摘要:

AbstractThis paper reports on measurements made during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment. Observations are shown from six wave cloud flights in the temperature range −12°C to −40 °C. In nearly all cases ice nucleation does not occur until after droplets have formed. Parcel trajectories that experience temperatures below −35°C are dominated by homogeneous nucleation, while parcel trajectories that stay above −35°C are affected by heterogeneous nucleation. Ice particles appear to be nucleated after the parent droplets reach their largest size and coldest temperature at the wave peak. The most likely heterogeneous nucleation mechanisms are immersion and/or contact

ISSN:0035-9009    

DOI:10.1002/qj.49712757502

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe Met Office Cloud Resolving Model (CRM) and the UMIST Explicit Microphysics Model (EMM) have been employed in the analysis of data from airborne studies of a multi‐thermal cumulus cloud which developed over New Mexico in the summer of 1987. The principal goal was to establish a quantitative understanding of the observed development of glaciation of this cloud.The EMM was utilized in a series of tests designed to assess the sensitivity of cloud glaciation via the Hallett‐Mossop (H‐M) process to cloud parameters such as the concentration of cloud condensation nuclei, the cloud‐base temperature, entrainment, and the freezing and splintering of supercooled raindrops. These tests with the EMM demonstrate that reductions in the mean droplet diameter can inhibit the rates of H‐M splinter production and auto‐conversion, reducing the rate of accumulation of precipitation at the ground and reducing the concentration of ice particles. The warm‐rain process in the EMM is fundamental to the production of graupel, H‐M splinters and precipitation.Good agreement was found between the predictions of the CRM and the available dynamical and microphysical field observations. Analysis of results from both models indicated that the cloud glaciation is explicable in terms of the H‐M process, with ice production being dominated by the freezing of supercooled raindrops in the H‐M band, and the immediate and continuous production of ice splinters as supercooled drople

ISSN:0035-9009    

DOI:10.1002/qj.49712757503

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis paper describes the use of two‐dimensional (2‐D) cloud‐resolving numerical model simulations of a period of active convection from the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment to generate statistics of the distribution of total ice water content (IWC) and of the relative contribution to IWC of small (sub‐200 μm) particles with temperature. The model data are sampled in a way which excludes contributions from active convective updraught regions, so that the model results may be compared with similar such distributions derived fromin situsampling of a number of tropical anvil ice clouds in the vicinity of Kwajalein in the west Pacific.The model gives a good description of the mean and mode IWC in each of seven temperature ranges and, for temperatures warmer than –40°C, correctly predicts the dominant contribution of large particles to the IWC. At lower temperatures, the model retains an excessive fraction of its IWC in large particles. Estimates of the maximum crystal length (MCL) that would be sampled by a 2‐D Optical Array Probe exceed the observed values in this temperature range but are otherwise in good agreement. Tests of the sensitivity of model results to the cloud ice bulk density, and the absence of a homogeneous freezing source of ice crystals, suggest that the excessive MCL values are due in part to excessive rates of autoconversion and aggrega

ISSN:0035-9009    

DOI:10.1002/qj.49712757504

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe mixing of air between clouds and their environment is investigated with the help of a simplified model problem that contains only a small number of dynamically relevant parameters whose influence can be systematically explored. The classical model of a buoyant thermal, extensively studied in laboratory experiments, is generalized to include the key moist processes of condensation and re‐evaporation of cloud water. We consider the evolution of an isolated thermal initiated from a spherical warm and moist bubble at rest in a moist neutral sounding. The moist thermal loses buoyancy through mixing with the unsaturated environmental air, and finally collapses. The effect of moist physics on the thermal is shown to be represented by only three scale‐independent parameters. We present large‐eddy simulations that exhibit an approximate dynamical similarity, in that the loss of buoyancy resulting from the mixing process is mainly described by a single scale‐invariant parameter which we call the effective buoyancy,Beff. In order for the thermal to ascend a distance significantly larger than its initial diameter,Beffmust be much greater than unity. This implies that the environment must be close to saturation if the bubble has an initial buoyancy that is typical of real clouds. It is also shown that the stratification of the environment induces a length scale,Lbuo, for a thermal with a given buoyancy excess. The resulting scale‐independent parameterLeff=L/Lbuo(whereLis the initial bubble radius) is shown to affect the geometry of the thermal. Indeed, the thermal may break up ifLeffis too large. These geometrical effects can be explained by relatingLbuoto the depth reached by forced downdraughts, which corresponds to the vertical extent of the largest eddies that entrain unsaturated air into th

ISSN:0035-9009    

DOI:10.1002/qj.49712757505

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe mass, heat and moisture budgets of a composite easterly wave over the eastern tropical Atlantic are analysed from simulations of a convection‐resolving model (CRM). The large‐scale data from the GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE) phase III are used to drive the CRM simulations. The simulated, composite easterly wave is reasonably comparable with the observed, except for the lack of shallow convection embedded in the simulated easterly waves. The mesoscale stratiform precipitation processes are found to be an inseparable component of cumulus convection embedded in the easterly waves. Their main differences from convective processes are twofold: (1) the distinct vertical structures of the budget components due to mesoscale updraughts and downdraughts, and (2) the phase lags relative to the convective processes in the temporal evolution of the easterly waves. Thus, theoretical studies should include both convective and mesoscale stratiform precipitation processes in order to further understanding of diabatic processes in the dynamics of the easterly waves and possibly other waves in the tropics. In addition, the budget analysis presented in this study also provides some details for improving parametrizations of cumulus convect

ISSN:0035-9009    

DOI:10.1002/qj.49712757506

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractLow‐level cloud fields with typical albedos between 50% and 70% contribute significantly to short‐wave cloud radiative forcing. Consequently, the earth's radiative budget is sensitive to extended stratocumulus which typically occurs in sub‐tropical subsidence regions over the oceans. It is therefore vital to represent the cloud albedo of such cloud fields accurately in climate models. Nevertheless, cloud fields are modelled as homogeneous plane‐parallel clouds, which leads to an overestimation of the albedo by up to 15% compared with real inhomogeneous clouds, both types of cloud fields having the same mean cloud optical depth. This so‐called plane‐parallel cloud albedo bias has given rise to a number of studies that present methods for reducing the albedo bias for stratiform clouds in climate models.Using Monte Carlo simulations and the Independent Pixel Approximation (IPA) the inhomogeneous cloud albedo is investigated with the aim of deriving a parametrization of the albedo bias. The new approach used here is that the variance of the cloud optical depth is parametrized as a function of grid size and mean cloud liquid‐water content. The variance of the cloud optical depth is derived from turbulence processes. For the determination of the albedo bias an analytical solution is presented which is based on a simplified distribution function of the cloud optical depth and the IPA method. When used in conjunction with the parametrized variance of the cloud optical depth, a box function, which replaces the distribution function, produces comparable results to Monte Carlo simulations within an acceptable accuracy of a

ISSN:0035-9009    

DOI:10.1002/qj.49712757507

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractNew laboratory measurements and theoretical calculations of integrated line intensities for water vapour bands in the near‐infrared and visible regions (8600–15 000 cm−1) show a systematic 6–26% increase in band intensities compared to the HITRAN96 database. We have used the GENLN2 line‐by‐line code to assess the effects of such changes in the water vapour spectral database on calculations of clear‐sky short‐wave fluxes and heating rates. Three standard atmospheres were used and it was found that, compared with HITRAN96 results, the absorbed downward solar fluxes increase (including the effects of theoretically predicted weak water lines in the region) by 5.5, 4.8 and 2.2 W m−2(solar zenith angle = 30°) and by 2.4, 2.1 and 1.1 W m−2(solar zenith angle = 75°) respectively. The maximum change in heating rate is about 4%. The effects are about five to eight times larger than those produced by using the Giveret al.corrections. The combined effects of the revised description of the spectroscopy of water vapour account for approximately 30–90% of the absorption currently ascribed to the water vapour continuum

ISSN:0035-9009    

DOI:10.1002/qj.49712757508

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractBased on the physico‐chemical properties of water dimers, their near infrared and visible absorption of solar radiation in the earth's atmosphere is calculated. The calculation uses equilibrium constants determined by statistical mechanics, and a vibrational absorption spectrum determined by a coupled oscillator quantum mechanics model andab initioquantum chemistry. The resulting total atmospheric absorption was calculated using a line‐by‐line radiative‐transfer model, and depends significantly on the dimer abundance, as well as on the frequency and line width given to vibrational transitions. The best estimate achieved for the possible range of total absorption from 400 nm to 5000 nm by water dimer in the tropics is 1.6–3.3 W m−2. In a global‐warming scenario, the increased temperature and water vapour partial pressure result in a nonlinear increase in the absorption of solar radiation by water clusters. Most of the energy from water dimer absorption is deposited in the lower troposphere, particularly in the tropics, tending to make it mo

ISSN:0035-9009    

DOI:10.1002/qj.49712757509

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe impact of polar stratospheric clouds (PSCs) on the stratospheric radiative heating rate is analysed by including a nominal PSC in heating‐rate calculations that incorporate realistic atmospheric variables including tropospheric clouds. The use of realistic atmospheric conditions constrains the possible radiative effects of PSCs, which previous studies have shown to be very sensitive to such variables as temperature, tropospheric clouds, and solar zenith angle. Over the pole, winter heating rates within the stratospheric polar vortex are decreased substantially by the presence of a PSC, while a PSC increases the heating rates equatorward of 75–85°S. Although the PSC always increases the short‐wave heating, the effect in the long‐wave region depends on the ground temperature, the stratospheric temperature, and presence of a tropospheric cloud. For the thickest PSCs (Type II), the effect in August 1994 varies from cooling by 0.25 K d−1(potential‐temperature difference Δθ = 0.5 K d−1) at the pole to heating by 0.3 K d−1(Δθ = 0.6 K d−1) at 65°S to slight cooling equatorward of 57°S. September 1994 results are similar. Calculated heating rates over the pole including PSCs are near –0.5 K d−1(θ = –1 K d−1) for both months, and positive heating rates of up to 0.25 K d−1(θ = 0.5 K d−1) occur near the vortex edge. Thinner PSCs (Type I) have less of an effect; for example, heating rates of 0.375 K d−1(θ = 0.75 K d−1) occur over the pole in August when a Type I PSC is included. These results should be viewed as an upper bound to the effect of PSCs since the calculations specify 100% PSC cover; satellite results show that this assumption is not unreasonable within the vortex during winter and early spring, however. The increased latitudinal gradient in descent rates in the presence of a PSC is consistent with the behaviour of long‐lived trace‐gas observations, and str

ISSN:0035-9009    

DOI:10.1002/qj.49712757510

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIn order to analyse the regional climate sensitivity in the Iberian Peninsula and surrounding areas to the global change related to an increase of CO2, two simulations (current climate and scenario) are performed using a regional‐climate model (RCM) one‐way nested in a global circulation model (GCM). Both simulations cover a ten‐year period. The results of the control simulation (current climate) are compared to a climatological database. RCM and GCM representations of the climate are realistic, but the RCM shows more detailed results. Following this, a statistical analysis of the differences between current climate and double CO2scenario simulations is carried out considering both 2 m temperature and precipitation. The analysis shows a significant warming in seasonal averages and important differences in the 2 m temperature interannual variability. Especially important is the strong increase in interannual variability of precipitation found in winter and autumn. Relationships between anomalies of general circulation (500 hPa geopotential field) and surface temperature, precipitation and cloudiness are also analysed, showing an important effect of cloud cover anomalies over the 2 m temperature interannual variab

ISSN:0035-9009    

DOI:10.1002/qj.49712757511

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY