The RHB continued on Leg 2 of its EPIC2001 cruise with the following research groups on board: the UCSB ocean radiant heating group, the UNAM air aerosol/chemistry group, a University of Washington group working with the C-band radar and radiosondes, the ETL cloud radar group, the ETL lidar group, the ETL flux group, and the WHOI surface mooring group.
At 1800 local on October 15, 20001 RHB reached the site of the WHOI IMET mooring and began its work in the vicinity of the mooring site that continued until 0400 local on October 22. At that time, RHB began its eastward transit along 20°S that would take it to Arica, Chile on the morning of October 25.
After arriving at the mooring site, the first full day (October 16) was dedicated to a comparison of shipboard and buoy sensors, two CTD casts to 4000 m, and sampling the atmosphere at this site under the stratus clouds. On October 17, the mooring deployed one year earlier was recovered without problem. The instrumentation and hardware were in good shape, with some biofouling seen in the upper 30 m. Atmospheric sampling continued at the site on October 18. In parallel, a SeaBeam survey of the ocean bottom was conducted and a practice run along the track chosen for the mooring deployment carried out. On October 20, the new surface mooring was deployed. Atmospheric sampling continued and buoy-ship comparisons were restarted; two 4000 m deep CTDs were taken on October 21. Tradewinds have been steady in direction with some variation in strength. Occasional breaks in the cloud cover have allowed some glimpse of blue skies and sun. There has also been light rain.
The WHOI surface mooring group has extracted all the data from the instruments deployed for the first year. Few problems were encountered and at this point data return rate is estimated to be around 90%. The first look at the oceanographic data ( Fig 1 ) shows a well-developed seasonal cycle, with warming and restratification of the upper ocean. That seasonal cycle is interrupted for several weeks to a month at a time by eddy variability, which is noted by large vertical displacements of the thermocline and by stronger currents. Of great interest will be the determination of the extent to which local air-sea interaction explains the observed temporal evolution of upper ocean structure.
The complement of meteorological and oceanographic sensors on the RHB provided an excellent opportunity to document the at sea performance of an IMET-equipped surface mooring both after one year in the water and just after deployment. Data provided by Jeff Hare (ETL) was over plotted with the meteorological data telemetered from the surface mooring ( Figure 2 ). This and more rigorous comparisons after the buoy sensors are post-calibrated will provide a sound basis for quantifying the accuracy of the measurements at the surface mooring.
The ETL Flux Group (Abbott and Hare) continues to maintain the flux system, ceilometer, and radar wind profiler, and all systems are operating at 100%. The weather has been somewhat variable, with stiffer winds than the previous week, with cloudy and cool mornings and sunny afternoons. The group has been working to examine the contrasts between the Leg I and Leg II flux data sets. North of the Equator, the Leg I conditions were characterized by a warm (26 C), humid (18 g/kg), and strongly
convective environment, with periods of deep clouds and variable radiative forcing. Leg II has been dominated by stratus clouds, dryer conditions (9 g/kg of water vapor and very little rain), and cooler air and sea temperatures (18 C and 19 C, respectively). Figure 3 shows a composite of mean wind, temperature, and turbulent heat fluxes, plotted with latitude. Notice the highly variable conditions encountered during Leg I (north of the Equator), and the much warmer air (green) and sea temperatures (blue) before entering the stratus region. Understanding the nature and quantifying the heat flux components within these distinct climatologies is an important goal of the Epic experiment. The combination of the flux data set with the cloud remote sensing systems data sets will be very effective in describing the radiative heat budgets for the two regimes. The next task for the flux group
will be to work with the WHOI Buoy group to intercompare the buoy and ship meteorological data sets.
During the second week of Leg 2, the UCSB group completed a total of 9 CTD stations, 23 SPMR profiles, and acquired 9 SeaWiFS overpasses. Four of the CTD stations were full water depth (~4000 m) profiles, both before and after the IMET buoys were recovered and deployed. Discrete measurements were taken from all CTD casts for a total of 62 chlorophyll a samples and 82 nutrient (NO3-, PO4-) samples throughout the upper 500 m. Eighteen of the SPMR profiles were collected over the course of 3 days, giving us fairly high temporal resolution during the daylight hours. Minimal analyses have been performed on the data collected, as much of it requires further processing not available on the ship. However, the deep mixed layers at the IMET site at this time of year (100-150 m) are also manifest in the lower, more uniform chlorophyll distributions throughout the upper 100 m (not shown). This is in marked contrast to the gradient in both surface and subsurface maxima observed along the 95W transect. This will undoubtedly impact oceanic heating rates.
The UNAM group has continued its monitoring of the atmospheric aerosols. However, the measurements have not carried out at 100% since the PMS Passive Cavity Aerosol Spectrometer (which measures the size distribution of particles) presented some problems and had to be turned off, and its repair cannot be done during the cruise; the data (from this equipment) of the last days are suspected to be wrong. The DMS and DMSP samples have been successfully continued. In addition to the daily water sample about noon, samplings at different times (morning and night) have been carried out in order to observe the behavior of the DMS and DMSP production. The HPLC samplings have been continued as well. The data processing is still pending.
Figure 4 shows a 24-hr time-height cross section of the radar return from the stratus and the cloud base heights detected with the ceilometer (white line), as well as annotations of visual observations. Clouds are persistent through the day and with thinning between 16:00 and 24:00 GMT (afternoon and early evening local) indicating diurnal effects on cloud structure. Typically the radar return extends several 100s of meters below the cloud base indicating persistent virga and in some cases light drizzle that can be detected at the surface. The University of Washington (UW) group is measuring drop size distributions at the surface using blue methylene paper which will be useful for interpreting the radar-radiometer retrieved cloud microphysics, in particular for light drizzle. It has been noted that the persistent radar echo below cloud base, even when extending to the surface with reflectivities approaching 10 dBz are rarely associated with any detectable reduction in horizontal visibility.
The ETL lidar, the mini-MOPA, had amplifier difficulties and is once again fully operational. We continue to add to our large data set with various scans for approximately 12 hours each day. Scanning routines include probing the atmosphere for cloud boundaries, sub-cloud wind structure and velocity, water vapor profiles. Our cloud base approximations continue to agree with ceiliometer data. We have also operated for 2 satellite overpasses for comparison and ground truth verification.
Since arriving on station at the WHOI IMET buoy (20 S, 85 W) the UW group has collected a five-day atmospheric data set near the heart of the climatological Sc region off the coast of Peru. We have had drizzle events every day since arriving on station ( Figure 5 ). Several of these events were heavy enough to be considered trace precipitation. Examination of the 31 methylene blue raindrop samples obtained to date indicates raindrops > 1 mm in diameter during heavier precipitation periods. The SST has been warmer than the air temperature. Daytime decoupling between the surface layer and the cloud layer has occurred intermittently. Upper-air soundings obtained during drizzle periods show RH = 100% to within 250 m of the surface. The C-band radar shows groups of drizzle cells moving with the prevailing wind. There have been a total of 91 launches, with 81 reaching at least 5 km and 54 reaching at least 54 km.
Figure 1 . Contour plot of upper ocean temperatures from the first year of the deployment of the WHOI IMET mooring u8nder the stratus. (R. Weller, WHOI).
Figure 2 . Comparison of shipboard and buoy observations of SST done between the moored instruments of the first deployment (up to yearday 290.7) and of the second deployment (starting 292.6). (R. Weller, WHOI AND J. HARE, ETL).
Figure 3 . Composite of mean wind, temperature, and turbulent heat fluxes versus latitude for Leg 1 and Leg 2 of EPIC 2001. (J. Hare, ETL).
Figure 4 . Ceilometer and radar observations o the stratus and the cloud base on October 17 at the buoy site. (T. Uttall, ETL).
Figure 5 . Cloud photo obtained on 21 Oct 2001 19 UTC (14 LT) showing drizzle falling from Sc cloud on the right side of the picture. (S. Yuter, UW)
