The status of the data sets is discussed. Progress  was made in both data analysis and modeling areas. The atmospheric and land surface contributions to the net radiation budget over the  Sahara-Sahel region is being decoupled. The interannual variability of these two processes was  investigated and this variability related to seasonal rainfall fluctuations. A modified Barnes objective analysis scheme was developed which uses  an eliptic scan pattern and a 3-pass iteration of  the difference fields

Nicholson, Sharon

Smith, Eric A.

English

NASA Technical Reports Server

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Semi-annual Report: NASA Grant NAG 5-849 J .I& .I] 
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Investigating the r o l e  of the Land Surface 
in Explaining the Interannual Variation of the 
Net Radiation Balance 
over the Western Sahara and Sub-Sahara 
I 
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Eric A. Smith 
Sharon Nicholson 
Dept. of Meteorology 
Florida State University 
Tallahassee, FL 32306 
July 1987 
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https://ntrs.nasa.gov/search.jsp?R=19870018764 2020-03-20T10:08:40+00:00Z
I 
Table of Contents 
Page 
1.0 Summary of Activity .............................................. 2 
2.0 Project Personnel ................................................ 2 
3.0 Status of Data Sets .............................................. 2 
4.0 Research Progress ................................................ 3 
5.0 Meetings, Conferences, Reports, and Publications .................. 5 
Appendix A: Report from the IRAP Rio Rico Meeting 
I 
2 
1.0 Summary of Activity 
This report covers project activity from 10-1-86 to 5-31-87 during the 
first year of the grant. Personnel supported under the auspices of the 
project are identified in Section 2. Sections 3 and 4 discuss the data 
sets and research progress. Section 5 provides a list of meetings attended 
and publications sponsored under the grant. 
2.0 Project Personnel 
One Ph.D. dissertation and one Master's Thesis are being directed in 
conjunction with this project. Mr. B.J. Sohn and Mr. Andrew Lare are the 
advisees of Professors Smith and Nicholson respectively. Computer 
programming support is being provided by Mr. Matthew Smith and Mr. Kung W. 
Oh. Ms. Patricia Williams provides part-time clerical support. 
3.0 Status of Data Sets 
Six data sets were identified as being essential for this project: 
1. Nimbus 7 ERB WFOV/NFOV EMST Data 11979-19831 
2. NOAA Heat Budget Tapes [1979-19831 
3. NMC Global Analyses of Atmospheric Profiles 11979-19831 
4. African Rainfall Station Data [1979-19831 
5. E. Matthews Surface Albedo Tape 
6. ISCCP Cloud Data (1983 July-December) 
Currently we have all data except for the - 1983 August-December ISCCP 
cloud tapes. In addition we have recently placed an order for a new NASA 
cloud data set obtained from the Nimbus 7 THIR/TOMS for the 1979-1985 
period (CMATRIX tapes). Since these data are contiguous with our complete 
study period, we will substitute them in place. of the ISCCP data. We 
1-_1 -_I-- __ _- .. 
expect to receive the CMATRIX data in mid-August, 1987 directly from the 
NASA Space Science Data Center. 
3 
We have operational format cracking software for all of the data sets 
excepting CMATRIX and have conducted preliminary analysis with all data 
sets mentioned in the above table. 
4 . 0  Research Progress 
We have made steady progress in both the data analysis and modeling 
areas. The science investigation proposed and supported by this grant is 
illustrated in the flow diagram shown in Figure 1. In essence, we are 
trying to decouple the atmospheric and land surface contributions to the 
net radiation budget over the Sahara-Sahel region, investigate the interan- 
nual variablilty of these two processes and relate this variability to 
seasonal rainfall fluctuations. We intend to use the principals of clima- 
tonomic modeling to examine the physical feedbacks taking place between 
precipitation, land surface changes and the surface energy budget. 
As a point of reference on Figure 1 we have completed the tasks asso- 
ciated with steps 1,2,5,10; are completing development of shortwave and 
longwave radiation transfer codes needed at steps 3,6,11, and 14; and have 
developed parts of the climatonomic model needed at step 15. 
In order to bring the precipitation data into a form consistent with 
the time-space grid used in the analyses, we -have developed a modified 
Barnes objective analysis scheme which uses an eliptic scan pattern and a 
3-pass iteration of the difference fields. Figure 2 provides an example of 
monthly station data over Northern Africa along with the objectively ana- 
lyzed grid field. The 9999 values represent missing data regions which 
will be replaced with subjectively analyzed estimates. 
4 
We are nearing completion of the computational work on a paper 
focused on minimum planetary albedo variability during the 1979-1983 
period. This study is based on synthasizing a 5-year time series of mini- 
mum planetary albedo from the NOAA heat budget data using the 18 month 
record of ERB-NFOV minimum albedo as the reference. 
Figures 3 - 6 illustrate some of the results of this study. In Fig. 3 
we examine the latitude-time behavior of minimum planetary albedo calcu- 
lated from a multiple-regression scheme using Nimbus 7 ERB-NFOV measure- 
ments as the training data set. It is easily noted that most of the 
variability in this quantity takes place in the Northern Sahel. Figure 4 
illustrates the composite seasonal analysis of minimum albedo (Winter, 
Spring, Summer, Autumn). These diagrams portray the dominant geographical 
features of North Africa. The Meteosat photo shown in Figure 5 can be used 
as a means of comparison. Figure 6 illustrates the interannual dispersion 
index. It presents our preliminary estimates of the source regions of 
variability in the minimum planetary albedo. The dominant regions are num- 
bered with numerals 1-4. 
5 
5.0 Meetings, Conferences, Reports, and Publications 
A. Meetings: 
1. Smith, E.A., and S.E. Nicholson, 1986: Attendance at the 
NASA-ISLSCP Retrospective Analysis Project (IRAP) Science Team 
report is given in Appendix A ] .  
Organizing Meeting, October 22-24, 1986, Rio Rico, AZ. [A 
2 .  Smith, M.R., 1986: Attendence at PLDS Team Meeting, September 
24-26, Ames Research Center, San Francisco, CA. 
B. Publications in Progress: 
Smith, E.A., and B . J .  Sohn, 1987: An extended time series of the 
minimum planetary albedo over Northern Africa from a com- 
bination of Nimbus 7 and NOAA satellite measurements. To be 
submitted to J .  Clim. A p p l .  Meteor. 
List of Figures 
6 
Figure 1: Flow diagram of diagnostic modeling process. 
Figure 2: Monthly precipitation accumulation (cm) at individual stations 
over Northern Africa during August, 1979 (top). Objectively ana- 
lyzed gridded representation using a modified Barnes scheme 
(bottom) . 
Figure 3: Time-latitude section of the minimum planetary albedo over North 
African latitudes for 60 months. 
Figure 4: Five year (1979-1983) seasonal composites of minimum planetary 
albedo over Northern Africa derived from multiple-linear 
regression scheme. 
Figure 5 :  METEOSAT visible satellite image from April 24, 1979. 
Figure 6 :  Dispersion index of minimum planetary albedo. The 4 dominant 
interannual variational centers are indicated with large numbers, 
SEPARATION OF SURFACE AND ATNOSPHERIC CONTRIBUTIONS 
SPECIFY CLOUD FREE 
TOA SOLAR FLUX WITH 
1 COMBINATION OF NOAA-HB 
ALBEDO ESTIMATES 
AND ERB-NFOV MINIMUM 
SPECIFY CLOUDY 
TOA SOLAR FLUX WITH 
WFOV DATA - 
COMPARE WITH 
4 E. MATTHEWS 
ALBEDO DATA 
DIAGNOSE BULK 
CLOUD LIQUID WATER 
PATH WITH SOLAR RTE MODEL 
T = EXP [ - , i:dz 
INVOKE AN 
7 INVARIANT 
CLOUD LWC 
SPECIFY ATMOSPHERIC 
PROFILE WITH 
NMC GLOBAL ANALYSES 
DIAGNOSE SURFACE 
ALBEDO (As) WITH 
SOLAR RTE MODEL 
CALCULATE A 
REPRESENTATIVE 
CLOUD THICKNESS 
Q 
SPECIFY CLOUDY INSERT CLOUP LAYERS 
WITH WFOV DATA ACCORDING TO FIXED 9 
IN IR RTE MODEL 
VAL IDATE WITH [from THIR/TOMS] 
NOM SATELL I TE 
MEASUREMENTS 
DIAGNOSE SURFACE CALCULATE SURFACE 
WITH IR RTE MODEL FLUX (Q*s) 
11 SKIN TEMPERATURE (Ts) NET RADIATION 
TIME SERIES ANALYSES 
BETWEEN Q*s and P 
STUDIES 
- 
Figure 1: Flow diagram of diagnostic modeling process.  
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Figure 2:- Monthly precipitation accumulation (cm) at individual stations 
over rjorthern Africa during August, 1979 (top). Objectively ana- 
lyzed gridded representation using a modified Barnes scheme 
(bottom). 
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Figure 6: Dispersion index of minimum planetary albedo. The 4 dominant 
interannual variational centers are indicated with large numbers. 
7 
Appendix A: Report from the IRAP Rio Rico Meeting. 
92693 
N 8 7 - 2 8 1 9 8  
Abstract 
Investigating the Role of the Land Surface 
in Explaining the Interannual Variation of 
the Net Radiation Balance over the 
Northwest Sahara and Sub-Sahara 
Eric A .  Smith 
Sharon E. Nicholson 
Department of Meteorology 
F l o r i d a  State University 
Tallahassee, FL 32306 
(904) 644-4253 
November 1986 
1 .  . .  
This investigation examines how much of the interannual variation in the 
satellite derived radiation balance can be purely attributed to changes taking 
place at the land surface. The basic scientific objective is to decompose the 
variance terms associated with the radiation budget parameters over Northwest 
Africa into the part due to surface induced variations versus the part due to 
atmospheric induced variations. Determining the relative magnitude of these 
partitioned terms is one of the central issues of the International Satellite 
Land Surface Climatology Project (ISLSCP) since part of the climate system is 
forced by the interactions between the surface radiation biidget and the plane- 
tary radiation budget in conjunction with feedbacks with the zonal energy 
transport process. Underlying this basic objective will be a concentrated 
effort to develop consistent time series of conditions at the surface of  desert 
and Sahelian regions of Northwest Africa. The principle parameters under study 
are surface albedo [A], skin temperature [T,], net radiation [ Q * s ] ,  surface air 
temperature [Tal, skin temperature jump [AT], precipitation [PI, dryness index 
[Q* ( L * P ) - l ] ,  soil water storage [dw/dt] , and evaporation [E]. 
Upon developing a 5-year time series of these parameters we intend to exa- 
mine the role of surface latent heating and its control of the precipitation 
pattern from one year to the next. Along this same line we will examine how the 
local moisture source acts as a memory mechanism in the Northwest African 
desert/semi-desert regimes. The latent heat exchange term will be arrived at 
diagnostically from a climatonomic modeling approach in which satellite 
radiation budget measurements and surface precipitation data are used to specify 
the forcing to the surface energy balance equation. 
The study region boundaries are 10°N - 30°N; 15OW - 30°E. The period of 
study extends from 1979 t o  1984 during which we have high quality coincident 
satellite radiation budget measurements, precipitation measurements and gridded 
vertical profiles of temperatures and mixing ratio; The space and time scales 
of  the analysis are 2 . 5  degrees and 30 days respectively. 
Figures 1 and 2 serve to emphasize the importance of understanding how sur- 
face processes operate over Northwest Africa. Figure 1 illustrates the portion 
of variance that can be attributed to interannual variation in the planetary 
albedo. There is a strong gradient from the Sahara, into the Sahelian zone 
where a closed high of  ~ 9 0 %  is located. The variance then decreases south of 
the Sahel into equatorial Africa. It is of interest to determine how much of  
this variance is explainable by interannual fluctuations in the cloud and preci- 
pitation systems which migrate through the Sahel and over the Southern Saharan 
fringe, and how much is due to surface fluctuations of vegetation and soil 
moisture. Both processes lead to variations in the planetary albedo. 
. .  
Figure 2 illustrates two key features of Northwest Africa vis-a-vis the 
radiation budget process. The top part of the figure depicts the difference in 
shortwave and near-infrared planetary albedo. This result is based on a 5-year 
average of Nimbus-7 ERB measurements. Note that Northern Africa is the only 
continental o r  oceanic region on the globe which exhibits negative differences. 
This emphasizes the fundamental contrast between the North African desert regime 
and other arid regions: it is drier, more cloud free and less vegetated than any 
of the other desert systems. These are the required conditions f o r  high near- 
infrared planetary albedo. This map is somewhat analagous to a Normalized 
- Difference Vegetation lndex (NDVI) map, but derived from and on the time-space 
scale of radiation budget measurements. 
The bottom panel illustrates the 5-year averaged difference field of day- 
night planetary emission temperature. Note that the three global maxima are 
centered over the African Sudd, the South African Kalahari desert, and the great 
Australian desert. A l l  three o f  these regions are large scale desert systems. 
Note that the position of the maximum diurnal temperature swing in Northern 
Africa corresponds with the position of the albedo variance maximum seen in 
Figure 1. This type of correspondence is not observed with respect t o  the other 
two major diurnal anomalies. This is an indirect indication that the interan- 
nual planetary albedo variation over the African Sudd is largely forced by sur- 
face processes. 
. .  
T h  
VFOV - .no spatial filtering 
WFOV - no spatial filtering 
F i g u r e  2 :  S p e c t r a l  a l b e d o  d i f f e r e n c e s  ( t o p ) :  d i u r n a l  t e m p e r a t u r e  d i f f e r e n c e s  (bottom) 


Investigating the role of the land surface in explaining the interannual variation of the net radiation balance over the Western Sahara and sub-Sahara

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