EPIC2001 Ronald H. Brown

Report #5

R. A. Weller et al.

October 14, 2001

The RHB entered Puerto Ayroa, Isla Santa Cruz, Galapagos on October 6, 2001 and departed on October 9, 2001. The science party exchanged personnel. All groups participating in the leg from San Diego continued except for the University of Washington ocean microstructure group. This group ended its work, and a group from WHOI (Woods Hole Oceanographic Institution) got on board to recover and redeploy a surface mooring at 20°S, 85°W. Thus, as RHB left Galapagos, the groups on board included: 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.

RHB steamed to 1°S, 95°W to begin a series of combined CTD (conductivity, temperature, depth) profiles to 500 m with water sampling along with SPMR (solar radiative flux) profiles every 0.5° southward along 95°W until 8°S. At 2°S and again at 8°S repairs were made to TAO buoys. Having completed the section south along 95°W, RHB is presently steaming southeast toward the WHOI mooring. CTD and SPMR profiles are being made at local noon. To date 3 ARGO floats have been deployed. Leaving the Galapagos sea surface temperature (SST) dropped to just under 17°C before recovering to close to 19°C along 95°W until roughly 5°S. Southward of 5°S SST warmed, reaching about 21.8°C near 8°S. Since turning toward the southeast, SST has steadily declined. True wind speeds were about 4 m/s along 95°W and increased to about 6 m/s as RHB steams to the southeast.

The UCSB group achieved one of their major goals by completing the sampling along 95°W at 0.5° resolution. Since leaving the Galapagos 16 CTD casts to 500 m, 8 SPMR casts during the daylight CTD stations, and approximately daily SeaWiFS images have been collected. Water samples have been collected at 12 depths for nutrient (192 samples) and chlorophyll (128 samples) analyses. Chlorophyll concentrations were highest near the equator ( Figure 1).

Since October 9, the UNAM group has been monitoring the atmospheric aerosols. Sulfate concentration, total particle concentration, and the size distribution of particles in a range from 0.1 to 3 microns, have been successfully measured. All the data have been collected for their post-processing. Water samples have been taken in order to analyze them for the DMS and DMSP produced by the phytoplankton by using gas chromatography. Four different depths ranging from surface to 100 m have been taken for the water samples once a day (about the time in which the chlorophyll peak is presented). As an additional project, HPLC of the phytoplankton has also sampled. During this leg, no data analysis has been carried out; the purpose is simply collecting the data for the post-processing in Mexico.

Data from the ETL cloud radar and the flux instrumentation clearly show that there has been a transition in cloud structure and air-sea fluxes as the ship has moved across and south of the equator. Promptly on October 1st, the deep convection and precipitation which characterized EPIC leg 1 (associated with the northeast pacific ITCZ) switched to low-level marine stratus (associated with the south-easterly tradewind and cross-equatorial flow) Figure 2.

Figure 3 shows a preliminary look at cloud effects on net heat flux. Prior to October 1st, a strong inverse relationship exists between net heat flux and the number of hours per day with deep cloud and/or precipitation as determined from the cloud radar. After October 1st, when cloud cover becomes shallow and nearly continuous, net heat flux has increased significantly and is positive, which indicates net heating of the sea surface.

The ETL lidar, the mini-MOPA, has been operating on a varied schedule, taking measurements from day to night and night to day in order to characterize the transitional periods of each day. The DIAL measurements, after averaging and quality control, will provide water vapor profiles and the initial data look acceptable. Additionally, wind velocity has been measured via Doppler, and cloud base heights are being measured. On October 13 the cloud base was around 800 m at 16:02 UTC and sub-cloud velocities were 1-2 m/s.

The UW group commenced our schedule of regular observations on 10 October. These observations include upper-air soundings every 3 hours, hourly cloud photos during daylight, hourly cloud coverage observations, and continuous scanning C-band radar data collection. Preliminary analysis of the 42 upper-air soundings obtained to date indicated less diurnal variability in cloud top height compared to similar data collected in the stratocumulus region off the coast of California in 1997. We have directly observed drizzle both during the afternoon and evening and have obtained 5 methylene blue samples of drizzle drops for use in constraining the drop size distribution. The C-band radar data reveal discernable drizzle cells (Figure 4 , 5 ) although at times sea clutter (a result of the radar beam grazing the ocean surface) overwhelms the returned signal. Of the 42 soundings, 35 reached at least 5km and 25 reached at least 15 km.

The RHB will reach the WHOI IMET mooring this evening and begin one day of shipboard sampling close to the buoy prior to recovering the mooring the following day. All instruments for the buoy and mooring to be deployed have been prepared.

Figure 1. From Toby Westberry, UCSB. Chlorophyll profiles in the upper 200 m at the stations from 1°S to 8°S.

Figure 2. From Jeff Hare and Tanneil Uttal, ETL. Comparison of net heat flux (blue) with deep echo (red) from the ETL Cloud radar. Net Heat Flux includes sensible and latent turbulent heat fluxes and upwelling and downwelling solar and infrared radiant flux. Deep echo represents the number of hours per day (times 10) with radar echo depth (cloud + precip) exceeding 10km.

Figure 3. From Tanneil Uttal, ETL. Time-height cross-section of radar reflectivities, Doppler velocities and Doppler spectral widths showing deep clouds evolving to boundary layer marine stratus at 22:00 GMT on October 1st

Figure 4. From Sandra Yuter, UW. Horizontal cross-section of C-band radar data from 02 UTC on 14 Oct 2001 showing drizzle cells in vicinity of ship. Red line is ship track. Grid marks are at 10 km intervals.

Figure 5. From Sandra Yuter, UW. Upper air sounding corresponding to time of C-band radar observation.