JULY 1962 MONTHLY WEATHER REVIEW 287
Correspondence
COMMENTS O N "ANALYSIS OF SATELLITE INFRARED RADIATION MEASUREMENTS ON
A SYNOPTIC SCALE" AND "SYNOPTIC USE OF RADIATION MEASUREMENTS FROM
SATELLITE TIROS 11"
PHILIP F. CLAPP
Extended Forecast Branch, US. Weather Bureau, Washington, D.C.
February 28, 1962; revlsed April 18, 1962
Two recent papers [I], [Z] revctd quite clearly a close
relationship between cloud cover and outgoing long-wnve
radiation to space from the earth-tttmospllere system, in
the sense that the outgoing radiation as measured by
satellites is largest, in areas of clear skies or low overcast,
and srnallest8 in regions of high cloudiness. Among other
things, these satellite radiation observations will be inval-
uable in supplementing the cloud pllotogrtLplls over dark-
ened areas of t'lle earth and will be useful in detertnining
the height' and type of c*louds.
I was especially interested in a remnrk 1))- Fritz and
Winston [Z] that the lowest d i a t i o n nwasurenwnts for
the case studied by t'lwrn were locttted i n :t rcgion of sub-
stantial precipitation. This suggests that it. is in such
regions that dense overcast conditions are likelj- to exttntl
to the greatest elevations, so that at letlst the portion of
outgoing long-wave rdi:Ltiotl in the water-vapor "win-
dow" is at a rninirnutrl. Weinstein tmd Suotni [I ] strongly
imply that such :L rclrttionship holds just as well for the
total infrared rnditrtion, sumrnctl over all wttvclengths,
since they show that the lowest vtrlues of r d i a t' 1011 occur
near intense Ilows and fronts.
It these findings are genertrlly vditl, then tl significant
positive correlation can be expected between outgoing
radiation (considered AS a heat loss, and therefore given
a negative sign) and the t'otal heuting tmd cooling of the
atmosphere, because :is Albrecht 131 htls pointed out,
there is a close positive re1:ltionship betwec.n total heating
and rainfall.
Some light is throw1 on this question 1))- a recently
completed (and tis >-et unpublished) study b ~- the writer
and his colleagues, in which estimates were r ~l t l t l r of the
vertical dist'ribution of the most irnportant conlponents
of atmospheric heating at W:~shingtorl, D.C., for the tropo-
spheric layer 1000 to 200 n1t)., and for cucllsuccacssive
12-hour period frotll JUW 1 to JulJ- 31, 1961. The esti-
mated heating components include cooling of the tropo-
sphere by long-wave nocturnal :n i t 1 heating by short-wave
solar radiation, net heating b ~- contlcllsation (lllctlsuretl
h; the lain falling in the Washington area), and heating
by direct turbulent contact with the ground. The esti-
mates were subject to many uncertainties, the most seri-
ous one being the rat'her crude determinations of cloud
mlount, type, base, and thickness. However, it is felt
that, the computations of each heating component give
reasonably rrleaningful results when averaged for the
entire troposphere.
A few preliminary results of this study are surnnlarized
here in table 1, where the various symbols are explained
in the legend.
The first row ol this table reveals a high positive correla-
tion between outgoing radiation a t 200 rnb. and cloudiness
(i.e., smaller negative values of radiation tend to be asso-
ciat'ed with large cloud cover). Because of low moisture
values in the stratosphere, the 200-mb. radiation is
probably only a litt'le smaller than the total outgoing
radiation R S measured by a satellite. Therefore, these
results confirm the close relat'ion between these paramet,ers
as revealed in thc two papers under discussion. The
correlation coefficient would have been larger if all cases
of low cloudiness had been removed, because, as pointed
out b ~- Fritz and Winston, in these cases the outgoing
rdiat,ion ~L J T he almost as large as that for clear skies.
TABLE 1 _- Iinear correlation and regression coe,fficients relating
selected heating parameters at Washington, D.C. First column
contains symbols ,for parameters: Itz, AR, and H are respectively
long-wave radiation at $00 mb.; difference in net outgoing radiation
200 mh. m i n u s 1000 mh.; and total individual heating from all
.sources in layer 1000 to 200 mb., all expressed in ly./day. (T h e first
two parameters always have a negative sign.) C i s total sky cover in tenths. The last three columns contain respectively the correlation
coe,fficient (tirnes loo), intercept, and slope o j a linear regression
rquetion, wh'ere the dependent and independent variables are respsc-
tively the first and second paraveter in column 1 .
288 JULY 1062
The second row shows that there is a fairly large positive
correlation between outgoing radiation and the net cooling
of the atmosphere by long-wave radiat’ion alone; i.e.,
the satellite radiation measurements probably give R fair
indication of that fraction of t’he total heat source or sink
due t’o long-wave radiation. This has been pointed out by
Professor Suonli in several recent talks.
The t’hird row shows that, as suggested intuitively, there
is a tendency for a positive correlation between out,going
radiation and total heating. Since the latter enters
directly into the t’hermodynarnic energy equation as used
in numerical models for predicting the general circulation,
these result’s may be of some assistance in establishing a
relation between total heat’ing and other parameters of
the circulat’ion. However, it should not be concluded from
this result’ that, on the average, warm air is cooled and
cold air warmed, a condition which would lead to R sys-
tematic destruction of t’he pot’ent’ial energy available for
maintaining the circulation (E. Lorenz [4]). This would
only be true if the atmosphere were always cloudless.
Act’ually, the Washington study indicates that there is ti
small but significant positive correlation between total
heating and temperature in the troposphere.
It should be pointed out that’ the correlation between
outgoing radiation and total heating would probably have
been higher were it not for certain cornputational diffi-
culties. Thus, the statistically-averaged cloud thicknesses
were probably too srllall on the days of heaviest precipita-
tion, resulting in too large values of outgoing radiation.
Also, on some days with high thin cirrus overcast and
lower stratiform clouds wit’h little rainfall, the computed
outgoing radiation was probably too small, because the
high clouds were always assumed to radiate as black
bodies. As pointed out by Fritz and Winston, high thin
clouds are not opaque, so t h a t t h e effect’ive radiation
comes from a level lower than the cirrus tops. Finally,
in the summer the average cloudiness rnay be small but
the rainfall large for a given 12-hour period. This diffi-
culty would not arise if it had been possible to make the
cornputations on n synoptic basis.
On the other hand, there are several reasons for ex-
pecting that the real correlation is far from perfect: On
clear days in summer the contribution to total heating
from turbulent interchange with t’he ground tends to be
larger th:tn on cloudy days; and thick high clouds occur on
some days with little or no rainfall. Tn winter, strong
heating from below often occurs in cold dry air flowing
over warlller land and ocean surfaces, while a t t h e same
t’irrre the outgoing radiation to space probably is large.
This tends to givc R negative correlation between radiation
and net heating. From this and other considerations, one
nlay conclude that the relat’ion between total heating and
outgoing long-wave radiwt’ion depends heavily on
geographical locution and season.
REFERENCES
1 . 11. Weinstein and V. Suomi, “Analysis of Satellite Infrared
Radiation hfcasurements on a Synoptic Scale,” Monthly
Weathcr Recirw, vol. 89, No. 11, Nov. 1961, pp. 419-428.
2. S. Fritz and J. S. Winston, “Synoptic Use of Radiation Measure-
ments from Satel!ite TIROS 11,” Monthly Weather Review,
vol. 90, No. 1, Jan. 1962, pp. 1--9.
3. F. Albrccht, “Untersnchungrn uber den Warmeha.ushalt der
Erdatmosphiire und seine Thcrmodynamische Bedeutung,”
Rerichte des Deutschrn Wetterdienstes in der U.S. Zone, vol. 3,
Yo. 17, 1950, 70 pp.
1. E. Lorenz, “Availablc Potential Energy and the hlaintenance of
the General Circulation,” TelZ?rs, vol. 7, No. 2, May 1955,
pp. 157-167.