ALBERT

Description: The importance of using a general circulation model that includes a well-resolved stratosphere for climate simulations, and particularly the influence this has on surface climate, is investigated. High top model simulations are run with the Met Office Unified Model for the Coupled Model Intercomparison Project Phase 5 (CMIP5). These simulations are compared to equivalent simulations run using a low top model differing only in vertical extent and vertical resolution above 15 km. The period 1960–2002 is analyzed and compared to observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. Long-term climatology, variability, and trends in surface temperature and sea ice, along with the variability of the annular mode index, are found to be insensitive to the addition of a well-resolved stratosphere. The inclusion of a well-resolved stratosphere, however, does improve the impact of atmospheric teleconnections on surface climate, in particular the response to El Niño–Southern Oscillation, the quasi-biennial oscillation, and midwinter stratospheric sudden warmings (i.e., zonal mean wind reversals in the middle stratosphere). Thus, including a well-represented stratosphere could improve climate simulation on intraseasonal to interannual time scales.

Description: Cloud simulations and cloud–climate feedbacks in the tropical and subtropical eastern Pacific region in 16 state-of-the-art coupled global climate models (GCMs) and in the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM) are examined. The authors find that the simulation of the present-day mean cloud climatology for this region in the GCMs is very poor and that the cloud–climate feedbacks vary widely among the GCMs. By contrast, iRAM simulates mean clouds and interannual cloud variations that are quite similar to those observed in this region. The model also simulates well the observed relationship between lower-tropospheric stability (LTS) and low-level cloud amount. To investigate cloud–climate feedbacks in iRAM, several global warming scenarios were run with boundary conditions appropriate for late twenty-first-century conditions. All the global warming cases simulated with iRAM show a distinct reduction in low-level cloud amount, particularly in the stratocumulus regime, resulting in positive local feedback parameters in these regions in the range of 4–7 W m−2 K−1. Domain-averaged (30°S–30°N, 150°–60°W) feedback parameters from iRAM range between +1.8 and +1.9 W m−2 K−1. At most locations both the LTS and cloud amount are altered in the global warming cases, but the changes in these variables do not follow the empirical relationship found in the present-day experiments. The cloud–climate feedback averaged over the same east Pacific region was also calculated from the Special Report on Emissions Scenarios (SRES) A1B simulations for each of the 16 GCMs with results that varied from −1.0 to +1.3 W m−2 K−1, all less than the values obtained in the comparable iRAM simulations. The iRAM results by themselves cannot be connected definitively to global climate feedbacks; however, among the global GCMs the cloud feedback in the full tropical–subtropical zone is correlated strongly with the east Pacific cloud feedback, and the cloud feedback largely determines the global climate sensitivity. The present iRAM results for cloud feedbacks in the east Pacific provide some support for the high end of current estimates of global climate sensitivity.

Description: The formation of pre–Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in Part II of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities. In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the “wave pouch” from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors’ earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.

Description: The climatology of a stratosphere-resolving version of the Met Office’s climate model is studied and validated against ECMWF reanalysis data. Ensemble integrations are carried out at two different horizontal resolutions. Along with a realistic climatology and annual cycle in zonal mean zonal wind and temperature, several physical effects are noted in the model. The time of final warming of the winter polar vortex is found to descend monotonically in the Southern Hemisphere, as would be expected for purely radiative forcing. In the Northern Hemisphere, however, the time of final warming is driven largely by dynamical effects in the lower stratosphere and radiative effects in the upper stratosphere, leading to the earliest transition to westward winds being seen in the midstratosphere. A realistic annual cycle in stratospheric water vapor concentrations—the tropical “tape recorder”—is captured. Tropical variability in the zonal mean zonal wind is found to be in better agreement with the reanalysis for the model run at higher horizontal resolution because the simulated quasi-biennial oscillation has a more realistic amplitude. Unexpectedly, variability in the extratropics becomes less realistic under increased resolution because of reduced resolved wave drag and increased orographic gravity wave drag. Overall, the differences in climatology between the simulations at high and moderate horizontal resolution are found to be small.

Description: This is the second of a two-part study examining the simulated formation of Atlantic Hurricane Felix (2007) in a cloud-representing framework. Here several open issues are addressed concerning the formation of the storm’s warm core, the evolution and respective contribution of stratiform versus convective precipitation within the parent wave’s pouch, and the sensitivity of the development pathway reported in Part I to different model physics options and initial conditions. All but one of the experiments include ice microphysics as represented by one of several parameterizations, and the partition of convective versus stratiform precipitation is accomplished using a standard numerical technique based on the high-resolution control experiment. The transition to a warm-core tropical cyclone from an initially cold-core, lower tropospheric wave disturbance is analyzed first. As part of this transformation process, it is shown that deep moist convection is sustained near the pouch center. Both convective and stratiform precipitation rates increase with time. While stratiform precipitation occupies a larger area even at the tropical storm stage, deep moist convection makes a comparable contribution to the total rain rate at the pregenesis stage, and a larger contribution than stratiform processes at the storm stage. The convergence profile averaged near the pouch center is found to become dominantly convective with increasing deep moist convective activity there. Low-level convergence forced by interior diabatic heating plays a key role in forming and intensifying the near-surface closed circulation, while the midlevel convergence associated with stratiform precipitation helps to increase the midlevel circulation and thereby contributes to the formation and upward extension of a tropospheric-deep cyclonic vortex. Sensitivity tests with different model physics options and initial conditions demonstrate a similar pregenesis evolution. These tests suggest that the genesis location of a tropical storm is largely controlled by the parent wave’s critical layer, whereas the genesis time and intensity of the protovortex depend on the details of the mesoscale organization, which is less predictable. Some implications of the findings are discussed.

Description: A framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP’s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced—namely, soluble organic aerosols. The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment–Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about −25° and −29°C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon. The EP’s predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP’s prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.

Description: At subzero temperatures, cloud particles can contain both ice and liquid water fractions. Wet growth of precipitation particles occurs when supercooled cloud liquid is accreted faster than it can freeze on impact. With a flexible framework, the theory of wet growth of hail is extended to the case of the inhomogeneities of surface temperature and of liquid coverage over the surface of the particle. The theory treats the heat fluxes between its wet and dry parts and radial heat fluxes from the sponge layer through the liquid skin to the air. The theory parameterizes effects of nonsphericity of hail particles on their growth by accretion. Gradual internal freezing of any liquid soaking the hail or graupel particle’s interior during dry growth (“riming”) is treated as well. In this way, the microphysical recycling envisaged by Pflaum in a paper in 1980 is treated, with alternating episodes of wet and dry growth. The present paper, the first of a two-part paper, describes the scheme to treat wet growth, accounting for dependencies on condensate content, temperature, and particle size. Comparison with the laboratory experiments is presented.

Description: Ice in atmospheric clouds undergoes complex physical processes, interacting especially with radiation, which leads to serious impacts on global climate. After their primary production, atmospheric ice crystals multiply extensively by secondary processes. Here, it is shown that a mostly overlooked process of mechanical breakup of ice particles by ice–ice collisions contributes to such observed multiplication. A regime for explosive multiplication is identified in its phase space of ice multiplication efficiency and number concentration of ice particles. Many natural mixed-phase clouds, if they have copious millimeter-sized graupel, fall into this explosive regime. The usual Hallett–Mossop (H–M) process of ice multiplication is shown to dominate the overall ice multiplication when active, as it starts sooner, compared to the breakup ice multiplication process. However, for deep clouds with a cold base temperature where the usual H–M process is inactive, the ice breakup mechanism should play a critical role. Supercooled rain, which may freeze to form graupel directly in only a few minutes, is shown to hasten such ice multiplication by mechanical breakup, with an ice enhancement ratio exceeding 104 approximately 20 min after small graupel first appear. The ascent-dependent onset of subsaturation with respect to liquid water during explosive ice multiplication is predicted to determine the eventual ice concentrations.