The cirrus cloud climatology of Wylie and Menzel (1988) and some ancillary studies of cirrus clouds that have been made using the same data are summarized. The Wylie and Menzel climatology is a data set of cloud statistics extracted from the GOES/VAS satellite. With these data the geographical distributions of clouds, the seasonal changes, some diurnal changes, and also what atmospheric conditions cause the clouds can be described

Wylie, Donald P.

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W H E ~  ARE CIRRUS CLOUDS AND WHY ARE THEY THERE? 
Donald Wylie 
Space Science and Engineering Center 
University of W i ~ o n s i n - M ~ i ~ n  
u 
This paper is a summary of the cirrus cloud climatology of Wylie and Menzel(1988) and 
some ancillary studies of cirrus clouds that have been made using the same data set. The Wylie 
an4 Menzel climatology is a unique data set of cloud statistics extracted from the GOES/VAS 
satellite. With these dava we can describe the geographical distributions of clouds, there seasonal 
changes, some diurnal changes, and also what atmospheric conditions cause the clouds. 
The GOESNAS cloud analysis algorithm separates cloud cover on the satellite imagery into 
two classes; 1) those clouds that itre opaque to terrestrial radiation, and 2) those that are semi- 
winsparent to the upwelling infriued radiation. The technique identifies the semi-transparent 
clouds using the multi-spectral channels of the GOES VISSR Atmospheric Sounder (VAS). Some 
of these channels are sensitive to different levels in the atmosphere. Ideally, a low altitude cloud 
caw not be seen by a channel sensitive only to the upper atmosphere while a high altitude cloud will 
be seen on all channels. The strength of the cloud signature from the cloud free background varies 
between the channels because each is sensitive to a different level in the troposphere. This method 
has also been called the T O 2  Slicing Method" (Menzel et al., 1983) because VAS channels are 
capable of viewing only "slices" of the troposphere. 
solved for the cloud top height independently from the emissivity of the cloud. This allows a 
segregation of clouds on the GOESNAS image into opaque clouds and partially transmissive 
Clouds which are called cirrus by Wylie and Menzel(1988). In reality, cirrus clouds are more 
correctly defined as ice clouds some of which are thick enough to be opaque to terrestrial radiation. 
A large number are sufficiently thin to upwelling terrestrial radiation. The thin clouds are very 
imponant to studies of the radiative heating of the earth because they allow solar radiation to 
penetrate through while partially blocking some of the upwelling infrared radiation. In this paper 
we use the term "cirrus" to mean only the thin and semi-vansparent clouds. 
Cirrus clouds are more frequent than previously estimated.. They are found over the 
Continental United States and bordering Oceans from 25 to 35 % of the time. Opaque clouds were 
found - 45% of the time and no clouds (clear skies) were found - 25% of the time. 
and 2). The "Sun Belt" where opaque clouds are infrequent and clear skies are frequently found, 
are obvious in the geographical summaries (Figures 1 and 2). They are found in southern Texas 
and Mexico during the winter and over most of the southwestern United States, Texas, New 
Mexico, Arizona, and California, in the summer. 
The differential sensitivity of the VAS channels allows the radiative transfer equation to be 
Opaque cloud cover has large geographical variations while cirrus do not (see Figures 1 
Cims clouds, on the other hand, have very small geographical variances. They were 
slightly less frequent in the southwest than the rest of the United States. The only area where few 
c h s  were found was in the Eastern Pacific Ocean. Here a Iarge seasonal change also can be seen 
because of the northward migration of the Subtropical High in the summer. 
The geographical bounds of this study are 250 to 500 north latitude and 600 to 1400 west 
longitude. The probability of finding both opaque and cirrus clouds increases in the winter. In the 
The seasonal changes over the area shown in Figures 1 and 2 are summarized in Table 1. 
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https://ntrs.nasa.gov/search.jsp?R=19910001137 2020-03-19T20:16:40+00:00Z
summer the average probability of opaque clouds is 42% which increases to 46% .in the winter. 
Cirrus clouds increased by a similar amount from a probability of 24% in the summer to 29% in 
the winter. The probability of clear skies correspondingly decreases from summer to winter, 34% 
to 25%. 
TABLE 1: THE PROBABILITY OF CLOUDS 
seaso n Cirrus aaue Clear skv 
Summer 24% 42% 34% 
Winter 29% 46% 25% 
To seek answers to the question of "why are the cirrus clouds present?" the GOESNAS 
cloud reports were compared to atmospheric dynamic and thermodynamic conditions which are 
suspected to cause clouds. To cause cirrus clouds, the upper troposphere has to be rising which 
can lead to saturation.. Moistening of upper layers also can lead to saturation which will cause 
cirrus generation. This can happen by a number of mechanisms. Cumulonimbus convection 
obviously is one mechanism which comes to mind because the large cirrus anvil clouds seen on top 
of the cumulonimbus clouds. 
To determine what fraction of the cirrus clouds are related to the cumulonimbus convection, 
we counted the percentage of cirrus reports near radar echoes using the echoes as indicators of the 
presence of the cumulonimbus clouds. The radar summary from the National Weather Service was 
used for locating the radar echoes (Figure 3). The radar summary is a pictorial summary of all 
echo reports from all weather radars in the United States. Regions where echoes are reported are 
shaded on these summaries along with numeric infoxmation on the maximum heights of the echoes 
and their speeds and directions of movement. 
cumulonimbus. The extended boundary (solid line) was drawn from 50 to 300 km outward from 
the echo summary boundary. The extended boundary was placed 100 to 300 km downwind of the 
echo boundary. The distance was apportioned using the 300 mb wind speed allowing for 
advection over three hours. On the cross wind and upwind sides of the echo boundary, the 
extended boundary was placed -50,km from the echo boundary to allow for lateral spreading of the 
cirrus anvil. 
The echo summary areas were extended to allow for cirrus blow off fiom the 
The GOESNAS cirrus analyses at 00:OO GMT and 12:OO GMT were used in this study. 
The extended echo boundaries for all N W S  radar summaries in the time period three hours prior to 
the GOESNAS analysis were used to define the extended boundary. This study was performed in 
two seasons. The month of June 1986 was chosen to represent a summer season and January 
1988 to represent a winter season. 
The cirrus reports were separated into either being close to the radar echoes (inside the 
boundary) or away from any echoes (outside of the boundary). The same separation also was 
made for all cloud reports (cirrus and opaque) of high cloud above 500 mb. The results are given 
in Table 2. 
In the summer approximately one-half of the cirrus and high cloud (both cirrus and opaque 
cloud) reports were close to or over radar echoes (52 to 57%). The other half were far away from 
the echoes (48 to 43%). This statistic implies that Cb convection only explains one half of the 
cirrus in the summer. 
In the winter a different picture was found. Only 22% of the cirrus and 24% of the high 
cloud reports were near the radar echoes. The other 76 to 78% were separated from the echoes. 
The amount of area covered by radar echoes decreased in the winter while the number of cirrus and 
opaque cloud reports increased. Thus in winter, most of the cirrus have no relation to Cb clouds. 
12 
The cirrus not directly caused by Cb clouds must be created and maintained by other 
motions in the atmosphere. We are currently examining vorticity and temperature advection for 
their relationships to cirrus ourd high clouds. The polar and sub-tropical jet streams also will be 
examined. 
TABLE 2: THE PROBABILITY OF FINDING CLOUD OBSERVATIONS Wmr 
RESPECT TO RADAR ECHOES. 
N a O r  Not Related 
3vm of C loud Echoes m 0 h ~  
Cirrus in summer 
Cirrus in winter 
52% 48% 
22% 78% 
All clouds <500 mb in summer 57% 43% 
All clouds 400 mb in winter 24% 76% 
October 86 (FIRE IFO) Cirrus 32% 68% 
October 86 (FIRE IFO) All clouds 34% 66% 
REFERENCES 
Menzel, W. P.,W. L. Smith, and T. R. Stewart, 1983: Improved cloud motion wind 
Wylie, D. P., and W. P. Menzel, 1988: Cloud cover statistics from GOESNAS, in 
vectos 
and altitude assignment using VAS. 377-384. 
preparation. 
1 
PROBABILITY OF CIRRUS IN SUMMER 
PROBABILITY OF CIRRUS IN WINTER 
bability of finding cims clouds in summer (June- August) and winter (December- 
February), Two winters and summers were averaged together. 
PROBABILITY OF OPAQUE CLOUD IN SUMMER 
PROBABILITY OF OPAQUE CLOUD IN WINTER 
Figure 2: Probability of finding Opaque clouds in summer and winter. 
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National Weather Service Radar Echo Summary 
Cirrus Cloud Analysis 
Figure 3: An example of a National Weather Service Radar Echo Summary used of defining 
locations of cumulonimbus clouds. The extended boundary of probable cims anvil is defined by 
the thick line. 
16 


Where are cirrus clouds and why are they there

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