Comments on “Tropical Convection and the Energy Balance at the Top of the Atmosphere” (original) (raw)
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Journal of Geophysical Research: Atmospheres, 2010
An insightful link of model performance to the physical assumptions in general circulation models (GCMs) can be explored if assessment of radiative fluxes and cloud radiative effects go beyond those at the top of the atmosphere (TOA). In this study, we compare the radiative flux profiles (at surface, 500 hPa, 200 hPa, 70 hPa, and TOA) and cloud effect profiles (500 hPa, 200 hPa, and TOA) from HadGAM1, using Surface and Atmospheric Radiation Budget (SARB) data from Clouds and the Earth's Radiant Energy System (CERES) on the TRMM satellite over the tropics (30°S–30°N). Comparison at TOA reveals that HadGAM1 agrees well with CERES for mean cloud height but lacks in cloudiness. Comparing to its predecessor, HadAM3, HadGAM1 agrees better with observations in TOA LW cloud effects, net cloud effects, and the ratio of SW to LW cloud effects. Extending the comparison to multiple levels, we gain additional insight into the vertical differences in clouds: for clouds at heights below 500 hP...
Journal of Geophysical Research, 2010
1] Using TRMM VIRS data, we attempt to replicate the analysis made by to quantify the effect of methodological choices on the magnitude of the observed correlations between upper-level cloud cover and SST. Using brightness temperature thresholds to identify upper-level cloud, we recover a relatively small change in the normalized area of cirrus clouds with SST ($À6%/K compared to $À2%/K found by Su et al. ). We discuss the effect of several methodological choices on the magnitude of the signal, namely, the classification of cloudy regions into convective updrafts and anvil, the use of cloud weighted SST, and the truncation and sampling error of the orbital satellite data with respect to the evolution of mesoscale convective systems. Accounting for some of these methodological differences could resolve the discrepancy between the weak signal documented by and the stronger signal documented originally by and others, including the results reported in this comment.
Journal of Geophysical Research, 1993
This paper describes calculations of the spatial and temporal variation of the radiation budget of a tropical mesoscale convective system (MCS). A combination of cloud model simulations, radiation model simulations, and analyses of observations obtained during the Equatorial Mesoscale Experiment (EMEX), the Stratosphere-Troposphere Exchange Program (STEP), and the Australian Monsoon Experiment (AMEX) are used to obtain these heating rates. The twodimensional version of the Colorado State University regional atmospheric modeling system is used to simulate a tropical MCS that occurred during EMEX mission 9 on February 2, 1987. The simulation is shown to broadly agree with the observations reported in a related paper. The spatial radiative heating distributions derived from a two-stream radiative transfer model corresponding to the mature stage of the simulated cloud system indicate that significant horizontal inhomogeneities exist. According to the model results the effects of the MCS are to (1) increase in the infrared emission to the surface and to decrease in the net infrared energy loss from the atmosphere relative to the clear sky emission and 2) change the transmission of solar flux to the surface, the shortwave albedo of the atmosphere, and the solar absorption in the atmosphere. The results show how the MCS significantly reduces the solar flux to the surface relative to the clear sky values and that the largest reduction occurs under the convective portions of the mature MCS. (3) The MCS creates a total (solar plus infrared) radiative warming in the atmosphere relative to the surrounding clear sky. The value of this total heating is governed by both infrared and solar absorption. Vertical profiles of this heating show the dominance of infrared cooling near cloud top and infrared heating inside and near cloud base. The shortwave heating rate can also be as large as the infrared cooling near the cloud top region of the tropical MCS, especially at a local noon. (4) The temporal changes in radiation profiles also demonstrate how the MCS modulates the radiation budget of the atmosphere. Specifically, the total radiation energy loss of the entire two-dimensional domain of the model atmosphere decreases and eventually becomes positive as the cloud system decays, becomes a stratiform in nature, and fills the domain. This change in the column divergence of flux translates into a total column radiative heating rate of approximately 1.7 K/d (relative to the clear sky radiative cooling rate). The solar component of this domain heating tends to be concentrated in the upper troposphere, whereas the infrared component of the heating is spread over the lower and middle troposphere. These results also show how tropical mesoscale cloud system provides an effective radiative heat source for the tropical atmosphere. sive upper tropospheric cloud shields [ Webster and Stephens, 1980]. Ackerman et al. [1988] have further demonstrated that radiative heating of the order of 20 to 30 K/day is expected in tropical anvil clouds. This radiative heating rate may be a source of buoyant turbulence in the anvils [Lilly, 1988] and perhaps produces significant mesoscale circula-
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.
Dynamic Effects on the Tropical Cloud Radiative Forcing and Radiation Budget
Journal of Climate, 2008
Vertical velocity is used to isolate the effect of large-scale dynamics on the observed radiation budget and cloud properties in the tropics, using the methodology suggested by Bony et al. Cloud and radiation budget quantities in the tropics show well-defined responses to the large-scale vertical motion at 500 hPa. For the tropics as a whole, the ratio of shortwave to longwave cloud forcing (hereafter N) is about 1.2 in regions of upward motion, and increases to about 1.9 in regions of strong subsidence. If the analysis is restricted to oceanic regions with SST > 28°C, N does not increase as much for subsiding motions, because the stratocumulus regions are eliminated, and the net cloud forcing decreases linearly from about near zero for zero vertical velocity to about −15 W m−2 for strongly subsiding motion. Increasingly negative cloud forcing with increasing upward motion is mostly related to an increasing abundance of high, thick clouds. Although a consistent dynamical effect o...
On the radiative processes associated with the tropical mesoscale convective systems /
The goal of this research is to improve our understanding of the spatial and temporal characteristics of the radiative budget of the tropical mesoscale convective systems (MCSs) and to explore any possible effects of radiation on the atmosphere and the evolution of the tropical MCSs. A combination of two dimensional cloud model simulations, two-stream radiative transfer model calculations, observational analyses, and comparisons with other published results are used to achieve this goal. Administration (NASA) grant NAG5-1592S-03. Computer calculations were partially carried out using the super computer facilities at National Center for Atmospheric Research (NCAR). NCAR is partially supported by NSF. EMEX aircraft data were provided by the National Hurricane Center of the National Oceanic and Atmospheric Administration (NOAA). The EMEX aircraft Doppler radar data were made available by Brian Mapes. The AMEX sounding and AMEX synoptic data set were furnished by the Australian Bureau of Meteorological Research Center (BMRC). The STEP radiation data. were obta.ined from the NASA AMES Research Center. The satellite data set were kindly made available by Raymond Zehr. The DUNDEE Radar and wind profileI' data set were provided by Professor Steven Rutledge and his students. vii References 212 A Tables of Spectral Optical Constants 220 B Individual breakdown of the Simulated Water Fields 225 x LIST OF FIGURES 1.1 Low level horizontal radar scan of a squall-type tropical MCS ... 1.2 A composited vertical structures of squall-type tropical MCS ... 1.3 Low level horizontal radar view of a nonsquall-type tropical MCS. 1.4 An estimated vertical structures of diabatic heating profile. 1.5 An estimated vertical structures of latent heating profile. 2.1 Comparison of longwave parameterization 2.2 Comparison of solar parameterization. 2.3 Schematic of microphysical interactions. 3.1 Normalized blackbody spectra representative of the sun 3.2 The terrestrial infrared spectra and various absorption bands.. 3.3 Spectral irradiance distribution curves related to the sun: 3.4 On the average, of all the solar energy that reaches the.. 3.5 Conceptual view of the flow of the radiation in the model 3.6 A conceptual view of the multiband radiation model. .. 3.7 Relative differences in shortwave radiative budget associated with the change
Tropical Convection and the Energy Balance at the Top of the Atmosphere
Journal of Climate, 2001
Earth Radiation Budget Experiment (ERBE) and International Satellite Cloud Climatology Project (ISCCP) data are used in conjunction with a radiative transfer model to estimate the effect of various cloud types on the top-of-atmosphere radiation budget in convective regions of the Tropics. This analysis shows that individual convective cloud elements can have strongly positive or negative effects on the radiation balance. Nonetheless, the ensemble of cloud types that occurs in association with deep convection in the Tropics arranges itself so that the individual positive and negative contributions cancel each other when averaged over the convective cloud system. This behavior of the cloud ensemble is extremely interesting, and the authors speculate that it is indicative of feedbacks in the climate system that have not been explored adequately. A simple model is introduced that includes feedbacks that drive the net radiation in convective regions toward the net radiation in adjacent nonconvective areas. If the nonconvective regions have small cloud forcing, then this model predicts small net radiative forcing by the convective cloud ensemble. This feedback process requires that the circulations in the Tropics be sensitive to small SST gradients and that the convective cloud albedo be sensitive to the vertical motion.
Atmospheric Chemistry and Physics, 2005
A decadal-scale trend in the tropical radiative energy budget has been observed recently by satellites, which however is not reproduced by climate models. In the present study, we have computed the outgoing shortwave radiation (OSR) at the top of atmosphere (TOA) at 2.5 • longitudelatitude resolution and on a mean monthly basis for the 17year period 1984-2000, by using a deterministic solar radiative transfer model and cloud climatological data from the International Satellite Cloud Climatology Project (IS-CCP) D2 database. Anomaly time series for the mean monthly pixel-level OSR fluxes, as well as for the key physical parameters, were constructed. A significant decreasing trend in OSR anomalies, starting mainly from the late 1980s, was found in tropical and subtropical regions (30 • S-30 • N), indicating a decadal increase in solar planetary heating equal to 1.9±0.3 Wm −2 /decade, reproducing well the features recorded by satellite observations, in contrast to climate model results. This increase in solar planetary heating, however, is accompanied by a similar increase in planetary cooling, due to increased outgoing longwave radiation, so that there is no change in net radiation. The model computed OSR trend is in good agreement with the corresponding linear decadal decrease of 2.5±0.4 Wm −2 /decade in tropical mean OSR anomalies derived from ERBE S-10N nonscanner data (edition 2). An attempt was made to identify the physical processes responsible for the decreasing trend in tropical mean OSR. A detailed correlation analysis using pixel-level anomalies of model computed OSR flux and ISCCP cloud cover over the entire tropical and subtropical region (30 • S-30 • N), gave a correlation coefficient of 0.79, indicating that decreasing cloud cover is the main reason for the tropical OSR trend. According to the ISCCP-D2 data
Testing the impact of clouds on the radiation budgets of 19 atmospheric general circulation models
Journal of Geophysical Research: Atmospheres, 2004
We compare cloud‐radiative forcing (CRF) at the top‐of‐the atmosphere from 19 atmospheric general circulation models, employing simulations with prescribed sea‐surface temperatures, to observations from the Earth Radiation Budget Experiment (ERBE). With respect to 60°N to 60°S means, a surprising result is that many of the 19 models produce unusually large biases in Net CRF that are all of the same sign (negative), meaning that many of the models significantly overestimate cloud radiative cooling. The primary focus of this study, however, is to demonstrate a diagnostic procedure, using ERBE data, to test if a model might produce, for a given region, reasonable CRF as a consequence of compensating errors caused either by unrealistic cloud vertical structure, cloud optical depth or cloud fraction. For this purpose we have chosen two regions, one in the western tropical Pacific characterized by high clouds spanning the range from thin cirrus to deep convective clouds, and the other in ...
Radiation and cloud radiative properties in the ECMWF forecasting system (1991)
The successive versions (EC1, EC2, and EC3) of the radiation scheme used in the European Centre for Medium Range Weather Forecasts (ECMWF) operational model, including the version which became operational on May 2, 1989, are reviewed and their results are compared to results of more detailed radiation models made available thanks to the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM) program. For clear-sky conditions, the shortwave H20 absorptivity is overestimated in EC1-EC2, which leads to too large shortwave atmospheric absorption (by up to 20%) and too small downward shortwave radiation at the surface (by 5-10%). EC1 and EC2 both significantly underestimate the longwave radiative cooling, with main errors in the lower troposphere with EC1, and between 700 and 300 hPa with EC2. Therefore EC1 and, to a smaller extent, EC2 underestimate the outgoing longwave radiation at the top of the atmosphere. In cloudy conditions, EC 1 shows an exaggerated sensitivity to small amounts of scatterer, a problem corrected in EC2. Due an unrealistic model cloud embedded in EC 1-EC2, these schemes cannot properly represent both the shortwave planetary albedo and outgoing longwave radiation for realistic cloud liquid water contents. EC3 corrects most of these deficiencies and gives results in better agreement with those of more detailed models.