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  • Author or Editor: V. Ramanathan x
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Rémy Roca
and
V. Ramanathan

Abstract

Deep convective clouds in the Tropics have long been recognized to occur on a wide spectrum of spatial scales, ranging from the individual cumulus to the meso- and synoptic-scale cloud systems. The objective of this paper is to examine the scale dependence of the properties of clouds embedded in the Intertropical Convergence Zone.

The Indian Ocean during the winter and summer monsoons offers an ideal domain to undertake this study, which uses INSAT-1B infrared imagery. The cloud systems are retrieved using the detect and spread algorithm and classified according to their top temperatures. Their spatial extension spans a continuous spectrum of individual clouds ranging from 500 km2 to 106 km2. The spatial distribution of these convective clouds over the Indian Ocean exhibits an increase in convective activity during boreal winter compared to summer. Despite the drastic modification of the synoptic environment over the seasonal cycle, intrinsic cloud properties in January and July are shown to be very similar.

The intrinsic cloud properties that are retrieved are the convective core area relative to the total cloud area, the area colder than 240 K (corresponding roughly to stratiform precipitation), the average cloud-top temperature of the entire cloud (core and anvil), and the minimum cloud-top temperature within a cloud that is assumed to denote the temperature of the overshooting cloud tops.

The analysis reveals a critical scale of about 104 km2, which distinguishes two separate convective regimes of scale-dependent cloud properties. Below the critical scale, the cloud mean effective temperature increases with cloud size and the relative core area decreases with the size. The overshooting cloud-top temperature is invariant to the cloud scale. For scales larger than the critical value, the scale dependence is reversed: the mean cloud temperature decreases, the fractional core area increases, and the overshooting cloud top strongly decreases as the cloud size increases. Essentially, the area of undiluted deep convective core increases with the total area of the cloud system, in turn affecting the macroscale properties such as cloud greenhouse effect and tropopause temperature, to name a few. In particular, it is the larger-scale (>104 km2) organized system that penetrates to the tropopause and determines the tropopause altitude, while the smaller scales (<104 km2) hardly reach the upper troposphere. Diurnal variations of the convective cloud cover are also presented with respect to the cloud size. The diurnal cycle of these systems depends significantly on their scale and exhibits complex patterns.

A discussion of these cloud statistics is then offered in the context of general circulation model parameterization.

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Baijun Tian
and
V. Ramanathan

Abstract

In this paper diagnostic estimates of cloud radiative forcing (CRF) and clear-sky radiation budget at the surface and in the atmosphere, based on satellite-observed radiation budget at the top of the atmosphere (TOA) and empirical parameterizations derived from radiation models and field observations, are presented. This analysis is restricted to the tropical Pacific. High clouds over the intertropical convergence zone (ITCZ), the South Pacific convergence zone (SPCZ), and the warm pool (WP) exert a positive CRF of about 70 W m−2 within the atmosphere and a negative CRF of about −70 W m−2 at the surface, although with a negligible net CRF at the TOA. On the other hand, low clouds over the eastern subtropical Pacific and the equatorial cold tongue exert a negative CRF of about −20 W m−2 at the surface as well as in the atmosphere. The spatial gradients of the clear-sky radiation budget at the surface and in the atmosphere are small. In particular, it is shown that the clear-sky radiative cooling in the atmosphere is larger over the ITCZ, the SPCZ, and the WP, when compared with that over the subtropics and the cold tongue. Next, based on these diagnostic estimates and available surface turbulent heat flux data, the role of atmospheric CRF in the large-scale atmospheric moist static energy (MSE) transport is quantified. It is found that the atmospheric CRF provides the major energy source for balancing the divergence of MSE transport (from the ITCZ, the SPCZ, and the WP to the subtropics and the cold tongue) by the large-scale atmospheric circulation. On the other hand, the clear-sky radiative flux convergence and the surface turbulent heat fluxes have just the reverse spatial pattern and hence cannot satisfy the large-scale atmospheric MSE transport requirements.

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Baijun Tian
and
V. Ramanathan

Abstract

A simple moist model for the large-scale tropical atmospheric circulation is constructed by combining the simple models of Gill and Neelin and Held. The model describes the first baroclinic mode of the moist troposphere with variable “gross moist stability” in response to given thermodynamic forcing from surface evaporation and atmospheric cloud radiative forcing (CRF), which is a measure of the radiative effects of clouds in the atmospheric radiative heating. When the present model is forced solely by the observed atmospheric CRF, quantitatively reasonable Hadley and Walker circulations are obtained, such as the trades, the ascending branches in the intertropical convergence zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), as well as the descending branches in the cold tongue and subtropics. However, when the model is forced only by the observed surface evaporation, the Walker circulation totally disappears, and the Hadley circulation reverses. These results indicate that, in the context of a moist dynamic model, the spatial variations of atmospheric CRF are more important in terms of driving and maintaining the Hadley and Walker circulations than the spatial variation of surface evaporation.

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Chul Eddy Chung
and
V. Ramanathan

Abstract

Sea surface temperatures (SSTs) in the equatorial Indian Ocean have warmed by about 0.6–0.8 K since the 1950s, accompanied by very little warming or even a slight cooling trend over the northern Indian Ocean (NIO). It is reported that this differential trend has resulted in a substantial weakening of the meridional SST gradient from the equatorial region to the South Asian coast during summer, to the extent that the gradient has nearly vanished recently. Based on simulations with the Community Climate Model Version 3 (CCM3), it is shown that the summertime weakening in the SST gradient weakens the monsoon circulation, resulting in less monsoon rainfall over India and excess rainfall in sub-Saharan Africa. The observed trend in SST is decomposed into a hypothetical uniform warming and a reduction in the meridional gradient. The uniform warming of the tropical Indian Ocean in the authors’ simulations increases the Indian summer monsoon rainfall by 1–2 mm day−1, which is opposed by a larger drying tendency due to the weakening of the SST gradient. The net effect is to decrease the Indian monsoon rainfall, while preventing the sub-Saharan region from becoming too dry. Published coupled ocean–atmosphere model simulations are used to describe the competing effects of the anthropogenic radiative forcing due to greenhouse gases and the anthropogenic South Asian aerosols on the observed SST gradient and the monsoon rainfall.

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C. P. Weaver
and
V. Ramanathan

Abstract

This paper examines the role of extratropical cyclones in determining the cloud radiative forcing over the North Pacific during summer. Specifically, this study uses daily and monthly ERBE cloud radiative forcing, monthly ISCCP cloud-type distributions and optical depth, daily ECMWF meteorological analyses, and a climatology of cloud-type distributions based on surface observations.

The geographic correspondence between monthly mean fields of cloud radiative forcing, cloud type and optical depth, and quantities such as baroclinicity and transient eddy flux suggests that large-scale, stratiform cloud systems associated with traveling cyclones are largely responsible for the band of strongly negative shortwave cloud forcing (Cs ) over the Pacific between 40° and 60°N. Analysis of daily ERBE cloud forcing for July 1985, in conjunction with daily ECMWF geopotential, demonstrates the evolution of highly reflective cloud systems associated with several traveling, closed lows. The southwest to northeast extension of the broad maximum in monthly mean Cs , magnitude is in agreement with the observed pattern of traveling cyclones whose genesis is near Japan and Korea and whose trajectory continues to the Aleutian Islands and the Gulf of Alaska.

The scale of each individual cloud system during July 1995 is a significant fraction of the entire Pacific basin north of 40°N. In addition, the mean shortwave cloud forcing of each system is approximately −150 W m−2. Over the month, most of the basin north of 40°N is covered by clouds associated with one or more such systems.

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A. K. Inamdar
and
V. Ramanathan

Abstract

Sea surface temperature (SST) in roughly 50% of the tropical Pacific Ocean is warm enough (SST > 300 K) to permit deep convection. This paper examines the effects of deep convection on the climatological mean vertical distributions of water vapor and its greenhouse effect over such warm oceans. The study, which uses a combination of satellite radiation budget observations, atmospheric soundings deployed from ships, and radiation model calculations, also examines the link between SST, vertical distribution of water vapor, and its greenhouse effect in the tropical oceans. Since the focus of the study is on the radiative effects of water vapor, the radiation model calculations do not include the effects of clouds. The data are grouped into nonconvective and convective categories using SST as an index for convective activity. On average, convective regions are more humid, trap significantly more longwave radiation, and emit more radiation to the sea surface. The greenhouse effect in regions of convection operates as per classical ideas, that is, as the SST increases, the atmosphere traps the excess longwave energy emitted by the surface and reradiates it locally back to the ocean surface. The important departure from the classical picture is that the net (up minus down) fluxes at the surface and at the top-of-the atmosphere decrease with an increase in SST; that is, the surface and the surface-troposphere column lose the ability to radiate the excess energy to space. The cause of this super greenhouse effect at the surface is the rapid increase in the lower-troposphere humidity with SST; that of the column is due to a combination of increase in humidity in the entire column and increase in the lapse rate within the lower troposphere. The increase in the vertical distribution of humidity far exceeds that which can be attributed to the temperature dependence of saturation vapor pressure; that is, the tropospheric relative humidity is larger in convective regions. The positive coupling between SST and the radiative warming of the surface by the water vapor greenhouse effect is also shown to exist on interannual time scales.

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C. P. Weaver
and
V. Ramanathan

Abstract

This paper identifies dynamical and thermodynamical factors that govern the seasonal and interocean differences in cloud cover and cloud radiative forcing (CRF) over the storm track regions of the northern extratropical Pacific and Atlantic Oceans. An outstanding problem of interest is the fact that cloud cover is larger in the summer than winter in the North Pacific, while the converse is true in the North Atlantic. This paper considers separately January and July in the North Pacific and North Atlantic and finds that, on daily timescales, rising motion associated with synoptic-scale events such as cyclones produces greater CRF. However, CRF does not vary much with vertical velocity in regions of subsidence. In addition, increased moist static stability is associated on daily and monthly mean timescales with increased cloud cover and shortwave CRF. These results imply that, on monthly mean timescales, if we hold moist static stability constant, CRF should increase with increasing vertical velocity variance. This effect, by itself, would tend to increase CRF during winter, since the variance of vertical velocity is much larger during winter than summer. This is consistent with what is observed in the North Atlantic. In the North Pacific, however, the mean moist static stability is much larger during summer, and this effect tends to counteract the summertime decrease in vertical velocity variance, resulting in greater summertime cloud cover. Extending the argument to explain interocean differences in cloudiness or CRF during the same season, this paper finds that the North Pacific and North Atlantic have approximately the same CRF (or cloud cover) during winter because the mean vertical velocity variance and moist static stability are approximately the same. The North Pacific is more cloudy than the North Atlantic during summer because, while the mean vertical velocity variance is approximately the same, mean moist static stability is much greater in the North Pacific. Finally, spatial variations in both parameters within a given ocean basin tend to either reinforce each other or compete in their effect on CRF.

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Eric M. Wilcox
and
V. Ramanathan

Abstract

Clouds exert a thermodynamic forcing on the ocean–atmosphere column through latent heating, owing to the production of rain, and through cloud radiative forcing, owing to the absorption of terrestrial infrared energy and the reflection of solar energy. The Tropical Rainfall Measuring Mission (TRMM) satellite provides, for the first time, simultaneous measurements of each of these processes on the spatial scales of individual clouds. Data from TRMM are used to examine the scale dependence of the cloud thermodynamic forcing and to understand the dominant spatial scales of forcing in monsoonal cloud systems. The tropical Indian Ocean is chosen, because the major monsoonal cloud systems are located over this region. Using threshold criteria, the satellite data are segmented into rain cells (consisting of only precipitating pixels) and clouds (consisting of precipitating as well as nonprecipitating pixels), ranging in scales from 103 km2 to 106 km2. For each rain cell and cloud, latent heating is estimated from the microwave imager and radiative forcing is estimated from the Cloud and the Earth’s Radiant Energy System radiation budget instrument.

The sizes of clouds and rain cells over the tropical Indian Ocean are distributed lognormally. Thermodynamic forcing of clouds increases with rain cell and cloud area. For example, latent heating increases from about 100 W m−2 for a rain cell of 103 km2 to as high as 1500 W m−2 for a rain cell of 106 km2. Correspondingly, the liquid water path increases tenfold from 0.3 to nearly 3 kg m−2, the longwave cloud forcing from 30 to 100 W m−2, and the diurnal mean shortwave cloud forcing from −50 to −150 W m−2. Previous studies have shown that in regions of deep convection, large clouds and rain cells express greater organization into structures composed of convective core regions attached to stratiform anvil cloud and precipitation. Entrainment of moist, cloudy air from the stratiform anvil into the convective core helps to sustain convection against the entrainment of unsaturated air. Thus large clouds produce more rain, trap more terrestrial radiation, and reflect more solar energy than do smaller clouds. The combined effect of increased forcing and increased spatial coverage means that larger clouds contribute most of the total forcing. Rain cells larger than 105 km2 make up less than 2% of the rain cell population, yet contribute greater than 70% of the latent heating. Similarly, the clouds larger than 105 km2, in which the largest rain cells are embedded, make up less than 3% of clouds, yet are the source of greater than 90% of the total thermodynamic forcing. Significant differences are apparent between the scales of latent heating and radiative forcing, as only about 25% of cloud area is observed to precipitate. The fraction of clouds that contain some rain increases dramatically from about 5% for the smaller scale (103 km2) to as high as 90% for the largest scale considered here (106 km2). The fractional area of the precipitating cloud ranges from 0.2 to 0.4 with a hybrid-scale dependence. Greater than one-half of radiative forcing is provided by nonprecipitating anvil portions of deep convective cloud systems. The results presented here have significant implications for the parameterization of clouds and rain in GCMs and washout of solute trace gases and aerosols in chemistry and transport models.

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Chul Eddy Chung
and
V. Ramanathan

Abstract

Aerosols are regionally concentrated and are subject to large temporal variations, even on interannual timescales. In this study, the focus is on the observed large interannual variability of the South Asian (SA) haze, estimating the corresponding variations in its radiative forcing, and using a general circulation model to study their impacts on global climate variability. The SA haze is a widespread haze, covering most of South Asia and the northern Indian Ocean during December–April. The southernmost extent of the haze varies year to year from about 10°S to about 5°N. In order to understand the impact of this interannual variation in the haze forcing, two numerical studies were conducted with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). The former has the forcing extending to 10°S, while the latter is confined to regions north of the equator.

Each of the two haze forcing simulations was implemented into a 3D global climate model (NCAR CCM3) with a prescribed SST seasonal cycle to estimate the sensitivity of the model climate to the aerosol forcing area. In both simulations, the haze forcing was prescribed only during the dry season between November and April. Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. In remote regions, however, the responses differ remarkably. Focusing on the remote effects of the haze, it is shown that some of the recent observed boreal-wintertime changes of the southwest Asian monsoon, El Niño–Southern Oscillation (ENSO), and the Arctic Oscillation (AO) could be explained by the SA haze forcing and its fluctuation.

First, both simulations reveal the wintertime drought over southwest Asia, with the EHF generating far more severe drought. Second, the EHF experiment simulates a poleward shift of the Northern Hemisphere (NH) zonal-mean zonal momentum during the winter season, while the SHF effect rather moves the NH extratropical zonal momentum only slightly equatorward. Thus, the interannual fluctuations in the extension of the haze forcing area can explain the recently documented increased variability of the AO.

Third, the EHF significantly suppresses the convection in the western equatorial Pacific during the boreal wintertime, and the SHF leads to much less suppression. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, it is proposed that the SA haze may be partially responsible for the observed El Niño–like warming during the recent decades. When the convection suppression in the EHF experiment is imposed in the Cane–Zebiak Pacific ocean–atmosphere model, the coupled model actually simulates a warm bias similar to the observed El Niño trends of the recent decades. These findings have to be verified with a fully coupled ocean–atmosphere climate model.

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F. Li
,
A. M. Vogelmann
, and
V. Ramanathan

Abstract

This study uses data collected from the Clouds and the Earth's Radiant Energy System (CERES) and the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments to determine Saharan dust broadband shortwave aerosol radiative forcing over the Atlantic Ocean near the African coast (15°–25°N, 45°–15°W). The clear-sky aerosol forcing is derived directly from these data, without requiring detailed information about the aerosol properties that are not routinely observed such as chemical composition, microphysical properties, and their height variations. To determine the diurnally averaged Saharan dust radiative forcing efficiency (i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m−2 τ −1 a ), two extreme seasons are juxtaposed: the high-dust months [June–August (JJA)] and the low-dust months [November–January (NDJ)]. It is found that the top-of-atmosphere (TOA) diurnal mean forcing efficiency is −35 ± 3 W m−2 τ −1 a for JJA, and −26 ± 3 W m−2 τ −1 a for NDJ. These efficiencies can be fit by reducing the spectrally varying aerosol single-scattering albedo such that its value at 550 nm is reduced from 0.95 ± 0.04 for JJA to about 0.86 ± 0.04 for NDJ. The lower value for the low-dust months might be influenced by biomass-burning aerosols that were transported into the study region from equatorial Africa. Although the high-dust season has a greater (absolute value of the) TOA forcing efficiency, the low-dust season may have a greater surface forcing efficiency. Extrapolations based on model calculations suggest the surface forcing efficiencies to be about −65 W m−2 τ −1 a for the high-dust season versus −81 W m−2 τ −1 a for the low-dust season. These observations indicate that the aerosol character within a region can be readily modified, even immediately adjacent to a powerful source region such as the Sahara. This study provides important observational constraints for models of dust radiative forcing.

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