The role of cirrus clouds, particularly in weather and climate processes, has been increasingly investigated. Numerical models have demonstrated the importance of the solar reflectivity and infrared radiation of cirrus clouds in the Earth's radiation budget and climate. These properties depend upon the cloud microphysical characteristics, density, and altitude and hence justify investigation. The results reported were obtained from cold clouds (-20 to -46 C) in the mid to upper troposphere during ten flights of the NCAR King Air as part of the First ISCCP Research Experiment (FIRE) in Wisconsin

Heymsfield, Andrew J.

Knight, Nancy C.

Sassen, Kenneth

English

NASA Technical Reports Server

E 
IIYDROMETEOR DEVELOPMENT IN COLD CLOUDS IN FIRE 
Andrew J.  Heymsfield and Nancy C. Knight 
National Center for Atmospheric Research* 
Boulder, Colorado 80307, USA 
Kenneth Sassen 
Department of Meteorology, IJniversity of Utah 
Salt Lake Cily, Utah 84112 
* The National Center for Atniospheric Research is sponsored b y  the National Science 
Fou ndatiori. 
55 
https://ntrs.nasa.gov/search.jsp?R=19910001146 2020-03-19T20:15:14+00:00Z
1. INTRODUCTION 
The role of cirrus clouds, particularly in weather and climate processes, has been 
increasingly investigated. Numerical models have demonstrated the importance of the 
solar reflectivity and infrared radiation of cirrus clouds in the earth's radiation budget and 
climate. These properties depend upon the cloud microphysical characteristics, density, 
and altitude and hence justify investigation. The results reported herein were obtained 
from cold clouds (-20 to -46°C) in the mid to upper troposphere during ten flights of the 
NCAR King Air as part of tlie First ISCCP Research Experiment (FIRE) in Wisconsin. 
2. LIQUID WATER CONTENT MEASUREMENTS 
The number of seconds during which liquid water was observed in FIRE clouds between -25 
and -35"C, is given in Fig. 1 as a function of amount detected. Data were obtained from 
a Rosemount Icing Detector (RICE), a Particle Measuring Systems' Forward Scattering 
Spectrometer Probe (FSSP), and two hot wire probes, the Johnson-Williams (J-W) and 
the Kiiig. Although the J-W and the King hot-wire probes have been the instruments of 
choice in past investigations of liquid water in cold clouds, they are limited by detection 
thresholds of 0.02 to 0.05 g an order of magnitude higher than the RICE or the FSSP. 
Tlie results given in Fig. 1 are taken from the FSSP data and illustrate the importance of 
measurements at  snialler LWC; most of the liquid water observed at temperatures cold'er 
than -20°C was below the detection limits of tlie hot-wire probes. The data from which 
Fig. 1 was derived show the largest amounts of liquid water between -30 and -35"C, 
indicating that in this range, meteorology is more important than temperature. 
The operating principles and calibration procedures of all but the RICE have been reported 
in the formal literature. The RICE collects water droplets arid measures corresponding 
changes in the frequency of a vibrating cylinder. The upper noise limit was determined 
by examining all output obtained from the instrument at temperatures lower than -40°C 
where all water is assumed to be frozen. At the true air speed appropriate for the King 
Air, - 100 In s-l, the h i i t  was found to be 3 mV s-I. FSSP spectra of tlie same 
temperatures were used to determine ice particle coritaminatiori in the data from that 
iiistrument, resulting in a conservative threshold of 1.5 C I I I - ~  particles per each size bin 
(3 to 45 pm). Examination of the data taken at warmer temperatures indicated that the 
RICE and FSSP corrections were valid to -20°C. 
As will be illustrated in another paper to be presented here, these measurernwls of 
supercooled liquid water were obtained within relatively thin altocuniulus cloud layers 
found in association with cirrus cloud systems. I'olarization lidar observat ions frnrti t Iw 
Wausau site reveal that the altocuniulus frcqueiit Iy i)ecairie incorporated iii deep cirrus ice 
clouds as precipitating cirrus particles reaclird t lie height of the liquid layer. This process 
was observed to occur during all four of the deep cirrus cloud occurrences studied from 
Wausau during the FIRE IFO, arid appears to have been important to the development of 
the cirrus at  low levels. 
56 
3. ICE PARTICLE EVOLUTION 
The types of ice crystals in the cold clouds and their size and number were determined 
from data obtained from two PMS imaging probes, 2D-C and P, and from collecting actual 
crystals in situ on oil-coated slides. 
The evolution of ice particles in the cloud layers was examined using patterns in which the 
aircraft performed Lagrangian spirals, slowly descending through the layers while drifting 
with the ambient wind. The particle-size spectra derived from the imaging probes during 
one such Lagrangian spiral are given in Fig. 2a. lnltoniogeneities have been removed by 
averaging each spectrum over the entire spiral. Meaauremenb taken from tire aircraft 
during the flight showed a zone of ice subsaturation between 7.4 arid 8.8 kin msl and ice 
supersaturations from 10 to 20% at other altitudes, all below the top of the cloud layer 
which the aircraft was unable to reach. In broadeitiirg with decreasing altitude the spectra 
shown are typical of those obtaitrecl in all of tJie flights. Again, typically, much of the growth 
occurs iri  the larger sizes. Examples of the particles, primarily bullet rosettes, collected 
during the same descent are also shown ia Fig. 2b. Bullet rosettes were predominant in 
many of the collections and columns or plates in others. 
4. ICE PARTICLE A@GR,EGATION 
At all temperatures the imaging probe data from every flight gave evidence of particle 
aggregation; usually of bullet rosettes joined at  their tips, as is illustrated in Fig. 2c and d. 
Aggregations of plates arid coluirins were also observed, the former joined at edges and tlie 
latter end-to-end. A t  temperatures lower tliaii -25°C tlie aggregations were almost always 
o f  two, equi-dimensional particles with cottceiitrabions uiivaried with altitude, typically 0.1 
to 0.5 t-'. A s  temperatures warmed above -25"C, the iiuinber of aggregates increased 
as did tlie number of particles coniyrisiiig tliern, their size, and their conceiitratioti in the 
total number of particles. 
5. CONCLUSIONS 
During the FIRE experiment, the microphysical characteristics of cold clouds have been 
examined, using tlie NCAR King Air. The cloucls iiivestigated ranged in temperature 
from -20 to -46°C. Jkiitid water waq detected i r r  these r.louds at -3S"C and may exist at, 
even colder temperatures. Evaluation of the conditiolis under wtiicli it exists at such low 
temperatures is coirtinuing. 
Aircraft patterns in the form of Lagrangian spirals were used to iiit,wprrt particle gIou 1 I t  
processes. Significaiit broadeiiitig of tlie ixirticle size spectra was ol,srrved nir,h t i i i t i o t .  
changes in the spectra at small sizes. Vitt.iia1l.v all of t tic l~roatletiitig ol>scr\t*cl \ \as  
attributable to ice particle aggregation wliicli occurred at all teitilwratures. ?'lie crystals 
comprising the aggregates were of comparable size, joiiied either at tips or edges, and were 
usually two in nurriber at teniperatures lower than -25"C, increasing to three or more at  
warmer temperatures. The data strongly suggest that siriteririg is the mecliariisni through 
57 
which the crystals aggregate. 
Aggregation appears to be important in the transfer of water mass from upper to lower 
levels in clouds. Investigation of bike aggregation process is continuing in greater detail. 
ACKNOWLEDGEMENTS 
The participation of the authors in FIRE was funded in part by NASA project order 
#L-98100B. University of Utah lidar observations were supported by the National 
Science Foundation Grant ATM-85 13975, and aircraft support waa obtained through 
the same NSF grant. The authors are grateful for Larry Miloshevich’s contribution in the 
preparation of Fig. 1. 
NUMBER OF SECONDS LW WAS DETTECTED 
IY 
IY 
am 
a'" 
a -  
1 -  
Y 
n 
.ol .01 .oa .m .a .a .n .m 
I* 1y-1. 
Fig. 1: Number or secondr in wlrlcli llqulcl wolar wm 
detected during FIRE flights as a function of LWC 
(g m-3) in temperatures between -25 and -35°C. Din 
widths are 0.001 g m". 
r.01 . . * * . * . . I  . * . L . - * . '  1 . . * . . . . a  1 . - . * a d  
18.' IO.' IO" I* IO' 
Ng. 3.: LACRANGlAN SPIRAL, 2 November 
1986. Purticle concentration (number per liter) u 
a function of altitude. Size range (microns): a) 75 - 
200; b) 200 - 500; c) 500 - 800; d) > 800; e) total. 
-22O - 
-45" 
500 p 
# 
Eig. 2b: Farticlea collected in situ at  temperatures 
given ("C). 
-46" -46" - 4 6 O  -31" .
500 IC 
Fig. 2c: 2D-C probe images at temperatures given 
("C). 
-22O U -22" 
500 j~ 
Pig. 2d: 2D-P probe imager at Lcnrperaburcr givcii 
("C). 


Hydrometeor development in cold clouds in FIRE

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