D - F
DYCOMS-II
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&#64atmos.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]