An assessment of the potential impact of a downward shift of tropospheric water vapor on climate sensitivity (original) (raw)
Related papers
Tropospheric Water Vapor and Climate Sensitivity
Journal of the Atmospheric Sciences, 1999
Estimates are made of the effect of changes in tropospheric water vapor on the climate sensitivity to doubled carbon dioxide (CO 2), using a coarse resolution atmospheric general circulation model coupled to a slab mixed layer ocean. The sensitivity of the model to doubled CO 2 is found as the difference between the equilibrium responses for control and doubled CO 2 cases. Clouds are specified to isolate the water vapor feedback. Experiments in which the water vapor distribution is specified rather than internally calculated are used to find the contribution of water vapor in various layers and latitude belts to the sensitivity. The contribution of water vapor in layers of equal mass to the climate sensitivity varies by about a factor of 2 with height, with the largest contribution coming from layers between 450 and 750 mb, and the smallest from layers above 230 mb. The positive feedback on the global mean surface temperature response to doubled CO 2 from water vapor above 750 mb is about 2.6 times as large as that from water vapor below 750 mb. The feedback on global mean surface temperature due to water vapor in the extratropical free troposphere (above 750 mb) is about 50% larger than the feedback due to the lower-latitude free troposphere water vapor. Several important sources of nonlinearity of the radiative heating rates were identified in the process of constructing the specified cloud and water vapor fields. These are (i) the interaction of clouds and solar radiation, which produces much more reflection of solar radiation for time mean clouds than for the instantaneous clouds; (ii) the correlation of clouds and water vapor, which produces less downward longwave radiation at the ground for correlated clouds and water vapor than when these fields are independent; and (iii) the interaction of water vapor with longwave radiation, which produces less downward longwave radiation at the ground for the average over instantaneous water vapor distributions than for the time mean water vapor distribution.
Multimodel Analysis of the Water Vapor Feedback in the Tropical Upper Troposphere
Journal of Climate, 2006
Relationships between the mean humidity in the tropical upper troposphere and tropical sea surface temperatures in 17 coupled ocean–atmosphere global climate models were investigated. This analysis builds on a prior study of humidity and surface temperature measurements that suggested an overall positive climate feedback by water vapor in the tropical upper troposphere whereby the mean specific humidity increases with warmer sea surface temperature (SST). The model results for present-day simulations show a large range in mean humidity, mean air temperature, and mean SST, but they consistently show increases in upper-tropospheric specific humidity with warmer SST. The model average increase in water vapor at 250 mb with convective mean SST is 44 ppmv K−1, with a standard deviation of 14 ppmv K−1. Furthermore, the implied feedback in the models is not as strong as would be the case if relative humidity remained constant in the upper troposphere. The model mean decrease in relative hu...
Atmospheric Chemistry and Physics
Variations in tropical lower-stratospheric humidity influence both the chemistry and climate of the atmosphere. We analyze tropical lower-stratospheric water vapor in 21st century simulations from 12 state-of-the-art chemistry–climate models (CCMs), using a linear regression model to determine the factors driving the trends and variability. Within CCMs, warming of the troposphere primarily drives the long-term trend in stratospheric humidity. This is partially offset in most CCMs by an increase in the strength of the Brewer–Dobson circulation, which tends to cool the tropical tropopause layer (TTL). We also apply the regression model to individual decades from the 21st century CCM runs and compare them to a regression of a decade of observations. Many of the CCMs, but not all, compare well with these observations, lending credibility to their predictions. One notable deficiency is that most CCMs underestimate the impact of the quasi-biennial oscillation on lower-stratospheric water ...
Journal of Climate, 1999
The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a twobox equilibrium model of the Tropics. One box represents the convecting, warm SST, high humidity region of the Tropics, and the other box represents the subsidence region with low humidity, boundary layer clouds, and cooler SST. The two regions communicate by energy and moisture fluxes that are proportional to the strength of the overturning circulation that couples the two regions. The boundary layer properties in the subsiding region are predicted with a mixing line model. Humidity above the inversion in the subsiding region is predicted from moisture conservation. The humidity above the inversion in the subsiding region increases rapidly with temperature, but this has less effect on the sensitivity than expected, because the inversion lowers as the humidity above the inversion is increased. Some of the increased greenhouse effect of the free troposphere can be offset by decreased greenhouse effect of the boundary layer. Increasing the area of the warm, convective region increases the SSTs, because of the greenhouse effect of the greater upper-tropospheric water vapor in the convective region. The circulation strength is constrained by radiative cooling in the cold pool. The strength of the circulation decreases with increasing convective area, because the increase in dry static stability overwhelms the increase in cooling rate. Although they have strong individual effects on longwave and shortwave radiation, high clouds in the convective region do not affect the tropical SSTs strongly, because their net radiative forcing at the top of the atmosphere is small. Low clouds in the subsidence region have a strong cooling affect on the tropical SST, because they strongly reduce net radiative heating at the top of the atmosphere. A negative feedback is produced if the low clouds are predicted from the observed relationship between stratus cloud amount and lower-tropospheric stability.
The Effect of Cumulus Convection on CO2-Induced Climate Change in the Tropics
Journal of the Meteorological Society of Japan. Ser. II, 1997
CO2-induced climate change related to the effect of cumulus convection on the vertical profile of water vapor was studied using a vertical one-dimensional radiative convective model with the Arakawa-Schubert cumulus parameterization that accounts for the drying effect of cumulus convection. Annual mean tropical solar forcing is used because cumulus convection is most active in the tropics. Since moisture and heat transport by large-scale motion (LS forcing) are important to moisture and heat balance in the tropics, two types of doubling CO2 experiments were conducted with and without considering LS forcing. The increase in specific humidity is small above the middle troposphere without LS forcing and the surface temperature increase is much smaller than that obtained in a three-dimensional CO2 experiment. When LS forcing is specified, however, water vapor increases significantly throughout the troposphere and the surface temperature increase is enhanced to a similar degree as in a three-dimensional experiment. LS forcing of heat enhances cumulus convection under control CO2 concentration, while LS forcing of moisture contributes to enhance changes due to CO2 doubling. Thus, the magnitude of cumulus convection response to change in radiative forcing is very sensitive to LS forcing. This means that it is important to incorporate the effects of moisture and heat transport by large-scale motion when investigating cumulus convection response to climate changes.
Convective-scale responses of a large-domain, modelled tropical environment to surface warming
Quarterly Journal of the Royal Meteorological Society, 2013
This article explores the response of convective-scale atmospheric characteristics to surface temperature through the lens of large-domain, cloud-system-resolving model experiments run at radiative convective equilibrium. We note several features reminiscent of the response to surface warming in atmospheric general circulation models. These include an increase in the rain rate that is smaller than the modelled increase in precipitable water, a systematic decrease in sensible heating and an increase in clear-sky cooling. However, in contrast to climate models, we note that tropospheric relative humidity increases and columnintegrated water vapour increases at the rate anticipated from the Clausius-Clapeyron relationship, but only when compared with the troposphere mean temperature rather than surface temperature. Also shown are results elucidating the changes in the vertically integrated water budget and the distribution of high precipitation rates shifting toward higher rates. Moist static energy distributions are analyzed and, from these, clouds are implicated in effecting the final equilibrium state of the atmosphere. The results indicate that, while there are aspects of the tropical equilibrium that are represented realistically in current general circulation model climate-change experiments, there are potentially influential local interactions that are sufficiently important as to alter the mean response of the tropical water and energy balance to changes in sea-surface temperature. Convection is shown to dictate the equilibrium state across all scales, including those unresolved in climate models, rather than only responding to surface-induced changes.
Upper-tropospheric humidity changes under constant relative humidity
Atmospheric Chemistry and Physics Discussions, 2015
Theoretical derivations are given on the change of upper-tropospheric humidity (UTH) in a warming climate. Considered view is that the atmosphere, getting moister with increasing temperatures, will retain a constant relative humidity. In the present study we show that the upper-tropospheric humidity, a weighted mean over a relative humidity profile, will change in spite of constant relative humidity. The simple reason for this is that the weighting function, that defines UTH, changes in a moister atmosphere. Through analytical calculations using observations and through radiative transfer calculations we demonstrate that two quantities that define the weighting function of UTH can change: the water vapour scale height and the peak emission altitude. Applying these changes to real profiles of relative humidity shows that absolute UTH changes typically do not exceed 1 %. If larger changes would be observed they would be an indication of climatological changes of relative humidity. As ...
Sensitivity of a global climate model to an increase of CO 2 concentration in the atmosphere
Journal of Geophysical Research, 1980
This study investigates the response of a global model of the climate to the quadrupling of the CO2 concentration in the atmosphere. The model consists of (1) a general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a simple mixed layer model of the oceans. It has a global computational domain and realistic geography. For the computation of radiative transfer, the seasonal variation of insolation is imposed at the top of the model atmosphere, and the fixed distribution of cloud cover is prescribed as a function of latitude and of height. It is found that with some exceptions, the model succeeds in reproducing the large-scale characteristics of seasonal and geographical variation of the observed atmospheric temperature. The climatic effect of a CO2 increase is determined by comparing statistical equilibrium states of the model atmosphere with a normal concentration and with a 4 times the normal concentration of CO2 in the air. It is found that the warming of the model atmosphere resulting from the CO2 increase has significant seasonal and latitudinal variation. Because of the absence of an albedo feedback mechanism, the warming over the Antarctic continent is somewhat less than the warming in high latitudes of the northern hemisphere. Over the Arctic Ocean and its surroundings, the warming is much larger in winter than summer, thereby reducing the amplitude of seasonal temperature variation. It is concluded that this seasonal asymmetry in the warming results from the reduction in the coverage and thickness of the sea ice. The warming of the model atmosphere results in an enrichment of the moisture content in the air and an increase in the poleward moisture transport. The additional moisture is picked up from the tropical ocean and is brought to high latitudes where both precipitation and runoff increase throughout the year. Further, the time of rapid snowmelt and maximum runoff becomes earlier. This paper is not subject to U.S. copyright. Published in 1980 by the American Geophysical Union. insolation. They conducted extensive studies of the thermal, dynamical, and hydrological response of the model. The present study is a natural extension of the studies by Manabe and Wetheraid. It investigates the CO2 climate sensitivity problem by use of a global circulation model of the atmosphere with a simple mixed layer ocean, realistic geography, and seasonal variation of insolation. In the interpretation of the results from this model, one should recognize that the additional complexity of the model does not necessarily guarantee the better simulation of the sensitivity of the actual climate. However, it is hoped that the present study identifies some specific mechanisms controlling the sensitivity of the climate. Special emphasis of the study is placed upon the investigation of the seasonal and interhemispheric asymmetries in the response of the model climate to an increase of the CO2 concentration in the air. Some of the results from this study were summarized briefly in an earlier publication by Manabe and $touffer [ 1979]. 2. MODEL STRUCTURE As the box diagram of Figure I indicates, the mathematical model of global climate used for this study consists of (1) a spectral general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a mixed layer model of the oceans. The description of those three parts of the model follows. Atmospheric Model The general circulation model of the atmosphere predicts the rates of the changes in the vertical component of vorticity and horizontal divergence, temperature, moisture, and surface pressure based upon the vorticity equation, divergence equation, thermodynamical equation, and continuity equations of moisture and mass. The prognostic equations assume the by-Paper number 80C0663. 5529 5530 MANABE AND STOUFFER.' GLOBAL CLIMATE MODEL OF CO 2 INCREASE
Relative humidity changes in a warmer climate
Journal of Geophysical Research, 2010
1] Key climate feedback due to water vapor and clouds rest largely on how relative humidity R changes in a warmer climate, yet this has not been extensively analyzed in models. General circulation models (GCMs) from the CMIP3 archive and several higherresolution atmospheric GCMs examined here generally predict a characteristic pattern of R trend with global temperature that has been reported previously in individual models, including increase around the tropopause, decrease in the tropical upper troposphere, and decrease in midlatitudes. This pattern is very similar to that previously reported for cloud cover in the same GCMs, confirming the role of R in controlling changes in simulated cloud. Comparing different models, the trend in each part of the troposphere is approximately proportional to the upward and/or poleward gradient of R in the present climate. While this suggests that the changes simply reflect a shift of the R pattern upward with the tropopause and poleward with the zonal jets, the drying trend in the subtropics is roughly 3 times too large to be attributable to shifts of subtropical features, and the subtropical R minima deepen in most models. R trends are correlated with horizontal model resolution, especially outside the tropics, where they show signs of convergence and latitudinal gradients become close to available observations for GCM resolutions near T85 and higher. We argue that much of the systematic change in R can be explained by the local specific humidity having been set (by condensation) in remote regions with different temperature changes, hence the gradients and trends each depend on a model's ability to resolve moisture transport. Finally, subtropical drying trends predicted from the warming alone fall well short of those observed in recent decades. While this discrepancy supports previous reports of GCMs underestimating Hadley cell expansion, our results imply that shifts alone are not a sufficient interpretation of changes.