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Mike Bauer
and
Anthony D. Del Genio

Abstract

The role of midlatitude baroclinic cyclones in maintaining the extratropical winter distribution of water vapor in an operational global climate model is investigated. A cyclone identification and tracking algorithm is used to compare the frequency of occurrence, propagation characteristics, and composite structure of 10 winters of storms in the Goddard Institute for Space Studies general circulation model (GCM) and in two reanalysis products. Cyclones are the major dynamical source of water vapor over the extratropical oceans in the reanalyses. The GCM produces fewer, generally weaker, and slower-moving cyclones than the reanalyses and is especially deficient in storms associated with secondary cyclogenesis. Composite fields show that GCM cyclones are shallower and drier aloft than those in the reanalyses and that their vertical structure is less tilted in the frontal region because of the GCM’s weaker ageostrophic circulation. This is consistent with the GCM’s underprediction of midlatitude cirrus. The GCM deficiencies do not appear to be primarily due to parameterization errors; the model is too dry despite producing less storm precipitation than is present in the reanalyses and in an experimental satellite precipitation dataset, and the weakness and shallow structure of GCM cyclones is already present at storm onset. These shortcomings may be common to most climate GCMs that do not resolve the mesoscale structure of frontal zones, and this may account for some universal problems in climate GCM midlatitude cloud properties.

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Anthony D. Del Genio
and
Jingbo Wu

Abstract

In continental convective environments, general circulation models typically produce a diurnal cycle of rainfall that peaks close to the noon maximum of insolation, hours earlier than the observed peak. One possible reason is insufficient sensitivity of their cumulus parameterizations to the state of the environment due to weak entrainment. The Weather Research and Forecasting (WRF) model, run at cloud-resolving (600 and 125 m) resolution, is used to study the diurnal transition from shallow to deep convection during the monsoon break period of the Tropical Warm Pool–International Cloud Experiment. The WRF model develops a transition from shallow to deep convection in isolated events by 1430–1500 local time. The inferred entrainment rate weakens with increasing time of day as convection deepens. Several current cumulus parameterizations are tested for their ability to reproduce the WRF behavior. The Gregory parameterization, in which entrainment rate varies directly with parcel buoyancy and inversely as the square of the updraft speed, is the best predictor of the inferred WRF entrainment profiles. The Gregory scheme depends on a free parameter that represents the fraction of buoyant turbulent kinetic energy generation on the cloud scale that is consumed by the turbulent entrainment process at smaller scales. A single vertical profile of this free parameter, increasing with height above the boundary layer but constant with varying convection depth, produces entrainment rate profiles consistent with those inferred from the WRF over the buoyant depth of the convection. Parameterizations in which entrainment varies inversely with altitude or updraft speed or increases with decreasing tropospheric relative humidity do not perform as well. Entrainment rate at cloud base decreases as convection depth increases; this behavior appears to be related to an increase in vertical velocity at downdraft cold pool edges.

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Anthony D. Del Genio
and
William Kovari

Abstract

A clustering algorithm is used to define the radiative, hydrological, and microphysical properties of precipitating convective events in the equatorial region observed by the Tropical Rainfall Measuring Mission (TRMM) satellite. Storms are separated by surface type, size, and updraft strength, the latter defined by the presence or absence of lightning. SST data and global reanalysis products are used to explore sensitivity to changes in environmental conditions. Small storms are much more numerous than mesoscale convective systems, and account for fairly little of the total rainfall but contribute significantly to reflection of sunlight. Lightning storms rain more heavily, have greater cloud area, extend to higher altitude, and have higher albedos than storms without lightning. Lightning is favored by a steep lower-troposphere lapse rate and moist midlevel humidity. Storms occur more often at SST ≥ 28°C and with strong upward 500-mb mean vertical velocity. In general, storms over warmer ocean waters rain more heavily, are larger, and have higher cloud tops, but they do not have noticeably higher albedos than storms over cooler ocean waters. Mesoscale convective system properties are more sensitive to SST. Rain rates and cloud-top heights increase statistically significantly with mean upward motion. Rain rates increase with albedo and cloud-top height over ocean, but over land there are also storms with cloud-top temperatures >−35°C whose rain rates decrease with increasing albedo. Both the fraction of available moisture that rains out and the fraction that detrains as ice increase with SST, the former faster than the latter. TRMM ice water paths derived from cloud-resolving models but constrained by observed microwave radiances are only weakly correlated with observed albedo. The results are inconsistent with the “adaptive iris” hypothesis and suggest feedbacks due primarily to increasing convective cloud cover with warming, but more weakly than predicted by the “thermostat” hypothesis.

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Michael P. Jensen
and
Anthony D. Del Genio

Abstract

The radiative and microphysical characteristics of 17 precipitating systems observed by the Tropical Rainfall Measuring Mission (TRMM) satellite over Manus, Papua New Guinea, and Nauru Island are modeled. These cases represent both deep and midlevel convection. Reflectivity data from the TRMM precipitation radar and Geostationary Meteorological Satellite infrared radiometer measurements are used to parameterize the three-dimensional cloud microphysics of each precipitating cloud system. These parameterized cloud properties are used as input for a two-stream radiative transfer model. Comparisons with measurements of broadband radiative fluxes at the top of atmosphere and the surface show agreement to within 20%. In cases in which the convective available potential energy (CAPE) is large, deep convective clouds with extended anvil decks form, containing layers of ice crystals that are too small to be detected by the TRMM radar but have a large optical thickness. This results in maximum shortwave heating and longwave cooling near cloud top at heights of 12–14 km. When CAPE is small, convective clouds extend only to midlevels (4–7 km), and there are no cloud layers below the detectability limit of the TRMM radar. Radiative heating and cooling in these cases are maximum near the freezing level. A sensitivity analysis suggests that the small ice crystals near the cloud top and larger precipitation-sized particles play equally significant roles in producing the high albedos of tropical anvil clouds. A comparison of the radiative heating profiles calculated in this study with latent heating profiles from previous studies shows that for cases of mature deep convection near local solar noon, the maximum radiative heating is 10%–30% of the magnitude of the maximum latent heating.

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Michael Allison
,
Anthony D. Del Genio
, and
Wei Zhou

Abstract

The diagnostic analysis of numerical simulations of the Venus/Titan wind regime reveals an overlooked constraint upon the latitudinal structure of their zonal-mean angular momentum. The numerical experiments, as well as the limited planetary observations, are approximately consistent with the hypothesis that within the latitudes bounded by the wind maxima the total Ertel potential vorticity associated with the zonal-mean motion is approximately well mixed with respect to the neutral equatorial value for a stable circulation. The implied latitudinal profile of angular momentum is of the form MMe (cosλ)2/Ri, where λ is the latitude and Ri the local Richardson number, generally intermediate between the two extremes of uniform angular momentum (Ri → ∞) and uniform angular velocity (Ri = 1). The full range of angular momentum profile variation appears to be realized within the observed meridional–vertical structure of the Venus atmosphere, at least crudely approaching the implied relationship between stratification and zonal velocity there. While not itself indicative of a particular eddy mechanism or specific to atmospheric superrotation, the zero potential vorticity (ZPV) constraint represents a limiting bound for the eddy–mean flow adjustment of a neutrally stable baroclinic circulation and may be usefully applied to the diagnostic analysis of future remote sounding and in situ measurements from planetary spacecraft.

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Anthony D. Del Genio
and
William B. Rossow

Abstract

Pioneer Venus OCPP ultraviolet images spanning eight years have been analyzed objectively to derive quantitative information on the properties of planetary-scale wave modes at the Venus cloud tops. We infer propagation characteristics for longitudinal wavenumber 1 by Fourier analyzing time series of longitudinal mean normalized image brightness. The dominant equatorial mode during 1979–80 was the 4-day periodicity associated with zonal motion of the Y-feature. The difference between this and the 4.7-day equatorial rotation period derived from the tracking of small cloud features implies that the Y is a propagating wave with a prograde phase speed of about 15 ms−1 relative to the wind. Simultaneous time series of cloud-tracked wind fluctuations also exhibit a 4-day periodicity, lending support to the wave interpretation. The prograde propagation and equatorial confinement of the wave, and the absence of analogous meridional wind fluctuations, identify it as a Kelvin wave. Zonal winds peak near the leading edge of the Y; gravity wave theory then implies that dark UV features at low latitudes are cold and produced by upwelling or convection associated with the wave. In 1982–83 the Kelvin mode was very weak or absent, replaced by a 5-day equatorial periodicity in brightness that is not significantly different from the 5.0-day cloud-tracked wind rotation period recorded during those years. Zonal wind fluctuations for 1982 show no obvious spectral peak, suggesting that brightness variations at this time are due to advection of a remnant albedo pattern rather than active wave propagation. The Kelvin wave amplitude and implied propagation characteristics suggest that it dissipates at the cloud tops and contributes significantly to the maintenance of the cloud top equatorial superrotation. The disappearance of the Kelvin wave between 1980 and 1982 may therefore explain the coincident 5–10 ms−1 decline in the equatorial zonal wind. The 1985–86 images indicate a return of the 4-day brightness periodicity and a restoration of equatorial wind speeds similar to those in 1979–80. Thus, the cloud level dynamics may be cyclic, with an apparent time scale of 5–10 years. A separate midlatitude planetary-scale transient mode with a period near 5 days also occurs when the 4-day equatorial wave is present. The midlatitude mode retrogrades with respect to the zonal wind and may be a slowly rotating analog to an internal Rossby-Haurwitz wave generated by shear instability of the midlatitude jet. If so, it too may accelerate the equatorial wind. Solar-locked diurnal and semidiurnal tidal modes are also present in both the brightness and cloud-tracked wind data during all imaging periods; their amplitudes appear to be similar to that of the equatorial Kelvin wave. The long-term evolution and maintenance of the Venus cloud top superrotation may therefore reflect a complex balance among at least four eddy momentum transport mechanisms.

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Anthony D. Del Genio
and
Mao-Sung Yao

Abstract

We examine the response of the GISS global climate model to different parameterizations of moist convective man flux. A control run with arbitrarily specified updraft mass flux is compared to experiments that predict cumulus mass flux on the basis of low-level convergence, convergence plus surface evaporation, or convergence and evaporation modified by varying boundary layer height. An experiment that includes a simple parameterization of saturated convective-scale downdrafts is also described. Convergence effects on cumulus mass flux significantly improve the model's January climatology by increasing the frequency of occurrence of deep convection in the tropics and decreasing it at high latitudes, shifting the ITCZ from 12°N to 4°5, strengthening convective heating in the western Pacific, and increasing tropical long-wave eddy kinetic energy. Surface evaporation effects generally oppose the effects of convergence but are necessary to produce realistic continental convective heating and well-defined marine shallow cumulus regions. Varying boundary layer height (as prescribed by variations in lifting condensation level) has little effect on the model climatology. Downdrafts, however, reinforce many of the positive effects of convergence while also improving the model's vertical humidity profile and radiation balance. The diurnal cycle of precipitation over the West Pacific is best simulated when convergence determines cumulus mass flux, while surface flux effects are needed to reproduce diurnal variations in the continental ITCZ. In each experiment the model correctly simulates the observed correlation between deep convection strength and tropical sea surface temperature; the parameterization of cumulus mass flux has little effect on this relationship. The experiments have several implications for cloud effects on climate sensitivity. The dependence of cumulus mass flux on vertical motions, and the insensitivity of mean vertical motions to changes in forcing, suggests that the convective response to climate forcing may be weaker than that estimated in previous global climate model simulations that link convection only to moist static instability. This implies that changes in cloud cover and hence positive cloud feedback have been overestimated in these climate change experiments. Downdrafts may affect the feedback in the same sense by replenishing boundary layer moisture relative to cumulus parameterization schemes with only dry compensating subsidence.

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Anthony D. Del Genio
and
Robert J. Suozzo

Abstract

As a preliminary step in the development of a general circulation model for general planetary use, a simplified version of the GISS Model 1 GCM has been run at various rotation periods to investigate differences between the dynamical regimes of rapidly and slowly rotating planets. To isolate the dynamical processes, the hydrologic cycle is suppressed and the atmosphere is forced with perpetual annual mean solar heating. All other parameters except the rotation period remain fixed at their terrestrial values. Experiments were conducted for rotation periods of ⅔, 1, 2, 4, 8, 16, 64 and 256 days. The results are in qualitative agreement with similar experiments carded out previously with other GCMs and with certain aspects of one Venus GCM simulation. As rotation rate decreases, the energetics shifts from baroclinic to quasi-barotropic when the Rossby radius of deformation reaches planetary scale. The Hadley cell expands poleward and replaces eddies as the primary mode of large-scale heat transport. Associated with this is a poleward shift of the baroclinic zone and jet stream and a reduction of the equator-pole temperature contrast. Midlatitude jet strength peaks at 8 days period, as does the weak positive equatorial zonal wind which occurs at upper levels at all rotation periods. Eddy momentum transport switches from poleward to equatorward at the same period. Tropospheric mean static stability generally increases in the tropics and decreases in midlatitudes as rotation rate decreases, but the global mean static stability is independent of rotation rate. The peak in the eddy kinetic energy spectrum shifts toward lower wavenumbers, reaching wavenumber 1 at a period of 8 days. Implications of these results for the dynamics of Venus and Titan are discussed. Specifically, it is suggested that the extent of low-level convection determines whether the Gierasch mechanism contributes significantly to equatorial superrotation on these planets.

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Joanna M. Futyan
and
Anthony D. Del Genio

Abstract

In the tropical African and neighboring Atlantic region there is a strong contrast in the properties of deep convection between land and ocean. Here, satellite radar observations are used to produce a composite picture of the life cycle of convection in these two regions. Estimates of the broadband thermal flux from the geostationary Meteosat-8 satellite are used to identify and track organized convective systems over their life cycle. The evolution of the system size and vertical extent are used to define five life cycle stages (warm and cold developing, mature, cold and warm dissipating), providing the basis for the composite analysis of the system evolution. The tracked systems are matched to overpasses of the Tropical Rainfall Measuring Mission satellite, and a composite picture of the evolution of various radar and lightning characteristics is built up.

The results suggest a fundamental difference in the convective life cycle between land and ocean. African storms evolve from convectively active systems with frequent lightning in their developing stages to more stratiform conditions as they dissipate. Over the Atlantic, the convective fraction remains essentially constant into the dissipating stages, and lightning occurrence peaks late in the life cycle. This behavior is consistent with differences in convective sustainability in land and ocean regions as proposed in previous studies.

The area expansion rate during the developing stages of convection is used to provide an estimate of the intensity of convection. Reasonable correlations are found between this index and the convective system lifetime, size, and depth.

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Mao-Sung Yao
and
Anthony D. Del Genio

Abstract

An improved version of the GISS Model II cumulus parameterization designed for long-term climate integrations is used to study the effects of entrainment and multiple cloud types on the January climate simulation. Instead of prescribing convective mass as a fixed fraction of the cloud base grid-box mass, it is calculated based on the closure assumption that the cumulus convection restores 0the atmosphere to a neutral most convective state at cloud base. This change alone significantly improves the distribution of precipitation, convective mass exchanges and frequencies in the January climate. The vertical structure of the tropical atmosphere exhibits quasi-equilibrium behavior when this closure is used, even though there is no explicit constraint applied above cloud base. Global aspects of the simulation using the neutral buoyancy closure are almost identical to those obtained in a previous study with a closure relating cumulus mass flux explicitly to large-scale forcing.

A prescription of 0.2 km−1 for the fractional rate of entrainment lower the peak of the convective heating profile, reduces equatorial specific humidifies in the upper atmosphere to more realistic values, and greatly increases eddy kinetic energy at the equator due to reduced momentum mixing. With two cloud types per convective event, each cloud type having a prescribed size and entrainment rate, a clear bimodal distribution of convective mass flux is obtained in strong convective events. At the same time, many of the desirable climate features produced by the neutral buoyancy and entrainment experiments are preserved.

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