Experiment Design and Observational Requirements

As noted above, EPIC2001 will study the atmosphere and the ocean mixed layer in a north-south section through the east Pacific Hadley circulation, with an extension into the stratus region to the southeast. This section will take advantage of the availability of data from the existing line of TAO buoys along $95^{\circ} \mbox{ W}$. The number and capabilities of these buoys will be enhanced in time for the EPIC2001 field phase, which is tentatively scheduled for the summer of 2001. Enhancements to the TAO buoys include measurements of precipitation, radiation, and high quality barometric pressure in the atmosphere and additional oceanographic measurements as well. In addition, three new buoys are planned so as to extend measurements as far north as $12^{\circ} \mbox{ N}$ and to fill a gap at $3.5^{\circ} \mbox{ N}$ (Meghan Cronin, personal communication).

  

Figure 8: Schematic illustration of core facilities requested for EPIC2001.

The EPIC2001 core effort plans to use two manned research aircraft, the NCAR Electra and a NOAA WP-3D. These will most likely be deployed from Huatulco, Mexico ( $16^{\circ} \mbox{ N}$, $96^{\circ} \mbox{ W}$), with Acapulco, Mexico as an alternate, but somewhat less desirable base. The Electra will be used to study atmospheric convection as well as mesoscale atmospheric boundary layer and ocean mixed layer structure in the ITCZ, while the WP-3D will document the ITCZ inflow region and ocean mixed layer as far south as the equator. In addition, pilotless Aerosonde aircraft will be deployed from the Galapagos Islands ( $1^{\circ} \mbox{ S}$, $91^{\circ} \mbox{ W}$) to study the atmospheric boundary layer both north and south of the equator. A NOAA ship, the Ron Brown, has been requested for atmospheric and oceanographic measurements in both the ITCZ region and its inflow, and in the southern hemisphere stratus region. A Mexican oceanographic ship will be used to deploy drifters in the east Pacific warm pool, to make atmospheric soundings, and possibly to deploy CORC floats. An additional ship may be requested to make measurements within $100 - 200 \mbox{ km}$ the Ron Brown in the ITCZ in order to understand the factors which produce small-scale variability in SSTs. Advantage will be taken of enhanced monitoring of the east Pacific from TAO buoys on the $95^{\circ} \mbox{ W}$ line and an IMET buoy to be deployed in the southern stratus region. The requested facilities are illustrated in Figure 8.

The scientific roles of the above-mentioned core platforms are as follows:

• NCAR Electra: Survey sea-air fluxes from Mexico south to the south side of the ITCZ. Make Eldora radar-based convergence boxes in the ITCZ so as to obtain overall mass fluxes in this region. Make cloud physical studies of cumulus clouds in the ITCZ using the Eldora radar and in situ measurements to understand the processes which either encourage or discourage these clouds to grow into deep cumulonimbus systems. Deploy AXBTs, ATCTDs, and AXCPs to probe the mesoscale structure and variability of oceanic temperature, salinity, and current in the ITCZ in conjunction with the above studies. Survey temperature, moisture, and wind in the boundary layer while outbound from Mexico to the ITCZ, and at mid-levels ( $3 - 5 \mbox{ km}$) from the ITCZ back to Mexico. Survey convection and stratiform precipitation on this transect with the Eldora radar. Measure upward and downward solar and infrared fluxes on both the outbound and return legs, allowing layer radiative flux divergence estimates to be made. Measurements will be made through the full diurnal cycle and in all phases of the easterly wave cycle. About 16 flights, each of $7.5 \mbox{ h}$ duration will be required, amounting to about $120 \mbox{ h}$of research time. The outbound and return legs together will take about $4 \mbox{ h}$, leaving about $3.5 \mbox{ h}$ for studies in and near the ITCZ. The Electra will operate in close collaboration with the ship Ron Brown where appropriate.
• NOAA WP-3D: Document atmospheric boundary layer turbulent fluxes, structures, and processes from the ITCZ to the equator, and document the concurrent upper ocean structure. Atmospheric dropsonde deployments will be made from $6 \mbox{ km}$ on outbound or southward tracks along $95^{\circ} \mbox{ W}$. Slow porpoise ABL soundings between $100 \mbox{ m}$ and about $2 \mbox{ km}$, will be made on inbound or northward tracks, interspersed with eddy correlation flux measurements on level tracks at the top and bottom of the boundary layer. In addition, oceanic AXBTs, AXCTDs, and AXCPs will be deployed on inbound tracks. The objective of these flights is to observe the modification of the ABL north of the oceanic cold tongue and the corresponding structure and evolution of the upper ocean. These observations will be used in budget studies of ABL mass, momentum, heat and moisture with emphasis on the role of entrainment and in studies of oceanic responses to transient atmospheric forcing. Comparisons will be made between different phases of the diurnal and easterly wave cycles. Support will also be provided to the NCAR Electra for some of its ITCZ investigations. Here the NOAA WP-3D will generally provide large-scale mapping of a convective disturbance on its southward passage to provide context for finer-scale surveys by the Electra. A total of about 16 flights, each of approximately $9 \mbox{ h}$ duration, or about $150 \mbox{ h}$ of research time, will be requested. The outbound leg will take about $4 \mbox{ h}$ to reach the equator from Huatulco. The return leg will take closer to $5 \mbox{ h}$ due to the lower altitude flown.
• Aerosondes: Deploy pairs of aerosondes along $95^{\circ} \mbox{ W}$, one proceeding north and the other south of the Galapagos islands, flying continuous ascents and descents through the ABL. This will yield a north-south cross-section with high time continuity of the wind and thermodynamic characteristics of the ABL flow in its cross-equatorial segment.
• NOAA ship Ron Brown: The ship will tentatively loiter near ( $8 - 10^{\circ} \mbox{ N}$, $95^{\circ} \mbox{ W}$) for 3-4 weeks in order to document convection and estimate rainfall in the ITCZ using the ship's C-band Doppler radar, make atmospheric soundings, and to study mixing processes in the upper ocean. The ship will also cruise south along $95^{\circ} \mbox{ W}$ and thence southeast through the east Pacific stratus region to the IMET buoy at ( $85^\circ \mbox{ W} , 18^\circ \mbox{ S}$), where atmospheric and oceanic measurements will also be made. Oceanographic measurements to be made include a program involving the Modular Microstructure Profiler (MMP). The MMPs carry air-foil probes for measuring the turbulent dissipation rate, FastTip thermistors for high-resolution temperature, and SeaBird CTDs for making temperature and salinity profile measurments. In addition, at least one will carry an acoustic current meter and a tracking pinger for shear profile measurements. During the segment through the stratus and ITCZ inflow regions, a millimeter-wave Doppler cloud radar, atmospheric lidar, and microwave and infrared radiometers will be used to remotely sense the characteristics of the clouds. The C-band Doppler radar will also be used to document the development of precipitation in the stratus. In addition, sea-air fluxes will be measured. This transect is considered to be exploratory, to see if insight obtained in other stratus and stratocumulus regimes can be applied here.
• Mexican ship: From Armando Trasviña:

  Our project's main aim is to study the horizontal heat advection in the upper ocean of the Mexican Warm Pool. The study includes 2 40-day oceanographic cruises, deployment of about 20+ WOCE-ARGOS surface drifters and AVHRR imagery analysis.

  Our main observation platform will be the B.O. Altair (Mexican Navy).

  SSTs from the drifters and other sources will be used to calibrate (MCSST-like) the AVHRR imagery. Radiative-convective models will then be used to subtract incoming heat from the SST imagery in weekly-averaged periods. Images ``rectified'' this way will be statistically analized to separate mesoscale and large-scale components of heat advection. Drifter data will be used to validate results.

  The atmospheric component will use these results to improve forcing mechanisms in climate prediction models.

  Both projects will start obervational campaigns in 2001. The oceanographic project will carry out two 40-day cruises. The first around May-June and the second around October-November.

  The atmospheric project will start deployment of land-based stations around January 2001. Atmospheric soundings from the ship are also planned but funding is still uncertain.

• Additional NSF-sponsored ship: An additional ship will be requested to make underway oceanographic and atmospheric measurements within 100 km of the Ron Brown while on station in the ITCZ. The measurements will include conductivity, temperature, and depth in the upper ocean made with a Seasoar towed at a speed of 4 m/s on a survey pattern centered on the Brown. The Seasoar will also measure conductivity microstructure and in situ optical properties. Additional oceanic measurements will include horizontal currents with the ship's ADCP and near-surface temperature and salinity. Meteorological measurements will be made to accurately estimate air-sea fluxes, including precipitation, from a combination of direct measurements and a bulk flux algorithm. The ship will also carry an upward-looking S-band radar to estimate rainfall. The underway measurements will complement measurements on the Brown and the nearby TAO mooring by providing horizontal gradients and their scales of variability.

Data from these facilities will be augmented by long-term monitoring efforts supported by NOAA's PACS (Pan American Climate Studies) program, by various satellite remote sensing tools, and by international efforts associated with the VAMOS (Variability of the American Monsoon System) project. These include the following:

• Enhancements to the TAO array at $95^{\circ} \mbox{ W}$ will consist of additional moorings at $3.5^{\circ} \mbox{ N}$, $10^{\circ} \mbox{ N}$, and $12^{\circ} \mbox{ N}$. All buoys at this longitude will gain shortwave and longwave radiometers, rain gauges, surface barometric pressure, extra thermistors, conductivity sensors, and 1-2 current meters. These observations are critical to achieving the objectives of EPIC2001.
• Deployment of an IMET mooring at $85^{\circ} \mbox{ W}$, $18^{\circ} \mbox{ S}$ is planned with the purpose of studying the energy balance in the stratus region.
• Enhanced soundings will be made from surrounding land areas under the auspices of PACS.
• GOES, TRMM, SSM/I, NOAA Polar Orbiting, and other satellite observing systems will be used in both the operational and data analysis phases of the project to provide context for the project.
• The Instituto Mexicano de Tecnologia del Agua (IMTA) in Cuernavaca, Mexico plans to make meteorological observations in the Isthmus of Tehuantepec.
• Instrumented buoys will be placed in the stratus regions of the eastern Pacific by investigators from Chile.
• Michael Douglas (personal communication) indicates that radiosonde soundings will soon be resumed from the Galapagos Islands. In addition, the wind profiler operated by NOAA's Aeronomy Laboratory will provide useful information.
• The ARGO program will deploy profiling floats throughout much of the Pacific ocean, resulting in ocean soundings to $2000 \mbox{ m}$. These data will provide a useful large-scale context to EPIC2001 ocean measurements.