We have used the Whole Atmosphere Community Climate Model to produce a small (three‐member) ensemble of simulations of the period 1950–2003. Comparison of model results against available observations ...shows that for the most part, the model is able to reproduce well the observed trends in zonal mean temperature and ozone, both as regards their magnitude and their distribution in latitude and altitude. Calculated trends in water vapor, on the other hand, are not at all consistent with observations from either the HALOE satellite instrument or the Boulder, Colorado, hygrosonde data set. We show that such lack of agreement is actually to be expected because water vapor has various sources of low‐frequency variability (heating due to volcanic eruptions, the quasi‐biennial oscillation and El Niño–Southern Oscillation) that can confound the determination of secular trends. The simulations also reveal the presence of other interesting behavior, such as the lack of any significant temperature trend near the mesopause, a decrease in the stratospheric age of air, and the rare occurrence of an extremely disturbed Southern Hemisphere winter.
The NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), is used to study the atmospheric response from the surface to the lower thermosphere to changes in solar and geomagnetic forcing ...over the 11‐year solar cycle. WACCM3 is a general circulation model that incorporates interactive chemistry that solves for both neutral and ion species. Energy inputs include solar radiation and energetic particles, which vary significantly over the solar cycle. This paper presents a comparison of simulations for solar cycle maximum and solar cycle minimum conditions. Changes in composition and dynamical variables are clearly seen in the middle and upper atmosphere, and these in turn affect terms in the energy budget. Generally good agreement is found between the model response and that derived from satellite observations, although significant differences remain. A small but statistically significant response is predicted in tropospheric winds and temperatures which is consistent with signals observed in reanalysis data sets.
A simulation of the middle atmosphere is presented using a general circulation model (GCM) forced with observed sea surface temperature for the period 1950–2000. The GCM extends to the lower ...thermosphere and reproduces realistic dynamical and temperature distributions. The period contains several El Niño and La Niña events, which are identified using the NINO3 index. Composite anomalies of relevant meteorological fields are obtained by stratifying the northern winter season according to the NINO3 index. These anomalies have the structure of vertically propagating planetary waves extending from the troposphere to the mesosphere. Circulation anomalies in the middle atmosphere are accompanied by large temperature anomalies that are of opposite sign in the stratosphere and mesosphere, the former being warmer and the latter colder during El Niño events. Near the summer mesopause, changes in momentum deposition by parameterized gravity waves results in warming during El Niño. Detailed statistical analysis is used to determine the significance of these anomalies. A chemical/transport simulation is carried out using output from the GCM. It shows that when the lower stratosphere is colder (as during La Niña events), some ozone depletion takes place. Conversely, when the lower stratosphere is warmer and more disturbed, as is the case during El Niño events, heterogeneous chemical processes are inhibited.
A new version of the Community Atmosphere Model (CAM) has been developed and released to the climate community. CAM Version 3 (CAM3) is an atmospheric general circulation model that includes the ...Community Land Model (CLM3), an optional slab ocean model, and a thermodynamic sea ice model. The dynamics and physics in CAM3 have been changed substantially compared to implementations in previous versions. CAM3 includes options for Eulerian spectral, semi-Lagrangian, and finite-volume formulations of the dynamical equations. It supports coupled simulations using either finite-volume or Eulerian dynamics through an explicit set of adjustable parameters governing the model time step, cloud parameterizations, and condensation processes. The model includes major modifications to the parameterizations of moist processes, radiation processes, and aerosols. These changes have improved several aspects of the simulated climate, including more realistic tropical tropopause temperatures, boreal winter land surface temperatures, surface insolation, and clear-sky surface radiation in polar regions. The variation of cloud radiative forcing during ENSO events exhibits much better agreement with satellite observations. Despite these improvements, several systematic biases reduce the fidelity of the simulations. These biases include underestimation of tropical variability, errors in tropical oceanic surface fluxes, underestimation of implied ocean heat transport in the Southern Hemisphere, excessive surface stress in the storm tracks, and offsets in the 500-mb height field and the Aleutian low.
The latest version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3) is described. The changes in both physical and dynamical formulation from CCM2 to CCM3 are ...presented. The major differences in CCM3 compared to CCM2 include changes to the parameterization of cloud properties, clear sky longwave radiation, deep convection, boundary layer processes, and land surface processes. A brief description of each of these parameterization changes is provided. These modifications to model physics have led to dramatic improvements in the simulated climate of the CCM. In particular, the top of atmosphere cloud radiative forcing is now in good agreement with observations, the Northern Hemisphere winter dynamical simulation has significantly improved, biases in surface land temperatures and precipitation have been substantially reduced, and the implied ocean heat transport is in very good agreement with recent observational estimates. The improvement in implied ocean heat transport is among the more important attributes of the CCM3 since it is used as the atmospheric component of the NCAR Climate System Model. Future improvements to the CCM3 are also discussed.
The Climate System Model, a coupled global climate model without “flux adjustments” recently developed at the National Center for Atmospheric Research, was used to simulate the twentieth-century ...climate using historical greenhouse gas and sulfate aerosol forcing. This simulation was extended through the twenty-first century under two newly developed scenarios, a business-as-usual case (ACACIA-BAU, CO₂ ≈ 710 ppmv in 2100) and a CO₂ stabilization case (STA550, CO₂ ≈ 540 ppmv in 2100). Here we compare the simulated and observed twentieth-century climate, and then describe the simulated climates for the twenty-first century. The model simulates the spatial and temporal variations of the twentieth-century climate reasonably well. These include the rapid rise in global and zonal mean surface temperatures since the late 1970s, the precipitation increases over northern mid- and high-latitude land areas, ENSO-induced precipitation anomalies, and Pole–midlatitude oscillations (such as the North Atlantic oscillation) in sea level pressure fields. The model has a cold bias (2°–6°C) in surface air temperature over land, overestimates of cloudiness (by 10%–30%) over land, and underestimates of marine stratus clouds to the west of North and South America and Africa.
The projected global surface warming from the 1990s to the 2090s is ∼1.9°C under the BAU scenario and ∼1.5°C under the STA550 scenario. In both cases, the midstratosphere cools due to the increase in CO₂, whereas the lower stratosphere warms in response to recovery of the ozone layer. As in other coupled models, the surface warming is largest at winter high latitudes (≥5.0°C from the 1990s to the 2090s) and smallest (∼1.0°C) over the southern oceans, and is larger over land areas than ocean areas. Globally averaged precipitation increases by ∼3.5% (3.0%) from the 1990s to the 2090s in the BAU (STA550) case. In the BAU case, large precipitation increases (up to 50%) occur over northern mid- and high latitudes and over India and the Arabian Peninsula. Marked differences occur between the BAU and STA550 regional precipitation changes resulting from interdecadal variability. Surface evaporation increases at all latitudes except for 60°–90°S. Water vapor from increased tropical evaporation is transported into mid- and high latitudes and returned to the surface through increased precipitation there. Changes in soil moisture content are small (within ±3%). Total cloud cover changes little, although there is an upward shift of midlevel clouds. Surface diurnal temperature range decreases by about 0.2°–0.5°C over most land areas. The 2–8-day synoptic storm activity decreases (by up to 10%) at low latitudes and over midlatitude oceans, but increases over Eurasia and Canada. The cores of subtropical jets move slightly up- and equatorward. Associated with reduced latitudinal temperature gradients over mid- and high latitudes, the wintertime Ferrel cell weakens (by 10%–15%). The Hadley circulation also weakens (by ∼10%), partly due to the upward shift of cloudiness that produces enhanced warming in the upper troposphere.
The parameterizations of clouds and precipitation processes have been revised considerably in the Community Atmosphere Model version 3 (CAM3) compared to its predecessors, CAM2 and the Community ...Climate Model version 3 (CCM3). The parameterizations in CAM3 are more realistic in their representation of processes affecting cloud liquid and ice particles and represent the linkages between processes more completely. This paper describes the changes to the representation of clouds in CAM3, including the partitioning of cloud water between liquid and ice phases, the determination of particle sizes and sedimentation rates, the phase and evaporation rate of precipitation, and the calculation of the cloud fraction.
Parameterization changes between CCM3 and CAM2 introduced a significant cold bias at the tropical tropopause, resulting in a dry bias for stratospheric water vapor. Tests of the sensitivity of the tropical temperature profile and the tropical tropopause temperature to individual process changes suggested that the radiative balance at the tropopause was altered by improvements in both clouds and relative humidity below. Radiative equilibrium calculations suggested that the cold bias could be removed by improving the representation of subvisible cirrus clouds. These results motivated the complete separation of the representation of liquid and ice cloud particles and an examination of the processes that determine their sources and sinks. As a result of these changes, the tropopause cold bias has been almost eliminated in CAM3.
The total cloud condensate variable, used in CAM2, has been separated into cloud liquid and cloud ice variables in CAM3. Both sedimentation and large-scale transport of the condensate variables are now included. Snowfall is computed explicitly and the latent heat of fusion has been included for all freezing and melting processes. Both deep and shallow convection parameterizations now detrain cloud condensate directly into the stratiform clouds instead of evaporating the detrained condensate into the environment. The convective parameterizations are not easily modified to include the latent heat of fusion. Therefore, the determination of the phase of convective precipitation, and of detrained condensate, is added as a separate step. Evaporation is included for sedimenting cloud particles and for all sources of precipitation.
Mesospheric thermal inversions are investigated in a numerical simulation with the Whole Atmosphere Community Climate Model, an upward extension of the National Center for Atmospheric Research's ...Community Climate Model. The seasonal character, spatial extent, and magnitude of the inversion layers are realistic during winter. In the model, the occurrence of wintertime inversions is a direct consequence of the rapid decay with height of vertically propagating planetary waves, which induces large temperature perturbations in the upper mesosphere to maintain hydrostatic equilibrium. The magnitude of the inversions is highly correlated with planetary wave amplitude, so that large inversions develop during episodes of planetary wave amplification. Gravity waves do not play a major direct role in the formation of the inversions because the largest thermal tendencies associated with gravity wave breaking occur well above the range of altitudes where inversions are found. However, gravity waves play an essential indirect role because they set up a critical line in the upper mesosphere where Rossby waves break in the mesospheric surf zone.
We compare two simulations of the middle atmosphere using the NCAR Whole Atmosphere Community Climate Model, which includes a comprehensive middle atmosphere chemistry model. Ozone is fully ...interactive in the first simulation. A zonal mean and monthly mean ozone climatology is constructed from the interactive simulation. That ozone climatology is used to determine the radiative heating rates in the second simulation. Comparison of the two simulations shows that using the zonal‐mean ozone climatology significantly affects the climate of the middle atmosphere. Differences between the two simulations are statistically significant throughout the tropics in the upper stratosphere and lower mesosphere.
The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a global climate model. The global model is an updated version ...of the NCAR Community Climate Model (CCM2). The local diffusion scheme uses an eddy diffusivity determined independently at each point in the vertical, based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in global atmospheric models. The nonlocal scheme determines an eddy-diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture. The two diffusion schemes are summarized, and their results are compared with independent radiosonde observations for a number of locations. The focus herein is on the temperature and humidity structure over ocean, where the surface temperatures are specified, since the boundary-layer scheme interacts strongly with the land-surface parameterization. Systematic differences are shown in global-climate simulations, with CCM2 using the two schemes. The nonlocal scheme transports moisture away from the surface more rapidly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically, which typically leads to clouds too low in the atmosphere. The nonlocal scheme has been chosen for CCM2 because of its more comprehensive representation of the physics of boundary-layer transport in dry convective conditions.
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