nsffog04
7 Nov 03 - DK
Lagrangian experiment:
Evolution of the cloudy marine atmospheric boundary layer along the California
Coast
Darko Koracin, Desert Research Institute, Reno, NV
Clive Dorman, Scripps Institution of Oceanography, San Diego, CA
Djamal Khelif, University of California, Irvine, CA
Joost Businger, University of Washington, Seattle, WA
Project summary
A fundamental understanding of the formation, evolution, and dissipation of
marine clouds and fog in coastal regions remains elusive. The problem of understanding
these phenomena is amplified in regions of complex coastal topography and sea-surface
temperature gradients where the land/sea differences strongly modify the dynamics and
thermodynamics of the cloudy atmospheric boundary layer and alter the effect of
synoptic forcing. The main reason for the insufficient knowledge of the structure and
evolution of the cloudiness and fog over the ocean is due to sparse critical measurements
over the ocean. Numerical weather prediction of atmospheric processes in coastal
regions is less accurate than over the land due to insufficient measurements over the
ocean to provide initial and boundary conditions as well as due to simplified model
physics and insufficient vertical and horizontal resolutions necessary to resolve the
land/sea and air/sea interactions, turbulence transfer, accurate radiation fluxes, cloud
processes, and cloud-top entrainment.
The main objective of the proposed study is to conduct a pilot field program and
use regional/mesoscale numerical simulations to investigate the roles of cloud radiative
heat transfer effects, surface fluxes, and cloud-top entrainment processes on the
modification of the cloudy marine atmospheric boundary layer along the long-overwater
trajectories that are crucial to the evolution of cloudiness and fog as viewed in a
Lagrangian frame of reference. The evolution of the marine layer will be tracked by
advanced “smart balloons” in conjunction with aircraft measurements of the dynamical
and thermodynamical properties of the marine layer. The study results will evaluate and
further develop a hypothesis proposed by Koracin et al. (2001a) that the cloud-top
cooling is the dominant effect for the generation of turbulence and the overall cooling of
the cloudy marine atmospheric boundary layer along multi-day offshore trajectories
during warm-season periods with significant advection along the Oregon and California
coasts. In addition, the study results will evaluate the Koracin and Dorman (2001)
hypothesis that the marine boundary layer depth is a first order function of the marine
layer convergence and divergence and evaluate its role on mesoscale cloud formation.
The field program and modeling will provide further evidence of the spatial and temporal
variability of the coastal dynamics and cloudiness including turbulence within the cloudy
marine atmospheric boundary layer.
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1.
Background and main objectives of the proposed study
The formation, evolution, and dissipation of coastal cloudiness and fog are
governed by marine atmospheric boundary layer processes, air-sea interaction, and
synoptic forcing, and influenced by coastal topography. The difficulty of understanding
these phenomena is made worse due to the lack of routine observations over the ocean
and inability of numerical weather models to accurately predict the structure and
evolution of the cloudy marine atmospheric boundary layer (Lewis et al. 2003a). Most of
the models’ inaccuracies and errors are caused by the uncertainties and errors in setting
the initial and boundary conditions as well as due to simplifications and assumptions in
the model physics, and generally insufficient vertical and horizontal resolutions necessary
to resolve the land/sea and air/sea interactions on various scales. Although satellite
measurements are providing an increased knowledge on the structure of the atmosphere
and the earth’s surface, they have inherent problem in resolving vertical profiles and
details of the shallow marine atmospheric boundary layer, especially near the coastline.
Major physical processes relevant to the evolution of the cloudy marine atmospheric
boundary layer are surface fluxes (Fairall et al. 1996), interaction of subcloud and cloud
layers (Tjernstrom and Koracin 1995), radiative heat fluxes (Nicholls 1984; Koracin and
Rogers 1990), turbulence transfer (deRodhe and Duynkerke 1997), coupling of radiation
and turbulence (Rogers and Koracin 1992), entrainment (Stevens et al. 2003), cloud
microphysical processes (Vali et al. 1998); dynamics and cloudiness (Koracin and
Dorman 2001), synoptic effects of cloudiness and fog (Lewis et al. 2003b), and transition
of marine cloud into fog (Koracin et al. 2001). Diurnal variation of the cloudiness and
associated radiative effects also alters the evolution of the marine layer (Betts 1990). In
some of the cases, the shortwave heating of the cloud can be greater than the heating of
the subcloud layer by the surface fluxes and lead to de-coupling of the cloud and
subcloud layers with respect to the turbulence structure (Nicholls 1984; Koracin and
Rogers 1990; Rogers and Koracin 1992; Tjernstrom and Koracin 1995). This effect can
lead to the daytime cloud dissipation due to the cloud shortwave heating and nocturnal
cloud growth due to the cloud top longwave cooling. Entrainment processes represent a
further determinant in the cloud evolution and they are difficult to measure and model
(Stevens et al. 2003). Additionally, near the coast the sea-surface temperature gradients
are of great magnitude and they can significantly modify the cloudy marine atmospheric
boundary layer (Koracin and Rogers 1990).
The United States West Coast is an area with complex topography, large coastal
gradients of the sea surface temperature, and seasonally variable setup of the synoptic
pressure systems. Earlier analyses were limited to large-scale structures and based on
sparse ship measurements (Neiburger et al. 1961; Nelson 1977). A network of automated
buoy stations that was established in 1980s, is providing a valuable data on synoptic and
diurnal wind and temperature structure near the shore (Dorman and Winant 1995).
Previous studies such as Koracin and Dorman (2001) have revealed significant and
persistent spatial and temporal variability of atmospheric flows and cloudiness along the
California coast. While approaching a cape, a supercritical flow decelerates and the
marine layer thickens – this area is referred to as “compression bulge.” As the flow
passes the cape, it accerelates and the marine layer thins – this area is referred to as
“expansion fan.” Further downwind of the cape the flow decelerates and marine layer
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thickens – this area is referred to as “deceleration region.” According to aircraft
measurements and modeling results, the coast from Cape Mendocino to Point Conception
is covered by alternating pairs of compression bulges and expansion fans. Moreover, in
addition to these mesoscale features, Koracin and Dorman (2001) suggest that the entire
northern and central California induce expansion-fan effect that can be detected by
analyzing longer time periods of winds and wind divergence field. The divergent flow in
the expansion fans and the convergent flow in the compression bulges and deceleration
areas exhibit diurnal trend. The MABL flow, stability, and cloudiness is generally
governed by these topographically-induced compression bulges, expansion fans, and
deceleration areas. In a shallow marine layer that is frequently close to the saturation,
even small vertical motions induced by the wind divergence in the marine layer will have
significant bearing on cloudiness evolution.
Main understanding of the structure and evolution of the coastal dynamics and
cloudiness has been gained from intensive field programs including aircraft (Beardsley et
al. 1987; Zemba and Friehe 1987; Enriquez and Friehe 1995; Rogers et al. 1998; Stevens
et al. 2002). Rogers et al. (1998) performed aircraft measurements over California and
Oregon coasts during June 1996. The observations evidenced a persistent shallow,
slightly stable, shear-driven MABL capped with strong elevated inversion and frequent
supercritical northwesterly flow with a low-level jet. The measurements focused on
clear-sky conditions, although there were cloudy conditions encountered in the second
part of the study period. Stevens et al. (2003) performed aircraft measurements off
southern California. They followed cloud parcels by means of circular aircraft patterns
and focused on roles of cloud-top radiative cooling, entrainment, and drizzle on the
evolution of the nocturnal marine stratocumulus. According to preliminary results,
Stevens et al. (2003) found that the role of entrainment is generally overestimated, while
the role of drizzle appears to be underestimated which confirms the importance of
tracking air parcels in understanding the evolution of the cloudy marine layer.
Studying the formation and evolution of the widespread cloudiness/fog along the
California Coast, Koracin et al. (2001a) and subsequent studies by Lewis et al. (2003a,b)
pointed out that it is crucial to investigate formation and evolution of cloudiness and fog
in the context of a Lagrangian frame of reference. Koracin et al. (2001a) performed a
numerical experiment emulating the modification of the marine layer along longoverwater trajectory as it is advected alongshore and evaluated modeling results using
constructed trajectories based on the buoy measurements and complemented by
radiosonde and satellite data. The cloud-top cooling in conjunction with the surface
moisture flux under conditions of great subsidence and strong marine inversion of about
10-15 K appear to be dominant effects in determining the formation, evolution, and
dissipation of cloudiness and eventual fog.
In order to prove the proposed hypothesis and quantify roles of each particular
component, detailed new measurements are necessary. The most appropriate
measurements should directly follow an air parcel and continuously monitor the
evolution of the thermodynamic and dynamic properties of the parcel. Development of
new balloon techniques in recent year offers us a possibility of investigating the
modification of the coastal marine layer over long over-water trajectories in a Lagrangian
framework. In particular, the technique based on a “smart balloon” tracking (Businger et
al. 1996; Johnson et al. 2000) appears to be most suitable for this type of an experiment.
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The smart balloons follow the air mass and provide direct evidence on the dynamical and
thermodynamical transformations it undergoes along the actual airflow trajectory. The
study results will answer crucial questions on the importance of the path history, diurnal
effects, and surface forcing on the fate of cloudiness and fog offshore. Since the smart
balloon measurements are lacking information on vertical profiles, turbulence fluxes,
capping air temperature inversion properties, and the depth of the marine layer, they will
be complemented with aircraft observations. A synthesis of smart-balloon and aircraft
measurements will be used as reference and evaluation evidence in investigating to what
extent regional and mesoscale models could be used to accurately predict evolution of the
cloudy marine layer and to what extent the accuracy of the modeling results could be
improved using data from the intensive field program such as the one we are proposing.
2.
Major determinants for the evolution of the MABL
To understand the modification of the MABL and associated evolution of the
cloudiness and fog, the origin of the cooling/heating processes and the generation of the
turbulence needs to be investigated. A series of 1D (Koracin et al. 2001a) as well as 3D
high-resolution numerical simulations using the regional/mesoscale model Mesoscale
Model 5 (MM5) (Koracin et al. 2001b) have been conducted to provide an understanding
of the determinants for this modification as well as the associated synoptic conditions
(Lewis et al. 2003a). The effect of the coastal dynamics on the coastal cloudiness has
been investigated by Koracin and Dorman (2001) who studied the wind divergence and
associated coastal cloudiness along the California coast. They simulated all of June 1996
using MM5 and evaluated model results by comparing with more than 18,000 points
from buoy and wind profiler data. They found significant modification of the MABL
flows due to the coastal influence. Their simulation results show that the major capes
cause a sequence of alternating upwind “bulges” characterized with low wind speed,
expansion fans in the lees associated with high wind speed and decreased cloudiness (as
compared to the upwind area), and deceleration area with possible hydraulic jump further
downwind. The bulge areas promote wind convergence and consequent deep marine
layer with frequent clouds, while the expansion fans induce wind divergence with mainly
cloud-free shallow marine layer. Diurnal variation of the mesoscale pressure gradient at
the coast alters the evolution of the coastal clouds and the wind divergence/convergence,
i.e., daytime marine layer is shallower and less cloudy as compared to nighttime. Diurnal
variation of the cloud radiative heat transfer effects further modifies the cloudiness
evolution.
Koracin et al. (2001a) studied a case of offshore fog that was wide spread along
the California waters. They have developed a conceptual model using results from a onedimensional turbulence-closure model in conjunction with data from buoys, land stations,
and satellites. They have found that the fog formed mainly due to the modification of the
cloudy MABL along long overwater trajectories under conditions of a significant surface
moisture flux, strong marine inversion and associated subsidence. A series of satellite
images for such a case illustrating the evolution and alongshore advection of the
fog/cloud layer from NW to SE is shown in Fig. 1. It should be noticed that for these and
similar conditions, the northern edge traveled about 8-10 hrs from the tip of Cape
Mendocino to the San Francisco Bay area. The travel time for the cloud/fog layer from
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the Monterey Bay area to the tip of Point Conception is generally 10-15 hrs. This is
confirmed in the next section by examining 18-hr forward trajectories from the Monterey
buoy area for every day in June 1996.
Fig. 1. GOES-10 visible images of clouds along the California coast on 13 Apr 1999
According to the results from observations and a one-dimensional model, Koracin
et al. (2001) suggest that the cloud-top cooling appears to be the main reason for the
overall MABL cooling, generation of the turbulence, and eventual surface saturation.
Koracin et al. (2001b, 2003) extended their study and used three-dimensional MM5
simulations to reveal the effects of advection. They performed a scale analysis of the
main components of the heat balance of the marine layer, i.e., surface fluxes, divergence
of the net radiative heat flux, and entrainment. Koracin et al. (2003) have found that the
cloud-top radiative cooling is a dominant process for overall cooling and induced
generation of turbulence. The partition of the main components of the thermodynamic
equation is shown in Fig. 2. The divergence of the net radiative heat flux and associated
heating/cooling for each vertical model layer was assigned for the entire marine layer by
multiplying with the ratio of that layer depth to the depth of the marine layer. This
provided information on the overall heating/cooling of the marine layer. The figure
clearly indicates that the radiative cooling is a dominant process for the cooling of the
marine layer during the first part of the period when the fog formed. For the later stage
when the fog dissipated, the net radiative heat flux overall induced heating that facilitated
fog dissipation. The smart-balloon measurements will evidence on the crucial issue of
evolving temperature, humidity, velocity, and underlying SST along the flow trajectory,
while the aircraft measurements will complement the balloon measurements with spatial
structure of the marine layer, turbulence fluxes, and radiative fluxes. The unique
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combination of the two high quality data sets will be used to evaluate mesoscale/regional
modeling. The modeling evaluation results will address the evolution of the marine layer
properties and should lead to improved forecasting of cloudiness and fog.
Fig. 2. Time series of the potential temperature tendency terms due to the surface heat
flux, the divergence of the net radiative heat flux, and entrainment simulated with MM5
for the case of offshore fog 15 April 1999.
3.
Preliminary modeling in support of the field program
To prepare and set up as comprehensive as possible a measurement plan, we have
used model results from Koracin and Dorman (2001) to simulate trajectories of the
possible “smart balloons” from the northern coastal waters to the area upwind of Point
Conception. This route coincides with the path of evolution of the advected mass as
discussed by Koracin et al. (2001a,b). The upwind area has quite few buoys (Clive – can
specify the number of buoys ???) and the radiosonde site and wind profilers in the
Vandenberg area. In particular, we predetermined possible location of the balloon starting
point, expected travel time, an optimum release height, an optimum offshore distance for
starting point, and possible spread and uncertainty of the ending point.
We focus on a location off the Monterey Bay as origin positions for balloon
launches and investigate how favorable this location provides long over-water trajectories
of the balloons. This location has been chosen due to three major reasons. First, the path
of the balloon is directed along the edge of Point Conception expansion fan and has a
chance to continue balloon travel offshore. The appropriate choice would then be along a
western edge of an expansion fan where a persistent northwesterly or north-northwesterly
flow could be expected. (this is explained further in the text). Second, this area is
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characterized by frequent cloudiness (Clive – reference ???). The third reason is practical
– the ship cannot position farther than 40-60 km from the coastline and the aircraft
measurements would have a short alongshore range. In any case, the balloons should not
be released from far-offshore locations since the balloon measurements should be
representative for a coastal zone where other reference measurements are being made.
Figure 3 shows a family of forward surface trajectories generated from the MM5
results with a starting point at the buoy 46042 (Monterey Bay) and also from the location
that is 27 km west from the buoy location.
Fig. 3. A series of MM5-generated forward trajectories at the surface that were started on every
day in June 1996 at 0000 UTC at the Monterey Bay buoy 46042 location (a); at 0000 UTC at the 27 km
west from buoy 46042 (b); at 0600 UTC at the 46042 buoy location (c); and at 0600 UTC at 27 km west
from the buoy 46042 (d).
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To examine the diurnal effects and daytime intensification of the on-shore flows that can
deflect the balloons over the land, we have generated the trajectories for these two
starting locations at two different initial times (0000 UTC /0400 LST/ and 0600 UTC
/2100 LST/). The surface trajectories (Fig. 3) show several interesting features. Moving
27 km to the west generally increases a chance of the trajectories to remain offshore.
Fig. 4. A series of MM5-generated forward trajectories at 500 m that are released on
every day in June 1996 at 0000 UTC at the Monterey Bay buoy 46042 location (a); at
0000 UTC at the 27 km west from buoy 46042 (b); at 0600 UTC at the 46042 buoy
location (c); and at 0600 UTC at 27 km west from the buoy 46042 (d).
There was only one flow reversal and on five days within the month the trajectories went
straight from the origin position over the land. This effect was quite similar for the
surface trajectories that were generated 27 km west of the buoy; however, the alongshore
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trajectories toward Point Conception were more over the ocean in the latter case. Starting
time at 0600 UTC significantly improves the position of the trajectories in the offshore
direction, especially for the case of the trajectories generated at the location 27 km west
of the buoy. The trajectories generated at 500 m show that in this case the time of the
release is more significant than the shift of 27 km in the origin location. According to the
simulations, both sets of trajectories (at the buoy and 27 km west from the buoy) that start
at 0600 UTC (2100 LST) have greater chance of staying offshore as compared to the
same trajectories that are generated at 0000 UTC (1600 LST). Further sensitivity studies
will be conducted to more precisely determine the location, the height, and the initial time
for the balloon releases. The starting location should be coordinated with the ship
capability to reach offshore destination.
4.
Specifics of the field program
Figure 1 shows a basic schematic of the field program setup.
Fig. 5. Schematic of the experimental setup and operational plan.
4.1
Ship operation
An NSF ship will depart Monterey harbor in the beginning of June and anchor about 50
km offshore for two 8-day periods, one at the beginning of June and the other starting the
third week of June. Smart balloon equipment (balloon launch, receivers, and controlling
unit) will be accommodated on the ship. To provide accurate information of the wind
profiles relevant for the launching decision, airsondes will be launched from the ship
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every 6-12 hours. These soundings will also evidence on diurnal evolution of the
offshore marine layer and will be used for mesoscale model verification. Clive Dorman
will be onboard and in charge of ship and balloon operations. The specifics of the ship
operations are as follows.
[….. ] /input from Clive/.
4.2
Smart balloon soundings
Lagrangian trajectories of the marine air parcels will be directly tracked using
smart balloons with supporting aircraft measurements. Smart-balloon measures pressure,
temperature, humidity, SST, GPS position, and transmits the information to satellite.
Sampling rate is usually about 5-10 seconds. Balloon buoyancy is set for a target height
(here the top of the marine layer) and is adjustable in flight. The smart balloons
launching from a UNOLS ship will be coordinated with aircraft measurements and by the
input from the forecasting and modeling results. Smart balloon will be carried rapidly
along by coast parallel winds in top of marine layer, passing Pt Conception in 8-12 hours
and well off San Diego in 16-24 hours. Clouds and fog tend to begin near Pt Conception.
[ …. ] /more input from Clive/.
4.3 Aircraft operations
4.3.1 Measurements requirements
It is well established that research aircraft, when instrumented appropriately, are capable
of measuring means and fluctuations of meteorological variables with accuracies
comparable to those of surface platforms (Burns et al. 1999, Khelif et al. 1999). In order
to obtain a detailed characterization of the MABL, we propose to use an aircraft to obtain
measurements that are simultaneous with those of the smart balloons while “loosely”
following their trajectory. Djamal Khelif will be in charge of the aircraft operations.
The candidate aircraft is required to be equipped with fast-response sensors to measure
humidity, temperature as well as 3-dimensional wind vector so that eddy correlation
fluxes of latent heat, sensible heat and momentum can be obtained. It should also be
equipped with downward and upward looking IR pyrometer (to measure SST corrected
for cloud reflection). Radiation sensors are required for the measurement of upwelling
and downwelling shortwave and longwave radiation fluxes. Aerosol measurements will
be very useful in the characterization of cloud-top entrainment. The capability to release
dropsoundes is very desirable especially on days when dense fog and or low stratus
clouds reduce the visibility to the point where the aircraft cannot operate in the lower part
of the MABL. Dropsoundes would also provide data below the 30-m lowest limit of
aircraft operation.
Both the NCAR C130 and the University of Wyoming King Air are part of NSF
deployment facility pool and the adjudication for their use is made by an independent
committee at no direct cost to the “science” part of the proposal budget. Of the two, the
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NCAR C130 is the one that meets best the requirements for our proposed study both in
terms of the scientific payload and the roughly 8-hours on-station time. However, we
learned that the C130 is scheduled for extended maintenance from February 1 to July 31,
2005. In the event its maintenance was to be completed ahead of schedule and in time for
our planned June 2005 experiment, we would prefer to use the C130. If this is doe not
materialize, we will consider using the King Air. Since this aircraft does not have a fastresponse humidity sensor and its fast-response temperature sensor is not reliable, UCI
will supply these two critical instruments and coordinate with the flight facility for their
integration and testing in time for the project. Another NSF project (HEAT) is on the
schedule of the King AIR with the starting date of July 1, 2005 and since 2 weeks of
preparation time are required it may impact the schedule of our experiment in the event
both projects were funded. A compromise schedule would need to be worked out to
accommodate the two projects. Else, an alternative would be to use the NPS/CIRPAS
twin Otter based in Marina, CA in the Monterey Bay area. It has all the measurements
capabilities required by this study including dropsounde launching. However, although
this aircraft recently became part of UNOLS pool it is not part of the NSF deployment
pool and therefore funding sources for its deployment would need to be identified.
4.3.2 Flight Track:
Figure 6 shows a sketch of a typical flight pattern for the proposed study. The aircraft
will be based at either Monterey or Marina airports. The flight will be coordinated with
the smart balloon launch. The aircraft will take off and head offshore toward the balloon
track and parallel it at the same level within a safe separation for 5-min (intercomparison,
radiation and entrainment run), then it will fly a stack of 4 levels cross-wind flux legs 5min each starting just above the MABL inversion and down to 30 m with the latter level
repeated once for better statistics on the estimate of the surface fluxes. This will be
followed by a sawtooth up-and-down sounding extending well above the MABL
inversion and down to 30 m. The aircraft will ramp up to the level of the smart balloon
and parallel its trajectory until it has traveled roughly the same distance from the location
of the first stack as the balloon which is estimated to be roughly 50 km. The sequence of
flux stacks and soundings pairs will be repeated until past Pt. Conception. Depending on
the aircraft allocated, landing in Santa Barbara for refueling may be required before the
return to base flight. The pattern for this portion (not shown on the sketch) will consist of
a series of saw tooth soundings separated by reciprocal cross-wind 5-min flux legs at 30
m. The latter pattern will help identify the “expansion fans” and “compressions bulges”
discussed above as well characterize the vertical structure of the MABL along a
streamline.
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Figure 6: Typical aircraft flight pattern for proposed Lagrangian Experiment in
June 2005.
4.3.3 Stress divergence
The stress divergence will be measured directly from the flux stacks. The proposed flight
pattern will help us test the validity of an alternative method to estimate the flux
divergence using only measurements of the mean pressure, P, and mean wind U along the
streamline at a constant height (lowest level runs for instance). Assuming steady winds
and x being along the streamline, the stress divergence is
dτ/dx = dP/dx + ρUdU/dx
(??)
With a well calibrated (trailing cone method) static pressure measurements combined
with the radar altimeter measurements which are very accurate over oceans (no
topography) the RHS of the equation can be readily determined using data from the 30-m
runs down the streamline. In deceleration regions downstream from expansion fans both
dP/dx and UdU/dx are expected to be negative resulting in some stress divergence.
4.4
Forecasting and modeling input
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An operational science-team base will be established during the field program in
Monterey, CA that will have access to observational data (coastal stations, buoys, and
satellites) and model results (global and regional models, as well as real-time forecasting
and trajectory simulations provided by the science team). The base team led by Darko
Koracin will coordinate launches of the smart balloons, ship-based soundings, and
planning of aircraft operations.
[ …. ] /input from Darko/.
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