Dynamics and Chemistry of Marine Stratocumulus Phase II

Project Description
The Dynamics and Chemistry of Marine Stratocumulus Phase II:
Entrainment Studies (DYCOMS-II) is the name given to a field
campaign which proposes to collect data for the purposes of
testing large-eddy simulations of nocturnal
stratocumulus. DYCOMS-II will be based on measurements taken from
the NCAR EC-130Q in and around nocturnal marine
stratocumulus. The experiment will consist of 9 flights out of
North Island Naval Air Station (just west of San Diego) between
July 7 and July 28, 2001. Eight of these flights will be
nocturnal. For more information about the experimental objectives
and strategy, as well as references and contacts click on the
appropriate link below.

Objectives:

1. The DYCOMS-II field program is designed to collect data to
test large-eddy simulations of stratocumulus

2. To test a recently proposed technique to measure large-scale
divergence

3. To test our ability to close scalar budgets under ideal
situations

4. To increase our understanding and of the statistical signature
of the diurnal cycle in marine stratocumulus

Strategies:

Our basic strategy is to make use of a unique combination of
instrumentation and flight plans to measure both the large-scale
environment and the turbulent dynamics of summertime, nocturnal,
subtropical marine stratocumulus.

Instruments:

1. EC-130Q Hercules: This four-engine, medium-size utility
turboprop has been modified from a U.S. military tactical
aircraft to a versatile and capable research platform that will
deliver the scientific instrumentation to the target area. The
Hercules has a 10-hour flight endurance, covers a 2,900 nautical
mile range at 20,000 ft, and carries a payload of up to 23,000
lb. In addition to the standard sensors that measure atmospheric
state parameters, cloud physics, and radiation, the C-130 will be
equipped with specialized instrumentation for measuring the state
of the atmosphere away from the aircraft. These latter
instruments include the Staring (Scanning) Aerosol Backscatter
Lidar (SABL), the ATD Dropwindsonde System, and the Wyoming cloud
radar.

2. GPS Dropsondes: These third-generation dropsonde, use a new
sensor module and a GPS receiver from Vaisala Inc. A unique
square-cone parachute is used to reduce the initial shock load
and slow and stabilize the sonde. The parachute is immediately
deployed on exit from the launch chute and streamers for about
five seconds until filled by ram-air. The stability of the square
cone parachute is very good during the sonde's descent and
reduces or eliminates any pendulum motion of the sonde. The fall
speeds of the sondes in the subtropical boundary layer are
estimated to be between 10 and 15 m/s, yielding profiles with a
resolution of less than 10m. Four sondes can be tracked from the
aircraft simultaneoulsy.

3. Scanning Aerosol Backscatter Lidar (SABL): The SABL lidar is
a compact and reliable instrument that detects backscatter from
air molecules, aerosols, and hydrometeors (water and ice) and is
used to measure and map distributions of relative aerosol
concentrations. The instrument operates at two wavelengths 532
(green) and 1064 nm (infrared). On the C130 aircraft, it operates
from zenith to nadir out to distances from 10 to 15 km with range
resolutions down to 7.5 meters and along-track resolution to 4
meters. The lidar is not eye-safe and thus its scanning
capabilities are currently limited. During DYCOMS-II it will be
mounted on a pod on a wing of the C130 and will be used primarily
in a downward staring mode to provide information about cloud top
structure. Craig Walther and Bruce Morley of NCAR lead the SABL
development.

4. Wyoming Cloud Radar (WCR): The Wyoming Cloud Radar is an
observational system for the study of cloud structure and
composition. It is intended for airborne use; principally on the
Wyoming KingAir. Operating at 95 GHz (3 mm wavelength), the radar
provides high-resolution measurements of reflectivity, velocity
and polarization fields in vertical or horizontal
sections. Coupled with the in situ observations of hydrometeors
and air motions from the same aircraft these data yield unique
information for analyses of cloud and precipitation
processes. During DYCOMS it is proposed that the radar will be
mounted on the C130. Gabor Vali of the University of Wyoming is
the primary contact for the implementation of the WCR during
DYCOMS-II.

5. Tunable Diode Laser (TDL): The tunable diode laser was
developed by Randall May of Spectra Sensors. The prototype
instrument is shown below mounted on the DC-8. The TDL on the
C-130 is mounted under a wing pod. The TDL is an open-path
instrument that makes independent water vapor measurements every
125 ms. Although as currently configured these measurements are
averaged together to provide data at 1Hz, during DYCOMS each
independent sample will be saved. The resultant 8Hz data should
be sufficient for measuring fluxes outside of the surface
layer. Previous experience with the prototype instrument during
CAMEX, and experience with the current instrument on 30 flights
during TOPSE suggests that it performs very well and should
provide unprecedented measurements of water-vapor in and around
the marine boundary layer, both in and out of clouds. Bruce
Gandrud of NCAR is a primary contact for the implementation of
the TDL.

6. Fast Ozone: The NCAR NO chemiluminesence techique for
measureing Ozone has been recently modified to increase its
frequency response. Preliminary tests with a cylindric reaction
chamber indicate a frequency response of 5.5Hz with 1 ppbv of
sensitivity. Tests with a conic reaction chamber have an slightly
improved frequency response (6.5Hz) and a slightly reduced (4
ppbv) sensitivity. The lead developer of this instrument is
Teresa Campos of NCAR.

7. Fast DMS (APIMS): The atmospheric pressure ionization mass
spectrometer (APIMS) has been developed by Alan Bandy and
colleagues at Drexel University. The method is based on mass
spectrometry using atmospheric pressure ionization as a source
of ions, and uses deuterated DMS as an internal standard. This
internal standard allows monitoring of the DMS mixing ratio
directly, which is a significant advantage in flux
determinations using eddy correlation. The instrument
sensitivity is about 100 counts per second per pptv. At a
typical DMS concentration of 100 pptv, the count rate is 104
counts per second. At a sample rate of 40 samples per second
this is 250 counts per sampling interval. This yields a
signal-to-noise of about 16 since the blank is negligible. The
instrument was first flown on test flights in November of 1999,
and we are hopeful that a second round of test flights
scheduled for August 2000 will demonstrate the capability of
the DMS instrument for entrainment mea! surements.

8. Particulate Volume Monitor (PVM-100A): The PVM is a cloud
microphysics probe designed to measure for small droplets the
liquid water content (LWC), droplet surface area (PSA), and
droplet effective radius (Re); see Fig. 1 (below). The PVM
makes these measurements optically on a cloud volume of about
10 cm^3, thus minimizing statistical sampling errors; and the
measurements are independent of air speed. The accuracy of the
PVM is estimated to be better than 10% for droplet spectra
with VMD (volume medium diameter) smaller than 30 um, and the
precision is on the order of 0.002 g/m^3. The PVM has the
unique capability of making these measurements at a rate
several orders of magnitude faster than other methods. An
example of 1000-Hz LWC measurements made with the PVM on the
C-130 during SCMS (Small Cumulus Microphysics Study) is shown
in Fig. 2 . This 10-cm resolution LWC data show a large amount
of fine scale in-cloud structure not seen in the 1-Hz data
often collected with! other probes. The planned PVM data
collection rate for DYCOMS-II is 2000 Hz, which gives 5-cm
in-cloud resolution. This will be useful in studying the fine
scales potentially associated with the entrainment
process. Also accurate and fast LWC measurements can be
combined with vertical velocity measurements from a gust probe
to estimate the entrainment velocity into the Sc using a
q-conservation method described earlier by Steve Nicholls.

Contact Information:

Bjorn Stevens (Principal Investigator)
Department of Atmospheric Sciences
405 Hilgard Avenue
Box 951565
Los Angeles CA 90095-1565
(310) 206-7428 (voice) -5219 (fax)
bstevens@atmos.ucla.edu
http://www.atmos.ucla.edu/~bstevens

DYCOMS-II Homepage: http://www.atmos.ucla.edu/~bstevens/dycoms/

[Summary provided by UCLA Department of Atmospheric Science]