fluxes in most cases.

Intercomparisons of surface longwave radiation are presented for the various longwave radiation schemes operational since the 15-yr ECMWF Re-Analysis (ERA-15) was performed. The Rapid Radiation Transfer Model of Mlawer et al., now operational at ECMWF, is shown to correct for the major underestimation in clear-sky downward longwave radiation seen in ERA-15.

Sensitivity calculations are also carried out to explore the role of the cloud optical properties, cloud effective particle size, and aerosols in the representation of the surface downward longwave radiation.

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Jean-Jacques Morcrette

Abstract

The author’s explore the implications of the temporal and spatial sampling of the radiation fields and tendencies upon the fields produced by the ECMWF system in operational-type forecasts, four-month seasonal integrations, and analyses. The model is shown to be much more sensitive to economies in the temporal than in the spatial description of the cloud–radiation interactions.

In 10-day forecasts, the anomaly correlation of geopotential shows little sensitivity to a more complete representation of the cloud–radiation interactions, but temperature errors display a stronger dependence on the temporal representation. The difference increases with height, particularly in the tropical areas where interactions among convection, clouds, and radiation dominate. In pointwise comparisons over five days, the approximate temporal representation introduces only small differences in total cloudiness, surface temperature, surface radiation, and precipitation.

In four-month seasonal simulations, the small errors seen in 10-day forecasts build up and a better temporal resolution of the radiation produces a colder stratosphere through cloud–radiation–convection interactions. The spatial sampling in the radiation computations appears beneficial to the operational model, inasmuch as, close to the surface, it smooths an otherwise wavy radiative forcing linked to the spectral representation of the surface pressure.

The impact of the temporal/spatial sampling in the radiation calculations is usually much weaker in the analyses when and where observational data are available, but can be felt if the density of observations becomes smaller. On the contrary, the effect of the temporal/spatial interpolation is important on the sensitivity parameters derived from perpetual July simulations with perturbed SSTs.

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Jean-Jacques Morcrette

Abstract

In an attempt to validate the ECMWF model’s cloudiness, model output has been processed to reproduce satellite measurements as closely as possible. Brightness temperatures in the longwave window channel of Meteosat are simulated from cloudiness, temperature, and humidity fields produced by the forecast model, and compared to the equivalent ISCCP-B3/CX observations over a 5-day period in July 1983.

On an instantaneous basis, clear-sky brightness temperatures from the model are generally in good agreement with observations, but errors arise from inaccuracies of the radiation transfer scheme, uncertainties in the input temperature and humidity profiles, and inadequate retrieval of the surface brightness temperature.

The diurnal cycles of surface temperature and cloudiness, over a number of limited 100° × 10° areas representative of different cloud regimes, are studied using evolution histograms of window brightness temperature. Comparison of histograms allows various deficiencies in the parameterization of the surface-cloud-radiation interactions to be pinpointed. Over tropical land areas, the model correctly reproduces the observed decrease in the amplitude of the diurnal cycle of surface temperature when the vegetation cover increases. The land surface temperature generally displays the correct phase but with too small an amplitude, linked to a combination of too little downward shortwave radiation, too large a surface albedo, or inadequate ground thermal resistance and surface-air coupling. Model brightness temperatures over high clouds are generally too high, showing deficiencies in the diagnosed cloud cover and cloud liquid-water content. The vertical extent of the convective cloudiness is rather well represented, but the deep clouds over land generally dissipate 6 h too early.

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Jean-Jacques Morcrette

Abstract

The cloud and radiation fields produced by the operational ECMWF forecasts are assessed using observations from the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) site over the April–May 1999 period. Over the first 36 h of the forecasts, most of the model fields, taken over a 24-h time window (either 0–24, 6–30, or 12–36 h) are generally in good agreement with each other. Comparisons of model fields taken from any such 24-h time window with observations are therefore representative of the quality of the ECMWF model physical parameterizations. The surface radiation fluxes are assessed separately for clear-sky, overcast, and whole-sky situations. For clear-sky fluxes, differences between model and observations are linked to differences in humidity and temperature profiles, the characterization of aerosols, and potential problems in the radiation schemes. For clear-sky conditions, the downward longwave radiation is usually within the accuracy of the measurements. For overcast conditions, the agreement with observations is also usually good. On the other hand, the downward shortwave radiation is overestimated, whatever the conditions. Although this might be partly due to uncertainties in the aerosol content, the clear-sky overestimation of the downward shortwave radiation, when aerosols are specified from climatic values or observations, indicates an underestimation of the gaseous absorption. Model cloud occurrences and boundaries over the SGP Central Facility are compared with similar quantities derived from radar and micropulse lidar observations. Model cloud water is tentatively assessed through comparisons with the radar reflectivity measurements. Systematic deficiencies in the surface radiation fields in the presence of clouds are discussed with respect to differences between the model and observed cloud characteristics. Given the TL319 resolution of the ECMWF model at the time of the comparisons, both the day-to-day and within-the-day temporal variability are captured reasonably well by 24-h forecasts that include cloud–radiation interactions with 1-h time resolution. However, most of the differences with observations can be traced back either to deficiencies in the clear-sky shortwave radiation scheme or to problems in the cloud fraction and/or cloud water content.

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Jean-Jacques Morcrette and Yves Fouquart

Abstract

Using the shortwave radiation scheme of Fouquart and Bonnel that accounts for scattering and absorption by gases and cloud particles, we study the effect of varying the assumption for the overlap of partially cloudy layers, and the resultant impact upon the heating rate profile, planetary albedo, net flux at the surface, and atmospheric net absorption. In this study, we consider the maximum, minimum, and random overlap assumptions and a radically simple scheme to approximate the radiative effects of a random overlapping of clouds. This simple scheme involves linear combinations of clear and cloudy reflectivities and transmissivities within a layer, and gives, respectively, fluxes and heating rates with maximum differences of 5% and 0.1 K day−1 compared to similar quantities obtained from a full calculation assuming a random overlapping of cloud layers. This former approach, however, is much more time efficient (five times faster for a 3-cloud atmosphere, three times faster in a full-size GCM).

Compared to the random assumption, the maximum overlap assumption gives smaller planetary albedo and larger net flux at the ground, whereas larger planetary albedo and smaller net flux at the ground result from the minimum overlap assumption. These differences tend to smooth out for larger values of the surface reflectivity. Systematic difference in the radiative forcings of a GCM due to these different cloud overlap assumptions largely vary with the cloud generation scheme.

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Jean-Jacques Morcrette and Christian Jakob

Abstract

The role of the cloud overlap assumption (COA) in organizing the cloud distribution through its impact on the vertical heating/cooling rate profile by radiative and precipitative/evaporative processes is studied in a series of experiments with a recent version of the ECMWF general circulation model, which includes a prognostic cloud scheme.

First, the radiative forcing initially obtained for different COAs (maximum, MAX; maximum-random, MRN;and random, RAN overlap) is discussed from results of one-dimensional radiation-only computations. Ensembles of TL95 L31 simulations for the winter 1987/88 (November–December–January–February) are then used, with the three different overlap assumptions applied on radiation only (RAD), evaporation/precipitation only (EP), or both (EPR). In RAD and EPR simulations, the main effect of a change in COA is felt by the model through the change in radiative heating profile, which affects in turn most aspects of the energy and hydrological budget. However, the role of the COA on the precipitation/evaporation, albeit smaller, is not negligible. In terms of radiative fluxes at the top and surface in the RAD and EPR simulations, RAN differs much more from MRN than MAX does, showing that for this vertical resolution, the majority of the clouds appear more in contiguous layers than as independent layers.

Given the large sensitivity of both the model total cloud cover and surface and top-of-the-atmosphere radiation fields to the cloud overlap assumption used in the radiation and cloud scheme, it is very important that these quantities are not validated independently of each other, and of the radiative cloud overlap assumption. The cloud overlap assumption for precipitation processes should be made consistent with that for radiation.

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Alberto Troccoli and Jean-Jacques Morcrette

Abstract

Prediction of direct solar radiation is key in sectors such as solar power and agriculture; for instance, it can enable more efficient production of energy from concentrating solar power plants. An assessment of the quality of the direct solar radiation forecast by two versions of the European Centre for Medium-Range Weather Forecasts (ECMWF) global numerical weather prediction model up to 5 days ahead is carried out here. The performance of the model is measured against observations from four solar monitoring stations over Australia, characterized by diverse geographic and climatic features, for the year 2006. As a reference, the performance of global radiation forecast is carried out as well. In terms of direct solar radiation, while the skill of the two model versions is very similar, and with relative mean absolute errors (rMAEs) ranging from 18% to 45% and correlations between 0.85 and 0.25 at around midday, their performance is substantially enhanced via a simple postprocessing bias-correction procedure. There is a marked dependency on cloudy conditions, with rMAEs 2–4 times as large for very cloudy-to-overcast conditions relative to clear-sky conditions. There is also a distinct dependency on the background climatic clear-sky conditions of the location considered. Tests made on a simulated operational setup targeting three quantiles show that direct radiation forecasts achieve potentially high scores. Overall, these analyses provide an indication of the potential practical use of direct irradiance forecast for applications such as solar power operations.

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Jean-Jacques Morcrette, George Mozdzynski, and Martin Leutbecher

Abstract

A specific interface between the radiation transfer calculations and the rest of the ECMWF model was introduced in 2003, potentially providing substantial economy in computer time by reducing the spatial resolution at which radiation transfer is evaluated, without incurring some of the deficiencies produced by the sampling strategy previously used in the ECMWF model. The introduction of a new more-computer-intensive radiation package (McRad) in June 2007 has led to a differentiated use of this interface depending on the applications. The history of the interface, how it is used, and its impact when using the new radiation scheme are discussed here. For a given model resolution, the impact of a lower-resolution radiation grid on the model behavior is studied here, in the context of 10-day forecasts at high resolution (TL799L91), of medium-resolution forecasts (TL399L62) used in the Ensemble Prediction System (EPS), and of low-resolution simulations (TL159L91) as used for model development and seasonal forecasts with an interactive ocean. Results for the high-resolution forecasts are compared in terms of objective scores and of the quality of “surface” parameters (total cloud cover, 2-m temperature and specific humidity, and 10-m wind) usually verified in a meteorological context. For the medium-resolution forecasts, the impact of the radiation grid is studied in terms of the potential increase in the efficiency of the EPS system without deteriorating the probabilistic skill. The impact of changes in the radiation grid resolution on the low-resolution versions of model is discussed in terms of cloud–radiation interactions and ocean surface temperature. For these operational applications, a radiation grid with a coarsening factor even as large as 2.5 for TL799L91 and TL159L91 and 4.2 for the EPS TL399L62 is shown to give results free of any systematic differences linked to the spatial interpolation and to the coarser resolution of both the inputs to and the outputs from the radiation transfer schemes.

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Marion Schroedter-Homscheidt, Armel Oumbe, Angela Benedetti, and Jean-Jacques Morcrette

The potential for transferring a larger share of our energy supply toward renewable energy is a widely discussed goal in society, economics, environment, and climate-related programs. For a larger share of electricity to come from fluctuating solar and wind energy-based electricity, production forecasts are required to ensure successful grid integration. Concentrating solar power holds the potential to make the fluctuating solar electricity a dispatchable resource by using both heat storage systems and solar production forecasts based on a reliable weather prediction. These solar technologies exploit the direct irradiance at the surface, which is a quantity very dependent on the aerosol extinction with values up to 100%. Results from present-day numerical weather forecasts are inadequate, as they generally use climatologies for dealing with aerosol extinction. Therefore, meteorological forecasts have to be extended by chemical weather forecasts. The paper aims at quantifying on a global scale the question of whether and where daily mean or hourly forecasts are required, or if persistence is sufficient in some regions. It assesses the performance of recently introduced NWP aerosol schemes by using the ECMWF/Monitoring Atmospheric Composition and Climate (MACC) forecast, which is a preparatory activity for the upcoming European Global Monitoring for Environment and Security (GMES) Atmosphere Service.

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