Simulated Radiative Fluxes Research Articles

A statistical and process-oriented evaluation of cloud radiative effects in high-resolution global models

Abstract. This study evaluates the impact of atmospheric horizontal resolution on the representation of cloud radiative effects (CREs) in an ensemble of global climate model simulations following the protocols of the High Resolution Model Intercomparison Project (HighResMIP). We compare results from four European modelling centres, each of which provides data from “standard”- and “high”-resolution model configurations. Simulated radiative fluxes are compared with observation-based estimates derived from the Clouds and Earth's Radiant Energy System (CERES) dataset. Model CRE biases are evaluated using both conventional statistics (e.g. time and spatial averages) and after conditioning on the phase of two modes of internal climate variability, namely the El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). Simulated top-of-atmosphere (TOA) and surface CREs show large biases over the polar regions, particularly over regions where seasonal sea-ice variability is strongest. Increasing atmospheric resolution does not significantly improve these biases. The spatial structure of the cloud radiative response to ENSO and NAO variability is simulated reasonably well by all model configurations considered in this study. However, it is difficult to identify a systematic impact of atmospheric resolution on the associated CRE errors. Mean absolute CRE errors conditioned on the ENSO phase are relatively large (5–10 W m−2) and show differences between models. We suggest this is a consequence of differences in the parameterization of SW radiative transfer and the treatment of cloud optical properties rather than a result of differences in resolution. In contrast, mean absolute CRE errors conditioned on the NAO phase are generally smaller (0–2 W m−2) and more similar across models. Although the regional details of CRE biases show some sensitivity to atmospheric resolution within a particular model, it is difficult to identify patterns that hold across all models. This apparent insensitivity to increased atmospheric horizontal resolution indicates that physical parameterizations play a dominant role in determining the behaviour of cloud–radiation feedbacks. However, we note that these results are obtained from atmosphere-only simulations and the impact of changes in atmospheric resolution may be different in the presence of coupled climate feedbacks.

Simulating Radiative Fluxes through Southeastern Pacific Stratocumulus Clouds during VOCALS-REx

ABSTRACTTime series of solar and thermal infrared radiative flux profiles are simulated with the Rapid Radiative Transfer Model (RRTM) using a hierarchy of constraints from radar reflectivity and passive microwave cloud remote sensing measurements collected over a ship in the southeastern tropical Pacific Ocean (20°S) during the second leg of the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx). Incorporating additional constraints results in simulations of physically consistent radiative profiles throughout the atmosphere, especially within the cloud, where they are difficult to observe precisely. Simulated surface radiative fluxes are compared with those observed on the ship and by aircraft.Due to the strong Rayleigh scattering of drizzle drops compared to cloud droplets that absorb, emit, and scatter natural radiation, cloud radar reflectivity overestimates cloud liquid water content (LWC). As a result, clouds are optically too thick and transmission ratios are too low in simulations using radar LWC. Imposing a triangular (increasing linearly with height from zero at cloud base) LWC profile in agreement with microwave liquid water path (LWP) improves the simulation of the transmission ratio. Constraining the corresponding microphysical cloud effective radius to that retrieved from optical depth, LWP, and cloud thickness results in additional improvements to the simulations. Time series, averages, and composite diurnal cycles of radiative fluxes, heating rates, and cloud radiative forcing are presented.

Assessment of 1D and 3D model simulated radiation flux based on surface measurements and estimation of aerosol forcing and their climatological aspects

Ground reaching solar radiation flux was simulated using a 1-dimensional radiative transfer (SBDART) and a 3-dimensional regional climate (RegCM 4.4) model and their seasonality against simultaneous surface measurements carried out using a CNR4 net Radiometer over a sub-Himalayan foothill site of south-east Asia was assessed for the period from March 2013–January 2015. The model simulated incoming fluxes showed a very good correlation with the measured values with correlation coefficient R2 ~0.97. The mean bias errors between these two varied from −40 W m−2 to +7 W m−2 with an overestimation of 2–3% by SBDART and an underestimation of 2–9% by RegCM. Collocated measurements of the optical parameters of aerosols indicated a reduction in atmospheric transmission path by ~20% due to aerosol load in the atmosphere when compared with the aerosol free atmospheric condition. Estimation of aerosol radiative forcing efficiency (ARFE) indicated that the presence of black carbon (BC, 10–15%) led to a surface dimming by −26.14 W m−2 τ−1 and a potential atmospheric forcing of +43.04 W m−2 τ−1. BC alone is responsible for >70% influence with a major role in building up of forcing efficiency of +55.69 W m−2 τ−1 (composite) in the atmosphere. On the other hand, the scattering due to aerosols enhance the outgoing radiation at the top of the atmosphere (ARFETOA ~−12.60 W m−2 ω−1), the absence of which would have resulted in ARFETOA of ~+16.91 W m−2 τ−1 (due to BC alone). As a result, ~3/4 of the radiation absorption in the atmosphere is ascribed to the presence of BC. This translated to an atmospheric heating rate of ~1.0 K day−1, with ~0.3 K day−1 heating over the elevated regions (2–4 km) of the atmosphere, especially during pre-monsoon season. Comparison of the satellite (MODIS) derived and ground based estimates of surface albedo showed seasonal difference in their magnitudes (R2 ~0.98 during retreating monsoon and winter; ~0.65 during pre-monsoon and monsoon), indicating that the reliability of the satellite data for aerosol radiative forcing estimation is more during the retreating and winter seasons.

The role of riparian vegetation density, channel orientation and water velocity in determining river temperature dynamics

A simulation experiment was used to understand the importance of riparian vegetation density, channel orientation and flow velocity for stream energy budgets and river temperature dynamics. Water temperature and meteorological observations were obtained in addition to hemispherical photographs along a ∼1km reach of the Girnock Burn, a tributary of the Aberdeenshire Dee, Scotland. Data from nine hemispherical images (representing different uniform canopy density scenarios) were used to parameterise a deterministic net radiation model and simulate radiative fluxes. For each vegetation scenario, the effects of eight channel orientations were investigated by changing the position of north at 45° intervals in each hemispheric image. Simulated radiative fluxes and observed turbulent fluxes drove a high-resolution water temperature model of the reach. Simulations were performed under low and high water velocity scenarios. Both velocity scenarios yielded decreases in mean (≥1.6°C) and maximum (≥3.0°C) temperature as canopy density increased. Slow-flowing water resided longer within the reach, which enhanced heat accumulation and dissipation, and drove higher maximum and lower minimum temperatures. Intermediate levels of shade produced highly variable energy flux and water temperature dynamics depending on the channel orientation and thus the time of day when the channel was shaded. We demonstrate that in many reaches relatively sparse but strategically located vegetation could produce substantial reductions in maximum temperature and suggest that these criteria are used to inform future river management.

Impacts of the direct radiative effect of aerosols in numerical weather prediction over Europe using the ALADIN-HIRLAM NWP system

Aerosol feedbacks are becoming more accepted as physical mechanisms that should be included in numerical weather prediction models in order to improve the accuracy of the weather forecasts. The default set-up in the Aire Limitee Adaptation dynamique Developpement INternational (ALADIN) — High Resolution Limited Area Model (HIRLAM) numerical weather prediction system includes monthly aerosol climatologies to account for the average direct radiative effect of aerosols. This effect was studied using the default aerosol climatology in the system and compared to experiments run using the more up-to-date Max-Planck-Institute Aerosol Climatology version 1 (MACv1), and time-varying aerosol data from the Monitoring Atmospheric Composition and Climate (MACC) reanalysis aerosol dataset. Accounting for the direct radiative effect using monthly aerosol climatologies or near real-time aerosol distributions improved the accuracy of the simulated radiative fluxes and temperature and humidity forecasts in the lower troposphere. However, the dependency of forecast meteorological conditions on the aerosol dataset itself was found to be weak.

RAMI4PILPS: An intercomparison of formulations for the partitioning of solar radiation in land surface models

[1] Remotely sensed, multiannual data sets of shortwave radiative surface fluxes are now available for assimilation into land surface schemes (LSSs) of climate and/or numerical weather prediction models. The RAMI4PILPS suite of virtual experiments assesses the accuracy and consistency of the radiative transfer formulations that provide the magnitudes of absorbed, reflected, and transmitted shortwave radiative fluxes in LSSs. RAMI4PILPS evaluates models under perfectly controlled experimental conditions in order to eliminate uncertainties arising from an incomplete or erroneous knowledge of the structural, spectral and illumination related canopy characteristics typical for model comparison with in situ observations. More specifically, the shortwave radiation is separated into a visible and near-infrared spectral region, and the quality of the simulated radiative fluxes is evaluated by direct comparison with a 3-D Monte Carlo reference model identified during the third phase of the Radiation transfer Model Intercomparison (RAMI) exercise. The RAMI4PILPS setup thus allows to focus in particular on the numerical accuracy of shortwave radiative transfer formulations and to pinpoint to areas where future model improvements should concentrate. The impact of increasing degrees of structural and spectral subgrid variability on the simulated fluxes is documented and the relevance of any thus emerging biases with respect to gross primary production estimates and shortwave radiative forcings due to snow and fire events are investigated.

The Ocean–Land–Atmosphere Model: Optimization and Evaluation of Simulated Radiative Fluxes and Precipitation

Abstract This work continues the presentation and evaluation of the Ocean–Land–Atmosphere Model (OLAM), focusing on the model’s ability to represent radiation and precipitation. OLAM is a new, state-of-the-art earth system model, capable of user-specified grid resolution and local mesh refinement. An objective optimization of the microphysics parameterization is carried out. Data products from the Clouds and the Earth’s Radiant Energy System (CERES) and the Global Precipitation Climatology Project (GPCP) are used to construct a maximum likelihood function, and thousands of simulations using different values for key parameters are carried out. Shortwave fluxes are found to be highly sensitive to both the density of cloud droplets and the assumed shape of the cloud droplet diameter distribution function. Because there is considerable uncertainty in which values for these parameters to use in climate models, they are targeted as the tunable parameters of the objective optimization procedure, which identified high-likelihood volumes of parameter space as well as parameter uncertainties and covariances. Once optimized, the model closely matches observed large-scale radiative fluxes and precipitation. The impact of model resolution is also tested. At finer characteristic length scales (CLS), smaller-scale features such as the ITCZ are better resolved. It is also found that the Amazon was much better simulated at 100- than 200-km CLS. Furthermore, a simulation using OLAM’s variable resolution functionality to cover South America with 100-km CLS and the rest of the world with 200-km CLS generates a precipitation pattern in the Amazon similar to the global 100-km CLS run.

Simulation and validation of radiative transfers in urbanised areas

The energy balance at the soil-atmosphere interface is the fundamental equation to simulate the microclimate of urban areas. It implies the equilibrium of several energetic terms, such as: radiation (in the solar and infrared wavelengths), conduction (soil heat flux), convection (sensible heat flux) and evapotranspiration (latent heat flux). This article deals with the 3D-numeric simulation of heat transfers in an urban canyon and the validation of the appropriate algorithms and results. The simulation procedure and parameterisation of the surface elements are briefly described. All the heat transfer algorithms are taken from the existing literature. Among the mechanisms, the terrestrial infrared radiation is the most difficult term to be simulated, because realistic surface temperatures must be obtained. For this reason, at each time step, a special iterative treatment is applied to solve the energy balance equation until complete stabilisation of the surface temperatures. As a result, the simulation model is able to give all the components of the energy balance equation for every mesh. The aim of the paper is to demonstrate the validity of this approach by comparing the simulation results with the experimentation. The reliability of the simulated radiative fluxes is judged against a data set specially acquired during a dedicated experimental campaign that took place in an urban canyon of a French city (Strasbourg) during summer 2002.

Simulated turbulent fluxes over land from general circulation models and reanalyses compared with observations

AbstractTurbulent fluxes over land in three general circulation models (GCMs) and the three reanalyses are compared with long‐term measurements. Observations were obtained from a variety of sources, including GEBA, Cabauw, The Netherlands, and two newly analysed sites. The intercomparison reveals a wide range of values between the models. The latent heat flux in two of the models shows good agreement with observations. These models benefit from accurately simulated radiative fluxes. The other models tend to overestimate the latent heat flux as long as there is sufficient soil moisture available. This overestimation is caused by a variety of reasons, including: a too intense zonal flow from the ocean to the interior of the continent, which produces an excessive moist advection, thereby increasing precipitation and evaporation; and a high insolation and soil moisture, producing excessive latent heat flux in summer. Where soil moisture is limited too early, an excessive decrease in latent heat flux is apparent towards the end of summer. The sensible heat flux in the models varies when compared with the observations due to the following possibilities: in models that exhibit an excessive summer drying, too much available energy will be partitioned into the sensible heat flux at the end of summer, since there is no water available for evaporation; in models where radiation is overestimated, the excessive available energy will lead to an overestimated sensible heat flux. Despite the continuous assimilation of observations from the global observing system in the reanalyses, the range of uncertainties is not reduced when compared with the GCMs. Thus, although the reanalysis project was developed to aid the validation of GCMs, it appears to be difficult to accomplish this in the case of the turbulent fluxes. Copyright © 2002 Royal Meteorological Society