EPIC2001 Workshop, Seattle, 25-26 March 2002

From notes by David J. Raymond, Meghan Cronin,
Steve Esbensen, and Robert Weller

Meeting Participants

25 March


John Molinari reported that mid-September 2001 was very active convectively in the east Pacific, while early October was inactive. The ITCZ shows up best (as an east-west line of convection at a definite latitude) in inactive periods. The easterly waves which passed through during the project tilted NE-SW.

Rob Cifelli reported that peak convective activity occurs about two days before easterly wave trough passage, as defined by low surface pressure. Three easterly wave passages were documented by the Ron Brown during it's residence at 95 W, 10 N. West of the trough in the northerly flow regime, strong convection exists. In and east of the trough the regime is much more stratiform in nature. The most intense convection in the diurnal cycle occurs near 0200 LT.

Darrel Baumgardner reported that aerosol concentrations varied widely, depending on wind direction, with the highest concentrations occurring in offshore winds. These high concentrations are sufficiently large to completely suppress warm rain production. Increasing wind speeds result in increasing concentration of giant nuclei.

Carlos López reported on progress in analyzing the Doppler radar data from the NOAA P-3 aircraft. This analysis is well under way.

David Raymond found that convection in the east Pacific ITCZ varied directly with surface wind speed, implicating surface total heat flux as a driving mechanism for deep convection.

Cross-Equatorial Flow

Nick Bond gave an overview of the C-130 95 W missions and indicated that, somewhat contrary to expectations, lots of variability existed in the flow even at the equator, in cloudiness, wind shear, turbulence kinetic energy, etc.

Simon de Szoeke noted that the TAO temperature at 5 N appeared to be too warm by about 1 K.

Steve Esbensen indicated that there was a strong correlation between surface stress and sea surface temperature (SST) near the SST front. Furthermore, the passage of easterly waves can be seen in the meridional wind at the Galápagos.

Chris Bretherton's large eddy simulations represent the cross-equatorial flow very well. NCEP reanalysis doesn't adequately capture the characteristics of this flow.

Mike McGauley presented latitude-height cross-sections of the thermodynamic fields and flows along 95 W from C-130 dropsondes.

Chris Fairall reported that the SST decreased by about 1 K during the Ron Brown's residence at 10 N. The EPIC region was found to have colder, drier air than the western equatorial Pacific or the Indian monsoon region.

Meghan Cronin indicated that ocean current speeds are a substantial fraction of the wind speed, with corresponding effects on fluxes. Maximum fresh water occurred at 3 N, maximum rainfall at 8 N. Extra maintenance costs induced by vandalism of TAO moorings may result in early shutdown of the EPIC components of the array. High time resolution of TAO data will be available in a few months.

Ocean Observations

Mike Gregg reported that very little mixing occurred below the thermocline. Deep water warmed during the observational period. Convective events produce near-inertial waves, which are responsible for mixing. Mixing rates are very low compared to middle latitudes.

Carter Ohlmann reported observations of solar flux divergence in the ocean, which is strongly controlled by biomass variations. Peak biomass occurs at the base of the mixed layer.

Dan Rudnick described the pattern followed by the New Horizon. The Sea Soar yielded 2 km horizontal resolution as it ascended and descended in a sawtooth pattern. A pattern was completed in half an inertial period, and 11 full patterns were done. At 10 m depth, the SST front exhibited a 2 K change in 100 m. Fresh water puddles were seen as a result of rain. There was almost no rain south of 6 N.

Nick Shay reported on expendable ocean probe observations. Strong shear was seen 5-7 Oct near 10 N.

Sean Kennan reported on the results of an experiment with the Revelle, 14 June - 26 July 2001. The equatorial counter current was very strong over a 3 week period, with a speed of 70 cm/s to E at 6 N. About 50% of the flow occurs in the upper 50 m.

Stratus Region

Chris Bretherton reported that the atmosphere above the marine layer was moist near the equator and dry near 20 S. Water path in the stratocumulus deck was minimum at local noon and maximum at night, with a factor of 4 variation. The drizzle was 6-10 times stronger at night. Cloud top height varies diurnally, but base height doesn't. The ECMWF vertical velocity agrees with these results, but the origin is uncertain.

Bob Weller reported that the ocean is heated by the atmosphere at 85 W, 20 S, except in the southern winter. This is a big surprise.

Darrel Baumgardner reported that there were problems with the DMS and sulfur measurements, but that the CN measurements appeared to be good.

26 March

Data and Instrumentation

Sandra Yuter reported problems with the Ron Brown C-band radar reflectivity calibration and pointing. These appear to be solvable.

Rob Cifelli reported that the C-band radar and the TRMM radar seem to agree fairly well. Soundings experienced about a 15% failure rate. Data are available from JOSS and CSU. Rain maps at 1 km will be produced, but the Z-R relationship needs to be determined. C-130 observations can be used, and Darrel Baumgardner is working on this.

Allen Schanot indicated that the unheated Rosemount probes on the C-130 were damaged by ice, but that the heated probe remained undamaged. This will be used as a reference temperature for the project. Comparison with dropsondes and the P-3 indicated that the C-130 had a low humidity bias. This will be corrected. The Ophir radiative thermometer doesn't work in rapid ascents and descents, and it exhibits isolated level shifts. However, it is still usable with care. The TDL instrument is not calibrated. A height-dependent correction for radiative SST has been developed.

David Raymond reported on a problem with the Ophir radiometer on the C-130 arising from the facts that about 5% of the photons reaching the device arrive from distances of 1 km or greater, and that it points down at an angle of about 20 deg from the horizontal, resulting in a warm bias outside of cloud. A fix for this problem has been developed using data from a downward-looking radiometer on the C-130.

Jeff Nystuen reported that moored acoustic rain gauges can be used to estimate both wind speed (from wave action) and rainfall rate.

Steve Williams described the JOSS webpage and data policies.

Plans for Collaborative Analysis

VAMOS-EPIC Report (Bretherton)

Chris Bretherton gave a brief report on recent meeting in San Jose, Costa Rica. The planning effort is pointing toward a three-pronged approach to work in VEPIC:

  1. Work to improve climate models' treatments of boundary layer clouds.
  2. Synthesize existing data sets in the eastern Pacific, bringing in the satellite data sets (TMI, Quikscat, ...) to add to the observational basis for characterizing the southeastern Pacific stratus region.
  3. Develop plan for extended time observations. Since the time scales for ocean variability and oceanic roles in coupled systems include monthly (coastal trapped waves, Rossby waves) and longer (seasonal, interannual), there is the need for observations that continue for at least several years to capture and explore this variability. At present, this is the priority rather than an short, intense IOP. Potential elements for extended time observations:
    1. San Felix Island observations.
    2. Airsea flux, cloud property observations on ship servicing the 95 W TAO line and the 85 W, 20 S mooring, and sometimes going to San Felix Island.
    3. Long-running observations, including stratus mooring at 85 W, 20 S, and other elements.
    There is also considerable interest in aerosols in the region, which gives rise to the possibility of joint or coordinated field work, perhaps using aircraft.


  1. March 2002: Set up ftp site for accepting and sharing results.
  2. Meeting report drafting. Each PI sends paragraph to session leader and submits key results to ftp site.
  3. April 2002: EPIC Power Point presentation for briefings.
  4. Agency briefings.
  5. Write CLIVAR Exchange articles (one each for stratus, cross-equatorial flow, ITCZ, ocean).
  6. Submit proposal to the Bulletin of the AMS for two articles, ITCZ/cross-equatorial flow/ocean and stratus.
  7. Start organizing EPIC special session at Fall AGU 2002.
  8. May 1, 2002: Deadline for CLIVAR Exchange articles.
  9. Late summer 2002: Two Bulletin of the AMS articles submitted.
  10. December 2002: EPIC working group meetings before Fall AGU.
  11. EPIC special session, joint Ocean and Atmospheric Sciences, at Fall AGU.

Climate Process Teams

Bretherton mentioned the possibility of forming Climate Process Teams consisting of a cross-section of EPIC participants and staff from major climate modeling centers, the idea being to hasten the introduction of new knowledge obtained from EPIC into climate models. He stressed that nothing would happen without funding for people dedicated to this project at the climate centers.

D. J. Raymond 2002-05-13