The Tropospheric Humidity Trends of NCEP/NCAR Reanalysis Before the Satellite Era (original) (raw)
Related papers
Geophysical Research Letters, 2003
1] The greenhouse effect of cloud may be quantified as the difference between outgoing longwave radiation (OLR) and its clear-sky component (OLRc). Clear-sky measurements from satellite preferentially sample drier, more stable conditions relative to the monthly-mean state. The resulting observational bias is evident when OLRc is stratified by vertical motion; differences to climate model OLRc of 15 Wm À2 occur over warm regions of strong ascent. Using data from the ECMWF 40-year reanalysis, an estimate of cloud longwave radiative effect is made which is directly comparable with standard climate model diagnostics. The impact of this methodology on the cancellation of cloud longwave and shortwave radiative forcing in the tropics is estimated. R. P., and M. A. Ringer, Inconsistencies between satellite estimates of longwave cloud forcing and dynamical fields from reanalyses,
Journal of Geophysical Research, 1993
To understand the role of clouds in the atmospheric circulation and in the modulation of energy available at the surface, their effect on the the atmospheric and surface absorption should be determined. The C1 data of the Intemational Satellite Cloud Climatology Project, along with the Satellite Algorithm for Shortwave Radiation Budget, are used to estimate the shortwave cloud effects in terms of the cloud-radiative forcing at the top of the atmosphere (TOA), at the surface and of the atmospheric column on a global scale for the July months of 1983-1985. Global means of TOA cloud forcing range from -43.6 (1983) to -39.1 Wm -2 (1985). The cloud forcing for July 1985 is underestimated, by about 8 Wm -2, compared with that obtained from the Earth Radiation Budget Experiment. The cloud forcing at the surface is almost identical to that at the TOA, indicating that the effect of clouds on the shortwave energy budget of the surface-atmosphere system is such that most of the cooling is at the surface. Regression analysis of the computed fluxes shows a strong linear correlation between the TOA and surface cloud forcing. The monthly averaged regional values of the atmospheric cloud forcing are generally less than the estimated uncertainty of 20 Wm -2. Assuming that the uncerhainties cancel, the global mean of the atmospheric cloud forcing is between 1 and 2 Wm -2, suggesting a slight warming owing to the presence of clouds.
Quarterly Journal of the Royal Meteorological Society, 1999
A simulation of the earth's clear-sky long-wave radiation budget is used to examine the dependence of clearsky outgoing long-wave radiation (OLR) on surface temperature and relative humidity. The simulation uses the European Centre for Medium-Range Weather Forecasts global reanalysed fields to calculate clear-sky OLR over the period from January 1979 to December 1993, thus allowing the seasonal and interannual timescales to be resolved. The clear-sky OLR is shown to be primarily dependent on temperature changes at high latitudes and on changes in relative humidity at lower latitudes. Regions exhibiting a 'super-greenhouse' effect are identified and are explained by considering the changes in the convective regime associated with the Hadley circulation over the seasonal cycle, and with the Walker circulation over the interannual timescale. The sensitivity of clear-sky OLR to changes in relative humidity diminishes with increasing relative humidity. This is explained by the increasing saturation of the water-vapour absorption bands with increased moisture. By allowing the relative humidity to vary in specified vertical slabs of the troposphere over an interannual timescale it is shown that changes in humidity in the mid troposphere (400 to 700 hPa) are of most importance in explaining clear-sky OLR variations. Relative humidity variations do not appear to affect the positive thermodynamic water-vapour feedback significantly in response to surface temperature changes.
Satellite observations of upper tropospheric relative humidity, clouds and wind field divergence
… to atmospheric physics, 1995
This paper presents / an observational study of upper level wind field divergence, upper tropospheric relative humidity (UTH) and high level cloud from satellite. Wind fields are derived from successive half-hourly images of the METEOSAT-4 water vapour (WV: 5.7-7.1 (im) channel. The UTH is inferred from the WV image data with a physical retrieval scheme based on radiative forward calculations. Upper tropospheric cloud amounts are based on the C0 2 -slicing technique applied to bi-spectral radiance observations from the High Resolution Infrared Sounder (HIRS) aboard the NOAA-11 and 12 polar orbiting satellites. The previous UTH retrieval has been simplified such that two radiative forward calculations define a linear relationship between observed WV brightness temperature and the UTH for each retrieval. The new approach is computationally more efficient, however, it retains the important dependency of the UTH on local mean temperature and lapse rate. The WV wind vectors provide a dense spatial coverage of the upper tropospheric flow and, thus, can be used to estimate monthly mean wind fields and associated large-scale divergence. Inferred values of the wind field divergence over a scale of 1500 km are between -5xl0 _6 s _1 and 5xl0~6s _1 which is consistent with other observations. The spatial patterns of monthly means of wind field divergence and UTH closely resemble each other although the quantitative relationship is non-linear. A comparison of the UTH with high-level cloud amounts from the HIRS instrument aboard the NOAA polar orbiting satellites also features a high spatial coherence between both data sets. This suggests that largescale dynamics is the governing factor for both the upper tropospheric relative humidity field and the cloud formation processes associated with the humidity fields. Some implications for largescale models are discussed.
Journal of Climate, 2006
Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed to determine the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (0-3 km), middle (3-6 km), and high clouds (Ͼ6 km) using ARM SCF ground-based paired lidar-radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up-and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with uncertainties of ϳ10 W m Ϫ2. The annual averages of total and single-layered low-, middle-, and high-cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Both totaland low-cloud amounts peak during January and February and reach a minimum during July and August; high clouds occur more frequently than other types of clouds with a peak in summer. The average annual downwelling surface SW fluxes for total and low clouds (151 and 138 W m Ϫ2 , respectively) are less than those under middle and high clouds (188 and 201 W m Ϫ2 , respectively), but the downwelling LW fluxes (349 and 356 W m Ϫ2) underneath total and low clouds are greater than those from middle and high clouds (337 and 333 W m Ϫ2). Low clouds produce the largest LW warming (55 W m Ϫ2) and SW cooling (Ϫ91 W m Ϫ2) effects with maximum and minimum absolute values in spring and summer, respectively. High clouds have the smallest LW warming (17 W m Ϫ2) and SW cooling (Ϫ37 W m Ϫ2) effects at the surface. All-sky SW cloud radiative forcing (CRF) decreases and LW CRF increases with increasing cloud fraction with mean slopes of Ϫ0.984 and 0.616 W m Ϫ2 % Ϫ1 , respectively. Over the entire diurnal cycle, clouds deplete the amount of surface insolation more than they add to the downwelling LW flux. The calculated CRFs do not appear to be significantly affected by uncertainties in data sampling and clear-sky screening. Traditionally, cloud radiative forcing includes not only the radiative impact of the hydrometeors, but also the changes in the environment. Taken together over the ARM SCF, changes in humidity and surface albedo between clear and cloudy conditions offset ϳ20% of the NET radiative forcing caused by the cloud hydrometeors alone. Variations in water vapor, on average, account for 10% and 83% of the SW and LW CRFs, respectively, in total cloud cover conditions. The error analysis further reveals that the cloud hydrometeors dominate the SW CRF, while water vapor changes are most important for LW flux changes in cloudy skies. Similar studies over other locales are encouraged where water and surface albedo changes from clear to cloudy conditions may be much different than observed over the ARM SCF.
The relationship between tropospheric wave forcing and tropical lower stratospheric water vapor
Atmospheric Chemistry and Physics, 2008
Using water vapor data from HALOE and SAGE II, an anti-correlation between planetary wave driving (here expressed by the mid-latitude eddy heat flux at 50 hPa added from both hemispheres) and tropical lower stratospheric (TLS) water vapor has been obtained. This appears to be a manifestation of the inter-annual variability of the Brewer-Dobson (BD) circulation strength (the driving of which is generally measured in terms of the mid-latitude eddy heat flux), and hence amount of water vapor entering the stratosphere. Some years such as 1991 and 1997 show, however, a clear departure from the anti-correlation which suggests that the water vapor changes in TLS can not be attributed solely to changes in extratropical planetary wave activity (and its effect on the BD circulation). After 2000 a sudden decrease in lower stratospheric water vapor has been reported in earlier studies based upon satellite data from HALOE, SAGE II and POAM III indicating that the lower stratosphere has become drier since then. This is consistent with a sudden rise in the combined mid-latitude eddy heat flux with nearly equal contribution from both hemispheres as shown here and with the increase in tropical upwelling and decrease in cold point temperatures found by . The low water vapor and enhanced planetary wave activity (in turn strength of the BD circulation) has persisted until the end of the satellite data records. From a multi-variate regression analysis applied to 27 years of NCEP and HadAT2 (radiosonde) temperatures (up to 2005) with contributions from solar cycle, stratospheric aerosols and QBO removed, the enhancement wave driving after 2000 is estimated to contribute up to 0.7 K cooling to the overall TLS temperature change during the period 2001-2005 when compared to the period 1996-2000. NCEP cold point temperature show an average decrease of nearly 0.4 K from changes in the wave driving, which is consistent with observed mean TLS water vapor changes of about −0.2 ppm after 2000.
Journal of Climate, 2006
The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolvedscale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability.
Cloud-Radiative Forcing and Climate: Results from the Earth Radiation Budget Experiment
Science, 1989
The study of climate and climate change is hindered by a lack of information on the effect of clouds on the radiation balance ofthe earth, referred to as the cloud-radiative forcing. Quantitative estimates ofthe global distributions of cloud-radiative forcing have been obtained from the spaceborne Earth Radiation Budget Experiment (ERBE) launched in 1984. For the April 1985 period, the global shortwave doud forcing [-44.5 watts per square meter (W/m2)] due to the enhancement of planetary albedo, exceeded in magnitude the longwave cloud forcing (31.3 W/m2) resulting from the greenhouse effect of douds. Thus, clouds had a net cooling effect on the earth. This cooling effect is large over the mid-and high-latitude oceans, with values reaching -100 W/m2. The monthly averaged longwave cloud forcing reached maximum values of 50 to 100 W/m2 over the convectively disturbed regions of the tropics. However, this heating effect is nearly canceled by a correspondingly large negative shortwave cloud forcing, which indicates the delicately balanced state of the tropics. The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO2. The shortwave and longwave components ofcloud forcing are about ten times as large as those for a CO2 doubling. Hence, small changes in the cloud-radiative forcing fields can play a significant role as a climate feedback mechanism. For example, during past glaciations a migration toward the equator of the field of strong, negative cloudradiative forcing, in response to a similar migration of cooler waters, could have significantly amplified oceanic cooling and continental glaciation. C LOUDS ARE REGULATORS OF THE RADIATVE HEATING OF the planet. They reflect a large part of the incoming solar radiation, causing the albedo of the entire earth to be about twice what it wouild be in the absence of clouds (1). Clouds also absorb the longwave (LW) radiation (also known as infrared or thermal radiation) emitted by the warmer earth and emit energy to space at the colder temperatures of the cloud tops. Cloud LW absorption and emission are, in a sense, similar to the radiative effects of atmospheric gases. The combined effect of LW absorption and emission-that is, the greenhouse effect-is a reduction in the LW radiation emitted to space. The greenhouse effect ofclouds may be larger than that resulting from a hundredfold increase in the CO2 concentration of the atmosphere (2).