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.

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.

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.

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.

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 M ≤ M_e (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.

Abstract

Climate changes obtained from five doubled CO₂ experiments with different parameterizations of large-scale clouds and moist convection are studied by use of the Goddard Institute for Space Studies (GISS) GCM at 4° lat × 5° long resolution. The baseline for the experiments is GISS Model II, which uses a diagnostic cloud scheme with fixed optical properties and a convection scheme with fixed cumulus mass fluxes and no downdrafts. The global and annual mean surface air temperature change (ΔT _s ) of 4.2°C obtained by using the Model II physics at 8° lat × 10° long resolution is reduced to 3.55°C at the finer resolution. This is due to a significant reduction of tropical cirrus clouds in the warmer climate when a finer resolution is used, despite the fact that the relative humidity increases there with a doubling of CO₂. When the new moist convection parameterization of and prognostic large-scale cloud parameterization of are used, ΔT _s is reduced to 3.09°C from 3.55°C. This is the net result of the inclusion of the feedback of cloud optical thickness and phase change of cloud water, and the presence of areally extensive cumulus anvil clouds. Without the optical thickness feedback, ΔT _s is further reduced to 2.74°C, suggesting that this feedback is positive overall. Without anvil clouds, ΔT _s is increased from 3.09° to 3.7°C, suggesting that anvil clouds of large optical thickness reduce the climate sensitivity. The net effect of using the new large-scale cloud parameterization without including the detrainment of convective cloud water is a slight increase of ΔT _s from 3.56° to 3.7°C. The net effect of using the new moist convection parameterization without anvil clouds is insignificant (from 3.55° to 3.56°C). However, this is a result of a combination of many competing differences in other climate parameters. Despite the global cloud cover decrease simulated in most of the experiments, middle- and high-latitude continental cloudiness generally increases with warming, consistent with the sense of observed twentieth-century cloudiness trends; an indirect aerosol effect may therefore not be the sole explanation of these observations.

An analysis of climate sensitivity and changes in cloud radiative forcing (CRF) indicates that the cloud feedback is positive overall in all experiments except the one using the new moist convection and large-scale cloud parameterization with prescribed cloud optical thickness, for which the cloud feedback is nearly neutral. Differences in ΔCRF among the different experiments cannot reliably be anticipated by the analogous differences in current climate CRF. The meridional distribution of ΔCRF suggests that the cloud feedback is positive mostly in the low and midlatitudes, but in the high latitudes, the cloud feedback is mostly negative and the amplification of ΔT _s is due to other processes, such as snow/ice–albedo feedback and changes in the lapse rate. The authors’ results suggest that when a sufficiently large variety of cloud feedback mechanisms are allowed for, significant cancellations between positive and negative feedbacks result, causing overall climate sensitivity to be less sensitive to uncertainties in poorly understood cloud physics. In particular, the positive low cloud optical thickness correlations with temperature observed in satellite data argue for a minimum climate sensitivity higher than the 1.5°C that is usually assumed.

Abstract

In this paper the coupling of the Goddard Institute for Space Studies (GISS) general circulation model (GCM) to an online sulfur chemistry model and source models for organic matter and sea salt that is used to estimate the aerosol indirect effect is described. The cloud droplet number concentration is diagnosed empirically from field experiment datasets over land and ocean that observe droplet number and all three aerosol types simultaneously; corrections are made for implied variations in cloud turbulence levels. The resulting cloud droplet number is used to calculate variations in droplet effective radius, which in turn allows one to predict aerosol effects on cloud optical thickness and microphysical process rates. The aerosol indirect effect is calculated by differencing the top-of-the-atmosphere net cloud radiative forcing for simulations with present-day versus preindustrial emissions. Both the first and second indirect effects are explored. The sensitivity of the results presented here to cloud parameterization assumptions that control the vertical distribution of cloud occurrence, the autoconversion rate, and the aerosol scavenging rate, each of which feeds back significantly on the model aerosol burden, are tested. The global mean aerosol indirect effect for all three aerosol types ranges from −1.55 to −4.36 W m⁻² in the simulations. The results are quite sensitive to the preindustrial background aerosol burden, with low preindustrial burdens giving strong indirect effects, and to a lesser extent to the anthropogenic aerosol burden, with large burdens giving somewhat larger indirect effects. Because of this dependence on the background aerosol, model diagnostics such as albedo-particle size correlations and column cloud susceptibility, for which satellite validation products are available, are not good predictors of the resulting indirect effect.

Abstract

The authors have analyzed global station data and created a gridded dataset of monthly precipitation for the period of 1900–88. Statistical analyses suggest that discontinuities associated with instrumental errors are large for many high-latitude station records, although they are unlikely to be significant for the majority of the stations. The first leading EOF in global precipitation fields is an ENSO-related pattern, concentrating mostly in the low latitudes. The second leading EOF depicts a linear increasing trend (∼2.4 mm decade⁻¹) in global precipitation fields during the period of 1900–88. Consistent with the zonal precipitation trends identified in previous analyses, the EOF trend is seen as a long-term increase mostly in North America, mid- to high-latitude Eurasia, Argentina, and Australia. The spatial patterns of the trend EOF and the rate of increase are generally consistent with those of the precipitation changes in increasing CO₂ GCM experiments.

The North Atlantic oscillation (NAO) accounts for ∼10% of December–February precipitation variance over North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is above normal in northern (>50°N) Europe, the eastern United States, northern Africa, and the Mediterranean, while below-normal precipitation occurs in southern Europe, eastern Canada, and western Greenland.

Wet and dry months of one standard deviation occur at probabilities close to those of a normal distribution in midlatitudes. In the subtropics, the mean interval between two extreme events is longer. The monthly wet and dry events seldom (probability < 5%) last longer than 2 months. ENSO is the single largest cause of global extreme precipitation events. Consistent with the upward trend in global precipitation, globally, the averaged mean interval between two dry months increased by ∼28% from 1900–44 to 1945–88. The percentage of wet areas over the United States has more than doubled (from ∼12% to >24%) since the 1970s, while the percentage of dry areas has decreased by a similar amount since the 1940s. Severe droughts and floods comparable to the 1988 drought and 1993 flood in the Midwest have occurred 2–9 times in each of several other regions of the world during this century.

Abstract

Precipitation processes in convective storms are potentially a major regulator of cloud feedback. An unresolved issue is how the partitioning of convective condensate between precipitation-size particles that fall out of updrafts and smaller particles that are detrained to form anvil clouds will change as the climate warms. Tropical Rainfall Measuring Mission (TRMM) observations of tropical oceanic convective storms indicate higher precipitation efficiency at warmer sea surface temperature (SST) but also suggest that cumulus anvil sizes, albedos, and ice water paths become insensitive to warming at high temperatures. International Satellite Cloud Climatology Project (ISCCP) data show that instantaneous cirrus and deep convective cloud fractions are positively correlated and increase with SST except at the highest temperatures, but are sensitive to variations in large-scale vertical velocity. A simple conceptual model based on a Marshall–Palmer drop size distribution, empirical terminal velocity–particle size relationships, and assumed cumulus updraft speeds reproduces the observed tendency for detrained condensate to approach a limiting value at high SST. These results suggest that the climatic behavior of observed tropical convective clouds is intermediate between the extremes required to support the thermostat and adaptive iris hypotheses.

Abstract

The conditions under which supercooled liquid water gradually gives way to ice in the mixed-phase regions of clouds are still poorly understood and may be an important source of cloud feedback uncertainty in general circulation model projections of long-term climate change. Two winters of cloud phase discrimination, cloud-top temperature, sea surface temperature, and precipitation from several satellite datasets (the NASA Terra and Aqua Moderate Resolution Imaging Spectroradiometer, and the Tropical Rainfall Measuring Mission) for the North Atlantic and Pacific Ocean basins are analyzed to better understand these processes. Reanalysis surface pressures and vertical velocities are used in combination with a synoptic storm-tracking algorithm to define storm tracks, create composite storm dynamical and cloud patterns, and examine changes in storm characteristics over their life cycles. Characteristically different storm cloud patterns exist in the Atlantic and Pacific and on the west and east sides of each ocean basin. This appears to be related to the different spatial patterns of sea surface temperature in the two ocean basins. Glaciation occurs at very warm temperatures in the high, thick, heavily precipitating clouds typical of frontal ascent regions, except where vertical velocities are strongest, similar to previous field experiments. Outside frontal regions, however, where clouds are shallower, supercooled water exists at lower cloud-top temperatures. This analysis is the first large-scale assessment of cloud phase and its relation to dynamics on climatologically representative time scales. It provides a potentially powerful benchmark for the design and evaluation of mixed-phase process parameterizations in general circulation models and suggests that assumptions made in some existing models may negatively bias their cloud feedback estimates.