The 7 July mission of the 1987 FIRE Marine Stratocumulus Intensive Field Observations was chosen for analysis because of a well-defined transition from stratocumulus to clear conditions sampled by the aircraft on this day. It is hoped that by studying this case something can be learned about the processes responsible for the maintenance and breakup of stratocumulus layers, a primary objective of FIRE. The preliminary analysis is based on data from the Electra flight of this day. The properties of turbulence elements, i.e., updrafts and downdrafts, are examined to gain information on the nature of the turbulent exchanges through the boundary layer and across the inversion. Since such exchanges in large measure determine the stability and structure of cloud layers, a study of draft properties should be informative. The results will also be useful in the development of boundary layer models that are based on draft circulations (e.g., Randall, 1988; Hanson, 1988)

Khalsa, Siri Jodha Singh

English

NASA Technical Reports Server

N90-28247
Turbulence Structure in Clear and Cloudy Regions
of the 7 July 1987 Electra Mission
Siri Jodha Singh Khalsa
Cooperative Institute for Research in Environmental Sciences
University of Colorado
Boulder, Colorado 80309-0216
I. Introduction
The 7 July mission of the 1987 FIRE marine stratocumulus intensive field observations has been
chosen by several researchers for analysis because a well-defined transition from stratocumulus
to clear conditions was sampled by the aircraft on this day. It is hoped that by studying this case
we can learn something about the processes responsible for the maintenance and breakup of
stratocumulus layers, a primary objective of FIRE.
The preliminary analysis reported on here is based on data from the Electra flight of this day.
The properties of turbulence elements, i.e. updrafts and downdrafts, are examined to gain infor-
mation on the nature of the turbulent exchanges through the boundary layer and across the inver-
sion. Since such exchanges in large measure determine the stability and structure of cloud layers,
a study of draft properties should be informative. These results will also be useful in the devel-
opment of boundary layer models that are based on draft circulations (e.g. Randall, 1988;
Hanson, 1988).
H. Methodology
The technique that we use is conditional sampling based on an indicator function. The indicator
function is set to +1 or -1 when the vertical velocity exceeds predetermined positive or negative
thresholds. Other variables are sampled based on this indicator function and conditional averages
computed. Prior to computing the indicator function all variables are linearly detrended and a
high pass filter is applied to remove fluctuations greater than about 8 km in length. As done is
previous studies (e.g. Khalsa and Greenhut, 1985) the thresholds are based on the one-sided
vertical velocity variances about zero. Also, no change of state that lasts less than 0.2 sec is
allowed.
HI. Conditions on 7 July 1987
Four turbulence measuring legs were flown on 7 July 1987 (Fig. 1). The first 50m leg was flown
towards the west, followed by a 370m leg just below cloud base and a 580m leg just below
cloud top. A leg at 50m was repeated approximately 2 1/2 hours after the first, but shifted to the
east because of the movement of the cloud boundary.
The cloud boundary that was the focus of this mission was aligned approximately with the mean
wind, making the cross-wind turbulence legs normal to it. The cloud boundary in the measure-
ment region was found to move to the east at approximately 3 ms 4. Each flight leg was divided
into cloudy, transition and clear segments. The cloudy region to the east was generally solid
stratocumulus, transition had approximately 50% cloud cover and "clear" had only widely scat-
tered small cumulus.
The wind speed at 50m was from 330 ° and varied from 15 ms -I in the east to 10 ms -I in the west.
Sea surface temperature was lower in the east by about I°C. Momentum flux and sensible and
latent heat fluxes at 50m were smaller in the cloudy region. The smaller stress in the east despite
greater wind speed implies a smaller transfer coefficient, in accord with the greater stability
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implied by cooler surface temperatures. The sensible and latent heat fluxes in all three regions
were much smaller during the second 50m leg.
Another difference between the first and second 50m legs was in skewness which was a factor of
two smaller during the second leg in both clear and cloudy regions. At 370m the second, third
and fourth moments are all larger in the cloudy region compared to the clear region, probably a
result of circulations originating in the cloud layer. In the cloudy region near cloud top the
skewness is negative and the kurtosis is 3.6 compared to a positive variance and a kurtosis of 9.5
in the clear region. Negative skewness probably arises from strong sinking motions. The large
kurtosis in the clear region is due to the fact that the latter part of the highest leg in the clear
region actually penetrated the inversion towards the end of the run (the inversion sloped down-
ward towards the west) resulting a high degree of intermittency in the time series.
IV. Conditional Sampling Results
Conditional sampling results from the four aircraft flight legs in the clear, transition and cloudy
regions are shown in Fig. 2. Lines show the vertical variation of conditional averages for up-
drafts, downdrafts and the environment (i.e. everything that is not updraft or downdraft). The
second 50m leg was slightly lower in altitude than the first.
The conditionally sampled vertical velocity (Fig. 2a) for updrafts and downdrafts ranges in
magnitude from 0.5 to 1 ms -I except for the 580m level in the clear region. Updrafts leave the
surface layer with positive buoyancy but soon begin to decelerate as they entrain environmental
air and do work against a slightly stable stratification.
The cloudy region has the smallest 50m magnitudes of w', a consequence of the cooler surface
temperatures and therefore weaker buoyant forcing. In contrast, this region has the largest
magnitudes of w' at 370m and magnitudes near cloud top exceed those of the clear region by
approximately a factor of two, with downdrafts having larger magnitudes than updrafts. This
suggests that circulations in the upper half of the boundary layer are being driven by local proc-
esses and not surface forcing.
In the transition region the magnitudes of w' at 50m are similar to those in the clear region but
w' does not decrease away from the surface. The magnitudes at cloud top are the largest of the
three regions. It may be that the vertical motions in this region were driven by both surface and
cloud layer processes.
Updraft temperature perturbations (Fig. 2b) are positive at 50m in the clear region and switch to
negative above. Downdrafts have negative T' at 50m and slightly positive T' at 370m. A nega-
tive value of T' near the inversion in the clear region is somewhat unusual in that downdrafts are
expected to contain warm air entrained from above the inversion. However, if entrainment is
weak, the downdrafts that are measured will be dominated by cool updrafts that have turned over
at the inversion. Further discussion of draft overturning is contained in the next section.
In the cloudy region updraft T' is near zero at all levels except near the inversion where it attains
the largest value of any class in all 3 regions. Downdraft T' at this level has the largest negative
value of any region. These magnitudes are approximately 50% of the standard deviation in T at
this level. The signs of the perturbations indicate that the circulations in the cloud layer are
thermally direct and are being driven by processes near the cloud top.
The transition region displays a mixture of properties found in the clear and cloudy regions. For
example, updrafts in the first 50m leg have positive T' and a negative T' at 370m, as in the clear
region but a negative T' again near the cloud top, as in the cloudy region.
132
Conditionally sampled absolute humidity perturbations are shown in Fig. 2c. All updrafts are
moist and all downdrafts are dry, as expected. Downdraft p, ' is nearly constant with height in the
...... V . • • ,
cloudy and transmon regions whereas it decreases wxth height m the clear region. As with the T
perturbation for downdrafts near the inversion in the clear region, this is contrary to what would
be expected if downdrafts were mainly air entrained from above the inversion. The large Pv'
perturbations for updrafts in the transition region are also puzzling. To explain these results we
have to look in more detail at the nature of the drafts near the inversion.
V. Draft Characteristics Near the Inversion
A greater understanding of the nature of turbulent elements can be gained by further classifying
updrafts and downdrafts by their mean temperature and moisture perturbations. In this way we
can distinguish, for example, between downdrafts that originate as entrained inversion air and
downdrafts that originate as overturning updrafts• In the first case the drafts will be dry and in
the second moist. In Tables 1, 2, and 3 the following conditional sampling results from the 580m
leg are given: number of events expressed as percentage of total events in each state (up, down
or environment); mean draft length; horizontal wind perturbation in the direction of the mean
wind; and virtual temperature perturbation. Only the dominant classes which together account
for 80-90% of the drafts in each region are given.
In the clear region the significant classes are warm/dry and cool/moist. Entrained air will be
warm and dry and will have a positive u' perturbation due to a positive shear across the inver-
sion. Warm/dry events make up 49% of all downdrafts at this height in this region. The most
common updraft class is cool/moist. These drafts carry a u' deficit. Both of these classes,
warm/dry and cool/moist exist as both updrafts and downdrafts, indicating that drafts can over-
turn and still remain distinct. Note also that in the clear region the size of the overturned drafts is
greater that the originating draft. The u' perturbation is not always preserved which may be a
result of vertical momentum being converted to horizontal momentum in the process of over-
turning. The small mean q' for downdrafts (Fig. 2c) probably results from the compensating
effects of dry entrained downdrafts and moist overturned updrafts in the average.
In the cloudy region cool/dry is by far the dominant downdraft class, accounting for nearly
three-fourths of all downdrafts. This air has a positive momentum perturbation, indicating it has
come from above the inversion, and is negatively buoyant. This entrained air is probably cooled
by evaporation of cloud liquid water making the parcel negatively buoyant. Radiative cooling
may also play a role. These downdrafts descend to below cloud base, as suggested by the sub-
stantial magnitudes of conditionally averaged vertical velocity at 370m. A fraction of the
cool/dry downdrafts turn over and are sampled as cool/dry updrafts. These retain, in the mean,
the positive u' and negative T v' perturbations they had as downdrafts.
The dominant updraft class is warm/moist, which has negative u' and positive buoyancy pertur-
bations. The downward flux of mass in sinking parcels produces compensating upward motion,
but the conditionally averaged w' for warm/moist updrafts is nearly a factor of two greater than
the threshold value used to define the updrafts making it unlikely that this is mere "compensat-
ing" motion. Moisture-driven updrafts from lower in the boundary layer may attain a positive
temperature perturbation upon reaching the cloud layer as latent heat is released. Overturned
warm/moist events are also seen as downdrafts. The portion of the time series not classified as
updraft or downdraft, the environment, has equal numbers of the dominant updraft and down-
draft classes.
Both clear and cloudy conditions are sampled in the transition region so some of each type of
thermodynamic class is represented. Cool/dry is the most common downdraft class, characteris-
tic of the cloudy region. The average negative buoyancy is half that for the cloudy region.
Cool/moist events are also present but the sizable positive u' perturbation suggest that they are
133
not overturned surface-based updrafts. The large negative u' for warm/moist events suggests that
these have come from below, as conjectured for the cloud region. Even in the environment state
warm/moist events have a u' that is an order of magnitude greater than for the other classes. The
warm/moist environment class is also distinguished by small mean size which more characteris-
tic of draft states.
Warm/dry downdrafts are also present in the transition region, indicating entrained air that has
not mixed with cloud layer air. Warm/dry air makes up over one third of the environment with
event sizes over twice as large as for the other classes. This air probably occurs in the region
between clouds.
VI. Conclusions and Future Work
A conditional sampling analysis has shown that the properties of lapdrafts and downdrafts reflect
the nature of the forcing of convective motions in the clear and cloudy boundary layers of the 7
July 1987 flight. In the clear region, draft statistics are characteristic of a boundary layer in
which convective motions are forced from the surface and entrainment is driven by impinging
surface-based updrafts (and perhaps also horizontal wind shear). In the cloudy region, the
strongest convective motions are driven by cloud layer processes, apparently evaporative cool-
ing of entrained parcels. If this were strong enough it would lead to cloud breakup. In the transi-
tion region there is a combination of clear and cloudy layer processes. Cool/dry downdrafts,
which we believe to be the "agents" of cloud-top entrainment instability (CTEI), are more preva-
lent and more negatively buoyant in the cloud layer than in the transition layer. Thus it is diffi-
cult to use CTEI to explain why the transition region was less cloudy that the region to the east.
Apparently the cloud layer is maintained through the resupply of moisture from lower in the
boundary layer evidenced by the greater number and larger vertical velocity of cool/moist up-
drafts at cloud base in the cloudy region compared to the transition region.
Further work needs to be done before we have a full understanding of the processes producing
the cloud conditions observed on this day. We plan to include cloud liquid water, particle size
and ozone in the conditional sampling analysis. A conserved parameter analysis of draft proper-
ties will be performed and results compared with the analysis of Betts and Boers (1989) who
looked at mean conditions in each region. We will also examine draft statistics for evidence of
differences in conditions above the inversion that may have been a factor in determining whether
or not a cloud layer could be maintained.
V. References
Betts, A.K. and R. Boers, 1989: A cloudiness transition in a marine boundary layer. Proceedings
of the FIRE Science Meeting, Monterey, CA, July 1989.
Hanson, H.P., 1988: Plume mechanics and stratocumulus convection. Proceedings of the FIRE
Science Team Workshop, Vail, CO, July 1988.
Khalsa, S. J. S., and G. K. Greenhut, 1985: Conditional sampling of updrafts and downdrafts in
the marine atmospheric boundary layer. J. Atmos. Sci., 42, 2550-2562.
Randall, D.A., 1988: Fractional cloudiness in shallow cumulus layers. Proceedings of the FIRE
Science Team Workshop, Vail, CO, July 1988.
Acknowledgments
This work was supported by the Marine Meteorology Program of the Office of Naval Research.
134
,,it ,4.
Geo
lo-i.
Figure 1, Turbulence light lell of the NCAR
Electra on 7 July 1987.
Significant Classes - CLEAR. 580m
Table I
DOWN
% (_, u'wsmx/cky 49 0.18 0.vo
coot/moist 38 100 0.00 -0,0B
ENVR
wimYdr.y 42 214 0.05 0.06
cool/moist 50 178 -0.06 -0.05
UP
werm/d_ 30 96 0.03 0.06
cool/moist 51 63 .0.18 -0.08
SignificantClasses- CLOUDY
Table 2
DOWN
% (m) u' Tv'
oool/dry 74 78 0.12 -0.06
warm/moist 20 45 .0.24 0.04
ENVR
oool/dry 46 121 0.10 -0.05
warm/moist 46 124 -0.10 0.05
UP
cool/dry 22 67 0.26 .0.05
wsrm/moiM 65 79 -0.18 0.07
Classes- TRANSmON
Table 3
DOWN
% (m) u' Tv'
73 0.10 .0.03cool/dry 38
cool/moisl 26 57 O.16 .0.07
warm/dry 25 76 0.00 0.06
warm/molst 11 39 -0.23 0.09
ENVR
cool/dry 23 167 0.03 -0.02
cool/molst 34 172 0.04 -0,04
warm/dry 23 412 0.02 0.02
warm/moist 20 68 .0.30 0.06
UP
cooPdry 24 49 049 .0,04
cool/moist 34 54 0.04 .007
warm/dry 6 26 -005 0.06
warm/moist 36 79 -0.46 O.11
Tabks. Conditional sampling statistics for
thermodyns.rnic_l]_ classified events in
the clear (,Table I), cloudy (Table 2),
and O'ansition (Table 3) regions. Data is
from the 580m/eg. The vir'iablesgiven
ire: numoer or events expresseai$ a
percentage.of the total number of
events in eacn state (updraft, aownoratt,
environment), the mean event length in
meters, and the along*wind velocity
perturbation in m/s, and the perturba-
tion vbtu*l temperature in °C.
a)
7C_
_0
_O
O ..... O
ck_r
f
,. _00 ¸
_CO
20O
co
0
-Io -S .0 .S 10
w (M/s)
o-- a environment
tronslt_on
..... ' - 7110
0
-10 -_, 0 S I0
w (,_/s)
ck_u_
i
O_
-10 -5 0 5 10
w (m/s)
b)
70O
aO0
2_0,
100
006
o ..... o
cleor
i , 700
• 600
. 500
0
O0 06 - 06
T ('C)
a .....o er'Mror'rn_t
lronsitio'_
, . 700
- eO0.
- SO0_
• leo0
• 300
i0
O0 i_ -06
T ('C)
A s
-- up
c_dy
00 _
T ('C)
e)
_o
v400
t30
o
o2
o ..... o dovw3
clecr
7C_ ....... 700
EO0 ' T_O
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3CO
20O
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o
-2
a.........o er_v_r_l
trmsil,o_
........ 700
., I!
300
/
0
-2.i o I
_q/_,,5
c_y
_a
I i
Figure 2. Vertical variation of conditionally sampled a) vertical velocity, b)
temperature and c) absolute humicfity from the four turbulence flight
legs shown in Figure 1. Statistics were computed separately for t'he
clear, transition and cloudy segments of each leg.
135



Turbulence structure in clear and cloudy regions of the 7 July 1987 Electra mission

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