Icing caused by supercooled clouds below 10,000 feet were characterized with a view toward a change in FAA standards for civil aircraft ice protection standards. Current techniques in cloud physics were employed

Masters, C. O.

English

NASA Technical Reports Server

'A NEW CHARACTERIZATION OF SUPERCOOLED CLOUDS 
3ELOW 10,000 FEET AGL" 
Charles 0. Masters 
Introduction 
The current atmospheric icing, supercooled cloud 
criteria for the design of U. S.  civil aircraft ice 
protection systems and equipments is presented 
in Appendix C of Federal Air Regulations (FAR) 
Part 25 [l]. These design criteria are based upon 
data developed by the National Advisory Com- 
mittee for Aeronautics (NACA) in the late 1940 
to early 1950 time frame, and were intended pri- 
marily for large, high-performance, fixed-wing air- 
craft of that era. They encompass both layer and 
convective clouds with altitudes from 0 to 22,000 
feet pressure altitude (PA), suggested tempera- 
tures as cold as -40" Celsius (" C), and liq- 
uid water contents (LWC) as high as 2.9 grams 
per cubic meter (gmm3 ). Since their generation, 
these criteria have been exacted upon all aircraft 
seeking U. S. certification for flight into known ic- 
ing conditions, including rotary and fixed wing, 
low-altitude, low- performance aircraft which typ- 
ically operate below 10,000 feet. Since the phe- 
nomenon which dictates the formation of cloud 
water droplets and their associated LWC are de- 
pendent upon horizontal mixing and the vertical 
development of the cloud above the surface, icing 
clouds developed within 10,000 feet of the surface 
under convective conditions will be less severe; i.e., 
a lower LWC than clouds with developments ex- 
tending to higher altitudes. Thus, in FY-1979, the 
Federal Aviation Administration (FAA) engaged 
the Atmospheric Physics Branch of the Naval Re- 
search Laboratory (NRL) to conduct studies and 
to gather data for a better characterization of the 
atmospheric icing environment below 10,000 feet. 
This effort has resulted in the data base employed 
in the generation of the new characterization of 
this presentation, and is described in the NRL Re- 
port Number DOT/FAA/CT-83/21 entitled, "A 
New Data Base of Supercooled Clouds Variables 
at Altitudes Below 10,000 Feet AGL and the Im- 
plications for Low Altitude Aircraft Icing" 121. 
This presentation introduces the new characteriza- 
tion of supercooled clouds below 10,000 feet above 
ground level (AGL), and presents the rationale, 
data analysis, and data reduction procedures em- 
ployed in the generation of the icing envelopes and 
other information which constitutes the new char- 
acterization. Also, potential applications of the 
new characterization will be discussed. 
The New Characterization 
The new characterization of supercooled clouds 
below 10,000 feet AGL is presented in Figure 1. 
In essence, it combines both layer and convective 
clouds, and encompasses three ambient tempera- 
ture (Ta) dependent icing envelopes of 0 to -15O C, 
-15 to -20" C, and -20 to -25" C. Associated with 
the two colder icing envelopes are cloud horizontal 
extents (durations) of 20 nautical miles (NM), and 
for the icing envelope of the warmer temperature 
range, cloud horizontal extents of 50, 20, 12, and 
6 NM for LWC ranges of .04 to .5, .5 to .75, .75 
to 1.0, and 1.0 to 1.74 gm-3 , respectively. Also, 
associated with the 0 to -15°C temperature enve- 
lope are median volume diameters (MVD) which 
range from 3 to 50 microns (pm) and LWCs which 
range from .04 to 1.74 gm-3; for the mid temper- 
ature envelope MVDs range from 5 to 38 pm and 
LWCs range from .04 to .66 gm-3 , and for the 
coldest temperature envelope, MVDs range from 
7 to 15 pm and LWCs range from .04 to .41 gm-3. 
The outermost edges of these envelopes and the 
horizontal extents represent extreme values of su- 
percooled cloud properties determined to a prob- 
ability level of exceedence of less than one part in 
a thousand; i.e., less than 0.001. 
d 
Figure 1.  The new characterization o f  supercooled 
clouds from ground level t o  10,000 
feet AGL 
56 
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General Approach 
The basic approach employed in these analyses 
for the new characterization was to determine val- 
ues of LWC, MVD, Ta, and event duration such 
that the probability of independently exceeding 
any one of these parameters would be less than 
one part in a thousand; Le.,< 0.001 for all atmo- 
spheric icing conditions up to 10,000 feet AGL over 
the conterminous U. S. and nearby offshore areas. 
The initial analysis effort consisted of reviewing all 
icing events in raw data form in 5 O  C temperature 
increments from 0 to -25" C for each parameter of 
interest. These parameters were then ordered by 
magnitude and the 99.9 percentile selected. 
Thus, values which exceeded the 99.9 percentiles 
would correspond to values of those paramters with 
a probability of exceedance less than 1 part in a 
thousand. Obviously, such a simplistic approach 
could only be employed and yield results with a 
high level of confidence in cases where there is a 
symmetrical, unimodal near-inhite data set from 
which to draw. However, in this case, the data 
base of 6,700 plus data miles representing some 
1,400 icing events was deemed marginal, especially 
for extreme parameter values which were typified 
by limited data miles. Thus, realizing the possible 
limitation of the raw data set, a least distribution 
was employed to predict the extreme values. De- 
tails of this procedure are contained in the techni- 
cal report noted in Reference [3]. 
A Combined Presentation for Laver and Convec- 
tive Clouds 
In FAR 25, Appendix C, the presentations of LWC, 
temperature, MVD, and horizontal extent (dura- 
tion) are presented separately for layer clouds (con- 
tinuous maximum conditions) and for convective 
clouds (intermittent maximum conditions). A re- 
view of the new characterization's data base in 
terms of layer clouds versus convective clouds in- 
dicates that the ranges of cloud properties were 
similar for both cloud types except for LWC's 1.0 
gm-3 which were found only in convective clouds 
and, for Ta colder than -17.5" C where only layer 
clouds were observed. This is delineated in the 
matrix of Figure 2, which shows Ta versus LWC for 
each cloud type. A further review of the horizon- 
tal extents (icing events durations) for each cloud 
type revealed that combining the two cIoud types 
into a single presentation would not be overly re- 
strictive provided due consideration was given to 
the proper cloud type; .e.g., the horizontal extent 
of 6 NM for LWC greater than 1.0 gm -' is based 
only upon convective cloud data. Thus, this was 
the approach taken. 
n 
M 
E 
m " 
I 
V 
3 
-I 
1.75 
1.5 
1.25 
1 .o 
.75 
.5 
-25 
0 
0 -5 -10 -15 -20 -2 5 
Ta - ( " C )  
'C = C o n v e c t i v e  C l o u d  
*L = L a y e r  C l o u d  
* 
Figure 2 .  Matrix of LWC versus ambient temperature 
(Ta) f o r  cloud types 
A Consolidated Temperature Range: 0 to -15O C 
Initially, raw data graphs were constructed for each 
of the 5' C temperature intervals between 0 and 
- 2 5 O  C in a manner similar to the LWC versus 
MED graphs of FAR 25, Appendix C. The max- 
imum observed values of LWC which occiirred in 
each 5 pm interval of MVD was used to establish 
an interim envelope outline for each of the temper- 
ature ranges. The one exception is the one lone 
maximum data point which occurred at 22 pm at 
a LWC of 1.7 gm-3, and a Ta of -6.5O C, which 
was omitted from the interim envelopes. These 
raw data graphs revealed very little differences be- 
tween the three envelopes in the 0 to -15" C tem- 
perature interval (see Figure 3). Consequently, it 
was decided to combine all data in the 0 to -15" 
C temperature range and establish one envelope 
which described these parameters. Rationale for 
the inclusion of the one lone data point of 1.7 gmP3 
to this temperature range could be supported if, 
during subsequent analysis, this point was found 
to lie within the Weibull 99.9 percentile. This 
semblance was not observed in the temperature 
ranges of -15 to -20" C and -20 to -25" C. Conse- 
quently, parameters in these ranges were treated 
separately. 
57 
Figure 3. Similarity of icing envelope o f  5°C 
intervals for the temperature range of 
0 t o  -15' C. 
Ambient TemDerature versus Altitude AGL 
An initial review of the data base indicated no ap- 
preciable altitude dependence for the cloud prop- 
erties of LWC and MVD. However, icing condi- 
tions were not observed at the colder temperatures 
which occurred at the higher and lower altitudes; 
Le., temperature in the range of -15 to - 2 5 O  C 
which occurred between ground level and 4,000 
feet AGL and between 6,000 feet and 10,000 feet 
AGL (Figure 4). However, this region constituted 
only a small portion, approximately 16 percent, 
of the total temperature versus altitude envelope 
and, for all practical purposes, could be accommo- 
dated by assuming the probable existence of super- 
cooled clouds at all temperatures of interest and at 
12,000 
- 0  -6 -10 -15 -20 -25 -30 
TEMPERATURE - "Celsius 
Figure 4. Ambient temperature versus a1 ti  tude 
above ground level for observed cloud 
types , 
all altitudes up to 10,000 feet AGL. (Possibly over 
the northernmost portions of the U. S. during out- 
breaks of extreme cold polar air masses.) Conse- 
quently, the new characterization does not present 
a temperature versus altitude chart, whereas FAR 
25, Appendix C ,  presents such a chart for both the 
continuous maximum and intermittent maximum 
criteria. 
The Weibull Distribution 
In these analyses, the Weibull distribution func- 
tion was employed to predict the extreme values 
of the supercooled cloud properties. This function 
reduced to the form 
was employed to establish the coordinates of the 
plot of the parameter of interest: where 
T = the ith percentile of an observed cloud 
property; Le., 20, 50, 60, . . . 99 
E = the value of an observed cloud property; 
e.g., LWC, associated wth the ith percentile. 
Most extreme values of the new characterization 
were determined by computer; however, for illus- 
tration purposes, Figure 5 graphically depicts the 
procedure employed in determining the extreme 
value of cloud horizontal extent (duration) associ- 
ated with the icing envelope of -15 to - 2 O O  C. 
-0.5 0. 0.5 1.0 1.5 2.0 2.5 
1 QnQn(-) 4 1 - a  
~ U K T I L E S :  45 62 81 94 99 99.94 99.4995 
X :  50 60 80 90 95 99 99.9 
MI: 2.1 2.6 5.0 6.8 9.5 15.0 18.6 
Figure 5. The determination o f  horizontal extent 
extreme: -15 "C t o  -ZOO C .  
Although in this case the observed 99.9 percentile 
value was 18.6 NM the Weibull predicted value 
was found to be 20.1 NM and was subsequently 
rounded off to 20 NM as depicted on the new char- 
58 
acterization (Figure 1). In a similar manner, the 
other extreme values of the cloud properties were 
determined, except that the Weibull predicted val- 
ues of LWC were determined for each 5 pm MVD 
interval of its associated icing envelope. 
A Final ComDarison 
Figure 6 presents a comparison of the new charac- 
terization, FAR 25, Appendix C, and the recently 
introduced FAA rotorcraft directorate's limited cri- 
teria. On this chart, all temperatures have been 
converted to Celsius, and the -40' F temperature 
contour line of the FAR 25, Appendix C, inter- 
mittent maximum criteria has been omitted, pri- 
0 rb 20 30 40 50 
MEDIAN VOLUME DIAMETER (pm) 
Figure 6. The new characterization superimposed 
on the far 25, Appendix C, and the 
rotorcraft directorate's limited inter- 
mi ttent maximum and continuous criteria. 
marily for clarity. Some of the readily apparent 
observations/conclusions that can be drawn from 
this chart are: 
1. The new characterization encompasses 
MVDs between 3 pm and 15 pm which were omit- 
ted from the FAR 25, Appendix C ,  and the rotor- 
craft directorate's limited criteria. 
2. The new characterization presents a maxi- 
mum LWC value of 1.74 gme3 at 22 pm, whereas 
the FAR 25, Appendix C, criteria depicts a maxi- 
mum value of 2.9 gm-3 at 15 pm, and the rotor- 
craft directorate's limited criteria depcits a maxi- 
mum value of 1.5 g ~ n - ~ .  
as -40' C, and the rotorcraft directorate's limited 
criteria coldest temperature is - 2 3 O  C. 
4. Jn the intermittent maximum criteria of 
both the directorate's limited criteria and the FAR 
25, Appendix C, criteria, all values of LWC asso- 
ciated with MVD's larger than 36 pm significantly 
exceeds those of the new characterization and are 
deemed excessively conservative for altitudes be- 
low 10,000 feet AGL. 
Concludinn Remarks 
Figure 1 depicts the final characterization of the 
atmosphere for supercooled clouds from ground 
level to 10,000 feet AGL. The envelope of each of 
the temperature ranges encompass values with a 
probability of exceedance greater than one part in 
a thousand, whereas the extremes of the envelopes 
represent exceedance probabilities less than or equal 
to one part in a thousand. Inherently, this char- 
acterization has parameters which may be em- 
ployed in subsequent design of ice protection sys- 
tems and equipments for aircraft which operate 
between ground level and 10,000 feet AGL. It is 
planned that this characterization will serve as an 
adjunct to the worldwide characterization of su- 
percooled clouds currently under development. 
References 
1. Federal Aviation Regulation Part 25 (FAR 25), 
Airworthiness Standards: Transport Category 
Airplanes, Appendix C, Washington, D. C. 20591, 
Department of Transportation, Federal Aviation Ad- 
ministration, 1974. 
2. Jeck, R .  K.,  A New Data Base of Supercooled 
Cloud Variables at Altitudes Below 10,000 Feet 
AGL and the Implications for Low Altitude Air- 
craft Icing, Department of Transportation, Federal 
Aviation Administration Report Number FAA- CT- 
88-21, 1988. 
8. Masters, C. O., A New Characterization of Su- 
percooled Clouds Below 10,000 Feet AGL, Depart- 
ment of Transportation/Federal Aviation Admin- 
datrutdon, Report Number FAA-CT-83-22, 1985. 
3. The new characterization depcits no tem- 
perature colder than -25O C, whereas the FAR 
25, Appendix C, criteria presents temperatures as 
cold as -30" C and suggests temperatures as cold 
59 
“DEVELOPMENT OF A PLAN FOR IMPROVED AIRCRAFT ICING 
FORECASTS AND ASS0 CIATED WARNING SERVICES” 
Ralph Pass 
I would like to describe a plan that has just re- 
cently started at the Office of the Federal Coor- 
dinator for Meteorology (OFCM). The plan was 
suggested by the National Transportation Safety 
Board (NTSB), and the goal is to provide inte- 
grated plans for improving aircraft icing forecasts. 
Before people panic and think we are going to  
come up with a new plan in a vacuum, I would like 
to say that I’m going to take whatever I can from 
various plans that already exist covering the var- 
ious phases of the aircraft icing forecast problem. 
Yesterday, we heard a description of the FAA’s 
plan from Loni Czekalski, which will be included 
in the OFCM plan. As a result, the aircraft cer- 
tification part of my effort will be rather straight- 
forward. Again, we are going to try to  develop a 
plan that will summarize a systems view of what 
the Federal Government should be doing in air- 
craft icing and associated warning service dissem- 
ination. We have broken it down into five major 
areas dealing with the data collection, forecasting, 
dissemination, display and education, and aircraft 
certification. 
Building on what has been said this morning, the 
FAA is now looking at  new characterizations of 
clouds. The question becomes, “How do you relate 
that to aircraft icing?”; “Does the aircraft manu- 
facturer have to give you a formula which says 
that given this droplet size and liquid water con- 
tent, this is the kind of icing you can expect for 
a given air speed?” This might be a reasonable 
thing to look at. If that is the case, then the ques- 
tion becomes, “How do you get information to the 
pilot relating to liquid water content and droplet 
size?” Currently there are no forecast procedures 
for that. There is currently no way to conveniently 
display it; and based upon disscussions between 
the Icing Committee and the Remote Detection 
Committee, there is no way to measure it. So, 
this type of plan with this kind of problem needs 
to be addressed coherently from a systems point 
of view. We are going to be looking at not only 
what goes into each of these five areas, but also 
their interconnection. If the FAA would like to 
require liquid water, for example, as one of the 
parameters, the pilot needs to know it before he 
takes off, and we are going to have to figure out 
how to get it to him. That is what the plan would 
like to address. 
I would like to give you a brief layout of what will 
be done. Task 1, which I have not yet addressed, is 
basically a literature search and interview period. 
Part of my reason for briefing here is to identify 
people to whom I should be talking in each of those 
areas we mentioned earlier. I certainly want to 
welcome anyone who would want to talk. Just let 
me know. 
Briefly, the project schedule goes like this. We 
started in the first of October through the collec- 
tion phase, Task 1, and it should be finished the 
end of this month or the first part of next month. 
At that point, we will start interviewing people 
throughout the country and throughout the vari- 
ous agencies interested in these areas, which will 
lead to a series of reports characterizing each of the 
individual areas we will address. We, then, have 
several months in order to put the report together 
and have it reviewed. Within approximately one 
year from that point, we hope to have a final copy 
out. 
A literature search has been run at the OFCM and 
at TASC. Anyone who would like to make sure 
that certain pieces of information are included are 
welcome to let me know. One of the things I would 
like to get hold of fairly soon is the AFGL report 
on comparing current procedures for forecasting 
icing. Again, the forecast procedures are proba- 
bly as conservative as the FAA characterization of 
clouds in the envelopes in FAR 25 Appendix C. 
I would like to interview relevant individuals. If 
you would like to be included or know of others 
you would like to have interviewed, please submit 
your name or names of all relevant individuals. 
Finally, we would like to prepare a plan outline, 
which will be available in November at  the OFCM. 
If you would like to see that, please contact either 
myself or M a y  Ballenzweig, and we will see that 
you get a copy of it. 
The first task is to see that we are pointed in the 
right direction. I don’t intend to work in a vac- 
uum. We would like to take the bits and pieces 
from the various groups and come up with a final 
integrated plan. Thank You. 
60 


A new characterization of supercooled clouds below 10,000 feet AGL

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