Increasing knowledge of cirrus cloud properties can contribute to general circulation model development and ultimately to a better understanding of climate. The objective was to gain a better understanding of cirrus cloud characteristics. Observations from different sensors during the First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) which took place in Wisconsin over Oshkosh together with pertinent calculations are used to understand the dynamical, microphysical, and radiative characteristics of these clouds

Gultepe, Ismail

Heymsfield, Andrew

English

NASA Technical Reports Server

" 90- 28279
MOISTURE AND HEAT BUDGETS OF A CIRRUS
CLOUD FROM AIRCRAFT MEASUREMENTS DURING FIRE
Ismail GULTEPE and Andrew HEYMSFIELD
National Center For Atmospheric Research 1
P.O. Box 3000
Boulder, Colorado 80307
1. INTRODUCTION
Increasing knowledge of cirrus cloud properties can contribute to general circulation
model development and ultimately to a better understanding of climate. The objective of
this study is to gain a better understanding of cirrus cloud characteristics. Observations
from different sensors during the FIRE (First ISCCP (International Satellite Cloud Cli-
matology Program) Regional Experiment) which took place in Wisconsin over Oshkosh
(Start, 1987) together with pertinent calculations will be used to understand the dynami-
cal, microphysical, and radiative characteristics of these clouds.
2. SYNOPTIC CONDITIONS
Cirrus clouds on 31 October over Wisconsin formed in a thermally stable atmosphere.
A cold front moved over the Wisconsin region at 0600 UTC. The winds were averaging 50
2W
455
[
tim
7m
m
im
O 600 Illl 1_0 ZIIIO Z_ll
O| $ TPlirJE i_M)
Figure I. A cross section parallelto the jet stream at 12:00 UTC on 31 October 1986.
Wisconsin region isbetween 1500 and 2000 km. The solidfinesare for iscntropicsurfaces
and the dashed linesare for relativehumidity with respect to water.
I The National Center for Atmospheric Research is sponsored by the National Science
Foundation.
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m s -1 at 300 mb at 1200 UTC over Wisconsin. There was a weak upper level trough which
deepened at 0000 UTC on I November. The cirrus cloud was situated between 400 and
300 mb over Wisconsin approximately between 1400 UTC and 1830 UTC on 31 October.
Moisture advection from the southwest probably played an important role in the cirrus
formation (see Figure 1) over Wisconsin.
3. MEASUREMENTS
The data for this research were taken by the NCAR King Air and rawinsondes on 31
October 1986 over Oshkosh. The primary measurements from the NCAR King Air were ice
crystal concentration, size, habit, temperature, pressure, dew point, horizontal and vertical
wind speed and direction, and infrared and solar radiances. The raw measurements were
available for calculations at a rate of 1 Hz.
Two different flight patterns are used in this study: the first is the step-up maneuver
and the second is the Lagrangian descent maneuver. The length of each constant altitude
penetration was about 30 km and the separation in the vertical between two penetrations
was about 300 m. The second maneuver called the Lagrangian spiral descent covered a time
period of about 20 minutes (1616-1633 UTC for spiral I and 1657-1720 UTC for spiral II)
for an entire cloud sounding on 31 October 1986. During these spirals, the aircraft descent
rate was about 1.5 m s -1.
4. METHODS OF ANALYSIS
In this section, the moisture and heat budget equations including internal sources
(e.g., ice crystal growth) and external sources (e.g., radiative flux divergence) are given.
The most important parameter in the budget equations is vertical velocity w. Therefore,
first we explain its calculation in the following subsection. Then, different terms (e.g.,
advection and turbulent flux divergence) in the budget equations are discussed.
a. Vertical velocity calculation from hydrometeor data
Vertical velocity w used in the budget equations is calculated from the particle size
spectra measurements. The simultaneous solution technique (Heymsfield, 1977) is used
to obtain w. This technique requires the following terms derived from the PMS 2D-
C probe measurements: ice water content (IWC), precipitation rate (PR), and terminal
velocity (Vt) which are all dependent on habit. This technique does not need dew point
temperature measurements.
b. Moisture budget calculation
The moisture budget for a cirrus cloud is calculated from the continuity equation of
the water vapor mixing ratio. To the equations used by Wilfried (1984), we add two more
terms: the vertical turbulent flux of the moisture and the ice crystal growth rate, and write
the equation as:
Oq --_ __Oq Ow' q' Owi 1 -- 0+ - , (i)
where t is the time, q the vapor mixing ratio, s the length along the aircraft longitudinal
axis, z altitude. In Equation (1), the first term is the local change of the vapor mixing
ratio, the second the horizontal advection, the third the vertical advection, the fourth the
divergence of the turbulent moisture flux, and the last term the loss of moisture because
334
0_._ The first term is calculated from two Lagrangian spiralof the growth of ice crystals ( 0, J"
descents which axe separated from each other by approximately 43 minutes. The second
term is obtained from the product of the longitudinal wind U with respect to cloud and
the change of the mixing ratio over the constant flight leg. The _ in the third term is
average vertical velocity. Perturbations (e.g., w') are obtained from the measured (w) by
substracting the average (e.g., _). The _ is obtained from q at two constant altitudes
(from the step-up maneuver). The fourth term is calculated from the vertical variation of
the turbulent flux at two constant altitudes. The depletion of vapor from the environment
because of ice crystal growth is determined from equations given by Heymsfield (1975).
The pa in the last term is air density.
c. Heat budget calculation
The calculation of the heat budget of a cirrus cloud is also obtained from the continuity
equations used by Telford and Warner (1964) with four additional terms as follows:
() (°')oo -Uoo ao ow,o, oo + Yi + - o, (2)o-i+ _+_+ 0----7-+ -_ _R sw a, p_cp
where the first term is the local change of potential temperature O, the second the horizontal
temperature advection, the third the vertical temperature advectlon, the fourth the vertical
divergence of temperature flux. The fifth and sixth respectively are the local change of
temperature due to the IR and SW radiative fluxes and the last term the latent heat remase
because of the ice crystal growth. In this term, L, is the latent heat of sublimation and
Cp the specific heat at constant pressure. The first term is obtained from the Lagrangian
spiral descent profiles as discussed previously for the moisture budget. The over bar in (2)
signifies the constant altitude flight leg (30 km) averages of the various parameters. The
radiative cooling rate due to the IR and SW radiative flux divergences is calcu!ated from
the IR and SW upward and downward looking radiometers. The radiative cooling rate is
obtained from the following equation (Welch et al., 1980):
( ) 0Fo0 _ (_)_R,sw (3)
-_ iR,sw c_p_
where F is measured irradiance.
5. RESULTS AND CONCLUSIONS
a. Relative importance of the moisture budget terms
The moisture budget of the cirrus on 31 October is calculated from (1). The terms used
in (1) are the local change, horizontal and vertical advections, turbulent flux divergence,
and ice crystal growth terms.
The values of the terms in the moisture budget equations are shown in Table 1. The
layers from I to V in this table are between 7.9 andg.4 km altitude with a vertical separation
approximately 300 m. The negative (positive) sign indicates the moisture convergence
(divergence) due to the advection. Moisture convergence is seen at low levels. At the
upper levels, moisture divergence is more important. The sum of the terms in the layers,
335
excluding the local change term, gives a divergence of 0.031 mg kg -1 s -1 which is very
close to t-he local change term. Molsture convergence is seen at layers II. III, and IV. The
sum of the vertical moisture advection values results in a convergence of about -0.095 mg
kg -1 s -1. Turbulent flux divergence is one order of magnitude lower than other terms in
the moisture budget equation. Turbulent flux divergence values add to about 0.003 mg
kg -1 s -1. As a negative source (moisture divergence), ice crystal growth plays relatively
an important role on the expenditure of vapor from the environment at mid-layers. The
small residual value of about 0.002 mg kg -1 s -1 is probably due to errors in the wind
component and the dew point measurements.
b. Relative importance of the heat budget terms
The heat budget of the cirrus cloud on 31 October is calculated from (2). The terms
in the heat budget equation (see Table 2) are the local change, horizontal and vertical ad-
vection, turbulent flux divergence, ice crystal growth rate, and IR and SW flux divergence.
A layer number in this table has the same meaning for those used in Table 1.
Negative values of the terms used in this section will represent cooling in the cirrus
layers. Cold air advection in the lowest layer is about 8°C day -1. Warm air advection is
seen in layers III and V. The net warming due to the horizontal advection term through the
layer is found to be about 0.5 °C day -1. The vertical heat advection is the most important
term in the heat budget equation. Values of the turbulent flux divergence and ice crystal
growth terms are about the same, but they are much smaller than the other terms in (2).
The magnitude of the growth term for the entire cirrus is greater than that of the absolute
radiative flux divergence terms in (2). The IR cooling rate shows that the cirrus cloud
is warming at cloud base with cooling in the upper levels. The maximum cooling rate is
found to be about 14°C day -1 in layers III and IV, and warming about 7°C day -1 in layer
I. The IR coolingrate for the entire cirrus cloud is about 5°C day_ -1. The SW coolin_
rate shows that t-he cirrus is cooling at cloud top and base. At mid-levels, a warming oI
about 8°C day -1 and a similar cooling occurs in the adjacent layers. The sum of the IR
and SW cooling rates for the entire cirrus cloud shows that the net cooling is about 1.1°C
day -1. Error in IR and SW irradiance measurements can be as much as 15% and 50%,
respectively. The residual of about 7.0×10-4°C s -1 between the local cooling rate and the
sum of the other terms in (2) can be attributed to errors in the aircraft measurements.
. Overall, it appears that the dynamic characteristics of the environment play a crucial
role in the structure of the cirrus cloud. The IR cooling rate and release of latent heat are
important in the upper layers, indicating higher ice crystal growth rates.
REFERENCES
Heymsfield, A.J., 1975: Cirrus uncinus generating cells and the evolution of cirriform
clouds. Part I: Aircraft measurements of the growth of the ice phase, d. Atmos. Sci.,
32, 789-808.
, 1977: Precipitation development in stratiform ice clouds: A microphysical and
dynamical study, d. Atmos. Set'., 34, 367-381.
Kahn_ P.M.. and H.K. Weickmann, 1969: High altitude radiometric measurements of
cirrus.._. Appl. Meteor., 147-154.
Starr D. O'C.. 1987: A cirrus cloud experiment: Intensive field observations planned for
_IRE. Bull. Amer. Meteor. Sot., 68, 119-124.
336
Telford, J.W., and J. Warner, 1964: Fluxes of heat and vapor in the lower atmosphere
derived from aircraft observations. 3. Atmos. Sci., 21,539-548.
Welch, R.M, S.K. Cox, and W.G. Zdunkowski, 1980: Calculations of the variability of ice
cloud radiative properties at selected solar wavelengths. Appl. Opt., 19, 3057-3066.
Wilfried, B., 1984: Evaporation Into the Atmosphere. Published by D. Reidel Publishing
Company, P.O. Box 17, 3300 AA Dordrecht Holland, 293 pp.
Table 1. The terms used in the moisture budget equation except the local cooling rate (see
text for detailed discussion).
LAYER
_a _t _z
_J
[rng kg -1 se¢ -1 ] [rag kg-' se¢-'] ires kg -1 se¢-'ll ling _ se¢
I -0.04 0.003 0.0062 -0.0098
II -0.018 -0.041 0.0073 0.0057
III 0.05 -0.028 0.0056 0.0047
IV 0.031 -0.049 0.0053 -0.0023
V 0.008 0.020 0.0034 0.0054
TOTAL 0.031 -0.095 0.0284 0.003
Table 2. The terms used in the heat budget equation except the local cooling rate (see
text for detailed discussion).
LAYER
- -- ,',0
r--_xlo-' _ xio-' _-_,,xlO-' _c,_._io-' (_),R_io-' (_)s__io-'
[,c ,_¢-'1 [oc,_¢-'] [oc,_¢-'_ Coc_¢-,' ?c _¢-'! 'oc_¢-'?
I -0,94 -1.31 0.20 0.18 0.80 -0.19
I1 -0.1 -3.10 0.04 0.21 - 1.39 1.05
III 0.29 -4.40 -0.07 0,16 1.50 -0.98
IV 0.05 -6.09 0.14 0.17 -1.45 0.97
V 0.85 -2.59 0,01 O.I0 -0.06 -0,38
TOTAL 0.05 -17.50 0.32 0.82 -0.60 0.47
337



Moisture and heat budgets of a cirrus cloud from aircraft measurements during FIRE

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