Differentiating the Role of Land Surface Variability and Cloudiness 

Variability on Global Energy Transpoft within the Atmosphere 

and Oceans 

March 1993 

vn 

Eric A. Smith, dnapal Investigator 

Professo 

Soc. sec. 

Telephone: (904) 644-4253 

Fax: (904) 644-9639 

EMail: esmith@metsat.met.fsu.edu 

<MAS&-CR-P952ZB) OIFFEREMTTATlNG M94-26222 

THE ROLE OF LAND SURFACE 

VARIABILITY AM8 CLQUDXWESS 

VARIABfLfTV ON GLOBAL ENERGY Uncl as 

TRANSPORT WHBHTN THE BTMQSBMERE 8Q0 

OCEANS Final Repor% (FBorida State 

Cilrriw,) 43 p 63/43 0208436 
1.0 Introduction 

The following provides the final report on NASA IDP Project NAGW-1840 

"Differentiating the Role of Land Surface Variability on Global Energy Transport 

within the Atmosphere and Oceans". The project was designed to investigate the 

role of regional perturbations in the earth radiation budget on atmospheric and 

oceanic energy transports on an interannual basis. We proposed a modeling strategy 

based on an entropy extremum principle that could be used to separate the 

transports into oceanic and atmospheric components so as to better understand the 

effects of regional perturbations at the distinct atmospheric and oceanic time scales. 

The original focus was to consider the maintenance and year-to-year modulation of 

a large-scale, low-latitude North African-West Pacific Ocean net radiation dipole, 

which we had detected in the Nimbus 6 and 7 record of earth radiation budget 

measurements, and which necessitated significant cross-meridional energy 

transports to maintain global equilibrium. In addition, perturbations in the 

radiation balance term were to be partitioned into cloud-induced and surfaceinduced components to better understand the feedbacks between clouds and surface 

boundary conditions on interannual variability of the radiation balance. 

The research was conducted over the period from 7-1-89 to 12-31-92. After the 

project commenced, we expanded the focus to include global scale processes as well 

as regional scale processes. The main data sets for the project were derived from the 

Nimbus 7 measurements and included ERB, THIR cloudiness, and SMMR MPD 

data sets. The research was ultimately divided into three basic topic areas: 

1. Coupling Between Surface and Top-of-Atmosphere Radiation Budgets in 

the African Sahel 

2. Cloud-Surface-Radiation Interactions and Their Impact on Required 

Energy Transports 

3. Two Dimensional Decomposition of Atmospheric and Oceanic Energy 

Transports 

Results from the project led to 10 publications, 7 of which were published in 

refereed journals. The references for these publications are given in Section 3. 

Section 2 provides a summary of the findings, along with the associated citations. 
2.0 Summary of Research Findings 

Cou~line Between Surface and Top-of-Atmosvhere Radiation Budvets in Sahel 

Smith and Choudhury (1993) conducted a study that showed interannual 

variations in the net radiation balance at the top-of-atmosphere over North Africa 

are linked to variations at the land surface within the Sahelian zone. The spatial 

distribution of the amplitude of interannual variability of net radiation at the top of 

the atmosphere was examined using coincident monthly time series of earth 

radiation budget and passive microwave measurements obtained from the Nimbus 

7 satellite over a five year period from 1979 to 1983. A strong association was found 

between the radiation budget anomalies derived from ERB-WFOV measurements 

and the anomalies of 37 GHz polarized brightness temperature difference derived 

from the SMMR measurements. Another study of Choudhury et al. (1993) was 

conducted to demonstrate the reliability of the 37 GHz MPD data set by considering 

the potential attenuation by the atmosphere on the estimates. Previous research by 

Smith and Sohn (1989)l had shown that the interannual anomalies in the top-ofatmosphere net radiation balance in the Sahelian zone mostly arise from changes 

taking place at the surface, not from variations in cloudiness. Since microwave 

polarization difference is related to vegetation cover, the new results indicated that 

the main control of the interannual modulation of North Africa's radiation balance 

is year to year fluctuations of vegetation cover within the Sahelian zone stemming 

from fluctuations in rainfall, which is intrinsically associated with cloud variability. 

However TOA interannual net radiation variability is more influenced on how 

cloud variability impacts the surface through rainfall, than it is through direct 

cloud-radiation feedback on TOA fluxes. This finding is considered to be relevant to 

an understanding of how cloud-radiation processes control and feedback on global 

climate through processes taking place at the surface of a semi-arid zone. 

Cloud-Surface-Radiation Interactions and Their Impact on Reauired Enerm Transports 

Based on the preliminary findings of Smith and Sohn (1990) and Sohn and 

Smith (1991), a series of studies were conducted by Sohn and Smith (1992a-c) to show 

Smith, E.A., and B.J. Sohn, 1990: Surface forcing of interannual variations in the radiation balance 

over North Africa. Part I: Partitioning the surface and cloud forcing. Climatic Change., 12, 5-51. 
how cloud-radiation and surface-radiation interactions influence the earth radiation 

balance and the requirements for energy transport on a global scale. Cloud-radiative 

forcing calculations based on the Nimbus 7 radiation budget and cloudiness 

measurements revealed that cloud induced longwave warming (cloud greenhouse 

influence) is dominant over the tropics whereas cloud induced shortwave cooling 

(cloud albedo influence) is dominant over the mid and high latitudes. The average 

SW cloud cooling taken over the area of the globe from 65ON to 65"s was found to 

be -27.8 Wem-2. This magnitude slightly overcomes LW cloud warming (-25.7 

Worn-2), resulting in a small net cooling effect of -2.1 Warn-2 over 93% of the earth. 

A six year zonally averaged mean cloudy and clear sky net radiation flux 

analysis showed that there are three distinct regimes in terms of net cloud warming 

or cooling, i.e., warming in the tropics (between 20°N and 20"s) and in the high 

latitudes (poleward of 55"), and cooling in the extratropical latitudes between 20" and 

55" in both hemispheres. These distributions reinforce the intensities of the Hadley 

and Ferrel meridional circulation cells. This stems from strong warming due to 

high-level clouds in the tropics and strong cooling due to middle and low level 

clouds at extratropical latitudes. The magnitude of the contribution by cloud forcing 

was found to be of the same order as eddy heat and momentum flux forcing to the 

maintenance of the mean meridional circulation. 

Surface-atmosphere forcing obtained by differentiating the cloud induced 

effects from the measured radiative fluxes indicated that an east-west coupled North 

Africa - Western Pacific energy transport dipole is maintained mainly by low 

latitude land-ocean contrasts associated with shortwave radiation but supported by 

cloud controls on tropical longwave radiation. This implied that interannual 

variations in the net radiation balance associated with these two regions can give 

rise to fluctuations of the basic dipole structure and thus fundamental changes in 

low-latitude climate. 

The source and forcing mechanisms of radiation budget variability over 

tropical latitudes were examined by separating the variations into cloud and surfaceforced components. A zonal harmonic analysis of emitted longwave radiation 

showed that these variations are largely controlled at the planetary wave scale. 

Positive total and cloud-forced longwave (LW) anomalies embedded within this 

planetary scale structure showed eastward movement from the Indian Ocean 

toward the Eastern Pacific together with the easterly displacement of negative 

anomalies from the Western Pacific toward Africa during the period prior to and 

after the active phase of the 1982-83 ENSO. The overall effect leads to an 

3 
approximately 50" per year propagation phase speed which is considerably slower 

than the oceanic Kelvin wave capable of driving east-west LW anomalies through 

SST feedback. The oceanic Kelvin wave speed is about 60' per month over the 

Pacific basin in the course of an ENS0 cycle. This pointed out that there are longer 

time scales of climatic signals in the tropical radiation budget. 

An examination of time dependent radiative energetics over the tropics 

revealed that the anomaly LW propagation is mainly due to cloud forcing associated 

with east-west circulation changes, although surface forcing was found to contribute 

within the Pacific basin. Since cloud amount changes are directly linked to 

variations in latent heat release, diabatic heating associated with coupled oceanatmosphere feedback appeared to be the mechanism responsible for the LW 

anomaly propagation. An examination of the complete radiation budget over the 

Maritime Continent and Equatorial Central Pacific during the 1982-83 ENS0 event 

demonstrated that radiative forcing produces positive feedbacks in conjunction with 

the sea surface temperature anomalies that develop in both regions. Furthermore, 

surface forcing was found to be an important control on net radiation variability 

within this teleconnection. An examination of two additional tropical east-west 

teleconnections showed that surface forcing is even more important than cloud 

forcing in controlling variations in the east-west net radiation gradients. 

The impact of differential net radiative heating on two-dimensional energy 

transports within the atmosphere-ocean system and the role of clouds on this 

process were then examined. The radiation budget data showed basic energy 

surpluses over the tropical oceans and relative or absolute energy deficits over low 

latitude continental regions. The two-dimensional mean energy transports, in 

response to zonal and meridional gradients in the net radiation field, exhibited an 

east-west coupled dipole structure in which the West Pacific acts as the major energy 

source and North Africa as the major energy sink. It was shown that the dipole is 

embedded in the secondary energy transports arising mainly from the differential 

heating between land and oceans in the tropics in which the tropical east-west 

(zonal) transports are up to 30% of the tropical north-south (meridional) transports. 

Thus, any perturbations to this dipole on an interannual basis due to regionally 

induced fluctuations of the net radiation balance, give rise to low latitude energy 

transport variations. In turn, the tropical variations lead to extratropical responses 

through alterations of requirements on both zonal and meridional transports at all 

positions on the globe. Cloud-induced transports, obtained by differentiating the 

cloud-free portion from the total transport field, indicated that year-to-year cloud 

4 
amount changes are contributing to fluctuations of the global climate system 

through these mechanisms. 

Increased cloudiness increases zonal available potential energy thus increasing 

the intensity of the north-south transports while slightly weakening the dipole 

intensity. It thus appeared that the basic role of cloudiness is to diminish the role of 

differential heating between continents and oceans and force the globe toward a 

more meridionally distributed energy imbalance. This implied that the radiative 

feedback effects of clouds, regardless of factors determining cloud amount 

variability, reduce the radiative decoupling of land and ocean. This conclusion 

cannot be arrived at heuristically because it pertains to the specific optical properties 

of continental and oceanic cloud systems and additional factors governing cloud 

amount variability over the land masses and oceans themselves. 

Two Dimensional Decomvosition of Atmos~heric and Oceanic Enernv Trans~orts 

Our final application of the Nimbus 7 cloud and radiation budget 

measurements concerned a new method to decompose the required oceanic and 

atmospheric energy transports in two dimensions. The studies of Sohn and Smith 

(1992d, 1993) showed how the total required transport field, as determined from net 

radiation measurements, could be separated into the separate components by 

applying a maximum entropy production principle to the atmospheric system. The 

results showed strong poleward fluxes by the oceans in the northern hemisphere 

exhibiting a maximum of 2.4 1015 W at 18ON, with maximum atmospheric 

transports found at 37ON with a magnitude of 4.5 1015 W. These results were found 

to be in good agreement with other published results. 

In the southern hemisphere, atmospheric transports were found to be 

considerably stronger than oceanic transports, corroborating findings based on other 

published direct estimates. Maximum atmospheric energy transports were found at 

37's with a magnitude of 4.7 1015 W; two local oceanic transport maxima were 

shown at 18's and 45"s with magnitudes of 1.3 1015 W and 1.1 1015 W, respectively. 

Results showed evidence of net cross-equatorial transport in which the southern 

hemisphere oceans give rise to a net transfer of heat northward across the equator, 

exceeding a net transfer from northern to southern hemisphere by the atmosphere. 

Since the southern hemisphere results should have the same degree of accuracy as 

the northern hemisphere results, our findings suggested that southern ocean 

transports are weaker than previously reported. A main finding of the study was 

5 
that a maximum entropy production principle can serve as a governing rule on 

macroscale global climate, and in conjunction with conventional satellite 

measurements of the net radiation balance, provides a means to decompose 

atmosphere and ocean transports from the total transport field. 

Furthermore, the modeling methodology provided a means to partition the 

transports in a two-dimensional framework. Sohn and Smith (1994) tested the 

approach on the separate ocean basins using the maximum entropy production 

(MEP) principle extended to two-dimensions. In terms of the meridional 

component of the ocean transport vectors, the results showed northward ocean 

transports throughout the entire Atlantic ocean from southern hemisphere high 

latitudes to northern hemisphere polar regions, southward transports throughout 

the entire Indian Ocean, and poleward transports separated at approximately 10"s in 

the Pacific Ocean. 

The ocean transport patterns were found to be consistent with well-known 

features concerning heat transport within the three ocean basins. However, 

uncertainty remained in the magnitudes of the transports. Because of the large 

remaining discrepancies between published estimates based on direct measurements 

and indirect estimates derived from energy budget methods, assessing the accuracy 

of the satellite-based magnitudes was difficult, although there was evidence that 

limitations in the spatial resolution of the model led to synergistic biases in the 

North Atlantic and North Pacific. 

In tenns of the cross-meridional energy transport component, the results 

suggested that most of the net energy transfer in the tropics takes place within the 

oceans. In the southern hemisphere high latitudes, the Pacific and Indian Oceans 

were found to export heat cross-meridionally to the Atlantic Ocean through the 

passages below Cape Horn and the Cape of Good Hope, although the magnitudes of 

these inter-ocean heat exchanges were small. Another important aspect of the 

southern hemisphere results was that poleward transports were dominated by the 

atmospheric component with strong zonal asymmetry. By contrast, in the northern 

hemisphere, atmospheric transports over the ocean were generally weaker than the 

corresponding southern hemisphere terms, indicating that the northern 

hemisphere oceans are relatively more effective in transferring heat poleward. 

Finally, poleward atmospheric transports over the continental areas exceeded those 

over the ocean at equivalent latitudes as a result of the generally greater energy 

deficits over the land areas. 
3.0 Publications Produced Under Grant 

Choudhury, B.J., E.R. Major, E.A. Smith, and F. Becker, 1993: Atmospheric effects on 

the SMMR and SSM/I 37 GHz polarization difference over the Sahel. Int. J. 

Rem. Sens., 13, 3443-3463. 

Sohn, B.J., and E.A. Smith, 1994: Energy transports by ocean and atmosphere based 

on an entropy extremum principle. Part 11: Two dimensional transports. 

Meteor. Atmos. Phys., 51, in press. 

Sohn, B.J., and E.A. Smith, 1993: Energy transports by ocean and atmosphere based 

on an entropy extremum principle. Part I: Zonal averaged transports. J. Clim., 

6,886-899. 

Sohn, B.J., and E.A. Smith, 1992a: The modulation of the low latitude radiation 

budget by cloud and surface forcing on interannual time scales. J. Clim., 5, 831- 

846. 

Sohn, B.J., and E.A. Smith, 1992b: Global energy transports and the influence of 

clouds on transport requirements -- A satellite analysis. J. Clim., 5, 717-734. 

Sohn, B.J., and E.A. Smith, 1992c: The significance of cloud-radiation forcing to the 

general circulation on climate time scales -- A satellite interpretation. J. Atmos. 

Sci., 49, 845-860. 

Sohn, B.J., and E.A. Smith, 1992d: Diagnosing ocean energy transports from earth 

radiation budget measurements. Preprint Vol., Sixth AMS Conf. on Satellite 

Meteorology and Oceanography [Jan. 5-10; Atlanta, GA], Amer. Meteor. Soc., 

Boston, MA, 367-370. 

Sohn, B.J., and E.A. Smith, 1991: Role of cloud-radiation interactions in the general 

circulation. Preprint Vol., Second International Symposium on Global Change 

Studies [Jan. 13-18; New Orleans, LA], Amer. Meteor. Soc, Boston, MA, 114-117. 

Smith, E.A., and B.J. Choudhury, 1993: The relationslup between anomalies of 

interannual net radiation and 37 GHz polarization difference over North 

Africa. Climatic Change, 23, 141-154. 

Smith, E.A., and B.J. Sohn, 1990: Interseasonal variability of the global net radiation 

balance and its consequence on global energy transport. Preprints, XXXXIst. Int. 

Astronautical Federation Congress [October 6-12; Dresden, G.D.R], Amer. Inst. 

Aero. Astronaut., Washington, DC, 10 pp.