Shortwave

Abstract: An overview of the Community Earth System Model Version 2 (CESM2) is provided, including a discussion of the challenges encountered during its development and how they were addressed. In addition, an evaluation of a pair of CESM2 long preindustrial control and historical ensemble simulations is presented. These simulations were performed using the nominal 1° horizontal resolution configuration of the coupled model with both the “low-top” (40 km, with limited chemistry) and “high-top” (130 km, with comprehensive chemistry) versions of the atmospheric component. CESM2 contains many substantial science and infrastructure improvements and new capabilities since its previous major release, CESM1, resulting in improved historical simulations in comparison to CESM1 and available observations. These include major reductions in low-latitude precipitation and shortwave cloud forcing biases; better representation of the Madden-Julian Oscillation; better El Nino-Southern Oscillation-related teleconnections; and a global land carbon accumulation trend that agrees well with observationally based estimates. Most tropospheric and surface features of the low- and high-top simulations are very similar to each other, so these improvements are present in both configurations. CESM2 has an equilibrium climate sensitivity of 5.1–5.3 °C, larger than in CESM1, primarily due to a combination of relatively small changes to cloud microphysics and boundary layer parameters. In contrast, CESM2's transient climate response of 1.9–2.0 °C is comparable to that of CESM1. The model outputs from these and many other simulations are available to the research community, and they represent CESM2's contributions to the Coupled Model Intercomparison Project Phase 6.

Abstract: The spaceborne Earth Radiation Budget Experiment was begun in 1984 to obtain quantitative estimates of the global distributions of cloud-radiative forcing The magnitude of the observed net cloud forcing is about four times greater than the expected value of radiative forcing from a doubling of CO2; the shortwave and longwave components of cloud forcing are about 10 times as large as those for a CO2 doubling Small changes in the cloud-radiative forcing fields can therefore play a significant role as a climate-feedback mechanism

Abstract: The purpose of this paper is to put forward a new estimate, in the context of previous assessments, of the annual global mean energy budget. A description is provided of the source of each component to this budget. The top-of-atmosphere shortwave and longwave flux of energy is constrained by satellite observations. Partitioning of the radiative energy throughout the atmosphere is achieved through the use of detailed radiation models for both the longwave and shortwave spectral regions. Spectral features of shortwave and longwave fluxes at both the top and surface of the earth's system are presented. The longwave radiative forcing of the climate system for both clear (125 W m-2) and cloudy (155 W m-2) conditions are discussed. The authors find that for the clear sky case the contribution due to water vapor to the total longwave radiative forcing is 75 W m-2, while for carbon dioxide it is 32 W m-2. Clouds alter these values, and the effects of clouds on both the longwave and shortwave budget are a...

Abstract: An urban surface scheme for atmospheric mesoscale models is presented. A generaliz- ation of local canyon geometry is defined instead of the usual bare soil formulation currently used to represent cities in atmospheric models. This allows refinement of the radiative budgets as well as momentum, turbulent heat and ground fluxes. The scheme is aimed to be as general as possible, in order to represent any city in the world, for any time or weather condition (heat island cooling by night, urban wake, water evaporation after rainfall and snow effects). Two main parts of the scheme are validated against published data. Firstly, it is shown that the evolution of the model-predicted fluxes during a night with calm winds is satisfactory, considering both the longwave budget and the surface temperatures. Secondly, the original shortwave scheme is tested off-line and compared to the effective albedo of a canyon scale model. These two validations show that the radiative energy input to the urban surface model is realistic. Sensitivity tests of the model are performed for one-year simulation periods, for both oceanic and continental climates. The scheme has the ability to retrieve, without ad hoc assumptions, the diurnal hysteresis between the turbulent heat flux and ground heat flux. It reproduces the damping of the daytime turbulent heat flux by the heat storage flux observed in city centres. The latent heat flux is negligible on average, but can be large when short time scales are considered (especially after rainfall). It also suggests that in densely built areas, domestic heating can overwhelm the net radiation, and supply a continuous turbulent heat flux towards the atmosphere. This becomes very important in winter for continental climates. Finally, a comparison with a vegetation scheme shows that the suburban environment can be represented with a bare soil formulation for large temporal or spatial averages (typical of global climatic studies), but that a surface scheme dedicated to the urban surface is necessary when smaller scales are considered: town meteorological forecasts, mesoscale or local studies.