A53F-01 INVITED
Decadal Increases in Upper Tropospheric Water Vapor and their Implications for Climate Change
The importance of water vapor in regulating climate is undisputed. It is the dominant greenhouse gas, trapping more of Earth's heat than any other gaseous constituent. As the climate warms in response to increases in other greenhouse gases such as carbon dioxide, the concentrations of water vapor are expected to increase. If water vapor concentrations do increase in a warmer world, the added absorption will act to further amplify the initial warming. Current projections indicate that the concentration of water vapor in the upper troposphere could double by the end of the century as a result of increases in greenhouse gases. Such moistening plays a key role in amplifying the rate at which the climate warms in response to anthropogenic activities, but has been difficult to detect because of deficiencies in conventional observing systems. We use satellite measurements to highlight a distinct radiative signature of upper tropospheric moistening over the period 1982 to 2004. The observed moistening is accurately captured by climate model simulations and lends further credence to model projections of future global warming.
A53F-02 INVITED
The Role of the Deep Outflow Layer in the Tropical Water Vapor Budget
I will review the evidence from rawinsonde arrays, chemical tracers, lidar measurements, and radiative mass flux calculations which support the existence of a deep outflow mode in the tropics extending from roughly 10 km to 17 km. The subsidence of this extremely dry air gives rise to a large negative tendency on the relative humidity of the lower tropical troposphere. To maintain relative humidities near their observed values, this negative tendency must be balanced by some combination of moistening from shallow convective outflow and evaporation of precipitation. Despite the clarity with which this mode emerges from observations, it is not always clearly represented in convective parameterizations. Parameterizations which underpredict the strength or height of this mode will tend to have unrealistically high precipitation efficiencies.
A53F-03 INVITED
Direct Assessment of Water Vapor and Cloud Feedbacks From Observations
Cloud and water vapor feedbacks in climate theory will be briefly reviewed, as will the relevant physical processes associated with both cloud cover and water vapor. We will focus on the tropics in this talk. Measurements of these quantities relevant to climate must take account of a number of factors that have often been ignored: 1. Water vapor in the tropics is extremely spatially heterogeneous, and one dimensional treatments using spatially averaged values will almost always be misleading. 2. Precipitation from cirrus detrained from cumulus towers is a major source of tropospheric humidity. However, the evolution of cirrus detrainment is a process which takes on the order of 6 hours. Given the nature of satellite orbits, this leads to statistical issues that must be handled with care. 3. For climate purposes, there is a major difference between changes in cirrus associated with the concentration of convection, and changes normalized by the amount of convection. It is the latter that is relevant to climate sensitivity. These factors will be explained, and in the light of these factors, we will describe new results from CERES, TRMM, and MODIS instruments (as well as the Kwajalein ground based radar) on temperature dependence of precipitation efficiency and cloud coverage and the radiative properties of cirrus (from the recent work of Choi and Ho).
A53F-04 INVITED
On the regulation of tropical tropospheric water vapor
Boundary layer air entering convective events is extremely moist, with mixing ratios of 20-30 g/kg. In the mid and upper troposphere, however, water vapor mixing ratios (H2O) are lower, with values of 0.5-5 g/kg. In this paper, we use a simple model of mid- and upper-tropospheric H2O to investigate the mechanisms by which H2O is removed from air (dehydration). We show that a model with the simplest possible microphysical assumption (instantaneous removal of H2O at 100% RH) reproduces observed H2O fields well. We will discuss the implications of this analysis for the water vapor feedback.
A53F-05
Correlation Between Precipitable Water and Temperature in Climate Simulations and Observations.
The Special Sensor Microwaver/Imager (SSM/I) series of microwave radiometers provide 19 years of precipitable water measurements over the oceans. We have recently completed a careful intercalibration of the data from these satellites resulting in a climate-quality dataset suitable for evaluating decadal-scale changes in precipitable water. We have investigated the correlation between precipitable water and lower-tropospheric and sea-surface temperature. For large spatial averages over the tropical oceans, we find that anomalies of these variables scale approximately as would be expected from the Clausius-Clapeyron relationship for seasonal, interannual, and decadal time scales in all IPCC AR4 20th Century climate simulations. When the SSM/I precipitable water data is compared to different observational temperature datasets, we find this time scale invariance for some temperature datasets, while other temperature datasets indicate divergence from the Clausius-Clapeyron scaling at the longest time scales.
A53F-06
Correlation Structure of Tropospheric Water Vapor and Temperature in AIRS Observations and ECMWF Reanalyses
The Atmospheric Infrared Sounder (AIRS) experiment on NASA's Aqua spacecraft observes the local three- dimensional distribution of tropospheric water vapor and temperature along the Aqua orbit track. We examine the correlation structure of the AIRS observations for a variety of geographic locations and seasons. The observations are compared with European Center for Medium-range Weather Forecasting reanalysis depictions of the same quantities.
A53F-07
Convective Detrainment and Control of the Tropical Water Vapor Distribution
Sherwood et al. (2006) developed a simple power law model describing the relative humidity distribution in the tropical free troposphere where the power law exponent is the ratio of a drying time scale (tied to subsidence rates) and a moistening time which is the average time between convective moistening events whose temporal distribution is described as a Poisson distribution. Sherwood et al. showed that the relative humidity distribution observed by GPS occultations and MLS is indeed close to a power law, approximately consistent with the simple model's prediction. Here we modify this simple model to be in terms of vertical length scales rather than time scales in a manner that we think more correctly matches the model predictions to the observations. The subsidence is now in terms of the vertical distance the air mass has descended since it last detrained from a convective plume. The moisture source term becomes a profile of convective detrainment flux versus altitude. The vertical profile of the convective detrainment flux is deduced from the observed distribution of the specific humidity at each altitude combined with sinking rates estimated from radiative cooling. The resulting free tropospheric detrainment profile increases with altitude above 3 km somewhat like an exponential profile which explains the approximate power law behavior observed by Sherwood et al. The observations also reveal a seasonal variation in the detrainment profile reflecting changes in the convective behavior expected by some based on observed seasonal changes in the vertical structure of convective regions. The simple model results will be compared with the moisture control mechanisms in a GCM with many additional mechanisms, the GISS climate model, as described in Rind (2006). References Rind. D., 2006: Water-vapor feedback. In Frontiers of Climate Modeling, J. T. Kiehl and V. Ramanathan (eds), Cambridge University Press [ISBN-13 978-0-521- 79132-8], 251-284. Sherwood, S., E. R. Kursinski and W. Read, A distribution law for free-tropospheric relative humidity, J. Clim. In press. 2006