The Interface or Air-Sea Flux Component of the toga coupled Ocean-Atmosphere Response Experiment and its Impact on Subsequent Air-Sea Interaction Studies




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The Interface or Air-Sea Flux Component of the TOGA Coupled Ocean-Atmosphere Response Experiment and its Impact on Subsequent Air-Sea Interaction Studies




Robert A. Weller+

Frank Bradley*

Roger Lukas#


+Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

*CSIRO Land and Water, Canberra, Australia

#Department of Oceanography, University of Hawaii, Honolulu, Hawaii, USA


September 9, 2002

The Interface or Air-Sea Flux Component of the TOGA Coupled Ocean-Atmosphere Response Experiment and its Impact on Subsequent Air-Sea Interaction Studies


Robert A. Weller+

Frank Bradley*

Roger Lukas#


+Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

*CSIRO Land and Water, Canberra, Australia

#Department of Oceanography, University of Hawaii, Honolulu, Hawaii, USA


Abstract

The interface or air-sea flux component of the Coupled Ocean-Atmosphere Response Experiment (COARE) of the Tropical Ocean-Global Atmosphere (TOGA) research program and its subsequent impact on studies of air-sea interaction is described. The field work specific to the interface component was planned to substantially enhance our understanding of air-sea interaction in the tropics by improving the methodology of flux measurement, and by collecting a comprehensive set of observations, with coverage of a broad range of time and space scales. The strategies adopted for the COARE measurement campaign, particularly on-site intercomparisons, together with post-experiment studies of instrument performance and bulk flux algorithm development, ensured the compilation of very high quality time series of basic near-surface meteorological variables and air-sea fluxes. Sustained collaboration by scientists engaged in studies of air-sea exchange enabled the goal of better than 10 W m-2 accuracy in mean net air-sea heat flux to be achieved, verified by closure of the ocean heat and freshwater budgets to within 10 W m-2 and 20% respectively. Reanalysis of in-situ rain gauge data confirms the findings by atmospheric thermodynamic budget analysis of a west-to-east gradient of IOP-average precipitation and that rainfall at the IMET site was closer to 9 mm day-1 than the radar value of 5.4 mm day-1. These results confirm that accurate in-situ observations of air-sea fluxes can be obtained during extensive measurement campaigns, and have established the foundation of current plans for global, long-term oceanic observations of surface meteorology and air-sea fluxes. They also point to uncertainties that remained after COARE, which must be, addressed in future studies of air-sea interaction.

1. Introduction

In this article we describe the interface or air-sea flux component of the Coupled Ocean-Atmosphere Response Experiment (COARE) of the Tropical Ocean-Global Atmosphere (TOGA) research program. The fieldwork of COARE was carried out in a domain centered near the equator, northeast of Papua New Guinea (Figure 1). The bulk of the observations were collected during the 4-month intensive observation period (IOP) of November 1992 through February 1993. Webster and Lukas (1992, WL92) reviewed the background for COARE and summarized its overall objectives. The scientific goals were to "describe and understand: 1) the principal processes responsible for the coupling of the ocean and the atmosphere in the western Pacific warm-pool system; 2) the principal atmospheric processes that organize convection in the warm-pool region; 3) the oceanic response to combined buoyancy and wind-stress forcing in the western Pacific warm-pool region; and 4) the multiple scale interactions that extend the oceanic and atmospheric influence of the western Pacific warm-pool system to other regions and vice-versa."

Six components of the experiment were identified: the interface or air-sea flux component, large-scale atmospheric circulation and waves, atmospheric convection, large-scale ocean circulation and waves, ocean mixing, and the modeling component. WL92 listed the scientific objectives specific to the air-sea flux component as:

1) "To provide a high-quality data set of heat, moisture, and momentum fluxes in the warm-pool region;

2) To understand the physics and thermodynamics of interfacial exchange processes that have particular behavior in this region of low wind speeds and strong atmospheric convection;

3) To improve various empirical formulae used to estimate net surface heat flux for use in warm-pool regions;

4) To determine the magnitude (and significance for longer-term models) of short time-scale variability of the fluxes of heat, moisture, and momentum - hourly, diurnal, and episodal, and;

5) To understand the impact of the full range of wind structures, from the ambient trade-wind regime through episodic westerly bursts, on the ocean-atmosphere fluxes of heat, moisture, radiation, and momentum."

The warm pool region of the western equatorial Pacific, with its low mean wind speeds, high tropical insolation, and intermittent, energetic forcing associated with convective systems presented a formidable observational challenge. WL92 highlighted the need for high quality flux measurements when they noted that five climatologies estimated net annual heat fluxes that ranged from 20 to 100 W m-2. The uncertainty was ascribed partly to the difficulty of making accurate and adequately sampled measurements of the surface meteorological variables (i.e. air temperature and humidity, precipitation, incoming shortwave and longwave radiation) in the low wind, convective conditions, and partly to the inadequacy of existing bulk formulae for wind stress and heat fluxes in these conditions. The latter had mostly been developed during windy, mid-latitude field experiments, and the applicability of the bulk transfer coefficients and parameterizations to tropical conditions was in some doubt. These uncertainties limited the usefulness of existing measurements and the derived flux data sets in the COARE region.

Planning for COARE recognized the need to improve the sampling and accuracy of the measurements and to develop appropriate bulk flux algorithms, adopting the TOGA goal to determine the net heat exchange across the air-sea interface to an accuracy of 10 W m-2 over monthly to seasonal time scales (WL92, Godfrey et al. 1999a). To help achieve this goal, results and experiences from pilot air-sea flux field studies in the warm pool region (Bradley et al. 1991, 1993; Chertock et al. 1993; Young et al. 1992) were used to help formulate an experimental strategy and improve the methodology for measurements.

Accurate rainfall measurements were of particular interest, because of the large annual precipitation, greater than 4 m yr-1according to Dorman and Bourke (1979), and its probable role in stabilizing the surface layer of the ocean (Lukas and Lindstrom 1991). There was, however, potential for large uncertainty and large inter-annual variability in the rainfall. Spencer (1993) pointed to differences of up to 0.7 m yr-1 in the COARE region among the Janowiak and Arkin (1991) satellite infrared estimates, an in-situ data set based on Legates and Wilmott (1990) and Jaeger (1983), and his own satellite microwave data sets and further showed inter-annual variability in the region of over 2 m yr-1.

Another major challenge was the known variability of conditions in the region, over a wide range of scales in space and time. Episodic westerly wind bursts, lasting up to two weeks in duration and with scales of many hundreds to a thousand or more kilometers, were recognized as a major forcing of the coupled air-sea system, and a possible important factor in the onset of the ENSO phenomenon. At the same time, however, COARE sought to determine the impact on air-sea exchange of squalls and small convective systems, down to individual elements a few kilometers in size, and to examine temporal variability over periods from hours to months.

Our purpose in writing this paper is to describe the fieldwork in COARE associated with the interface component, to document the steps taken to ensure the accuracy and compatibility of the diverse surface meteorological and air-sea flux measurements, and to summarize how these data were used to refine the bulk formulae. It is our belief that the effort invested in the COARE air-sea flux component has paid large dividends, a notion reinforced by many contributions to the recent COARE98 conference (WCRP 1999), by the adoption of the COARE bulk formulae by many researchers, and by the interest in the COARE interface component shown by analysts and modelers alike. COARE contributions to the state of the art were also discussed at the workshop organized by the WCRP/SCOR Working Group on Air-Sea Fluxes (WCRP 2001). We assess the impact of this effort on the planning and execution of subsequent field programs where accurate surface measurements and air-sea fluxes have been needed and on the thrusts now developing to further extend our understanding to very low wind and to high wind regimes. We submit that the successes of the COARE air-sea flux effort have created a foundation upon which to build strategies for sustained flux observations on a global basis.


2. Observational approach during COARE

During the planning for COARE, an air-sea interaction (Flux) working group was established to guide the formulation of the flux components of the scientific and operational plans. The Flux group recognized that success in addressing the five objectives of the flux component of COARE would require the coordinated use of multiple measurement platforms in the field, emphasis on comparison and inter-calibration across these platforms, and a concentrated effort following the field work to identify and resolve data quality problems and to improve flux estimates by bulk methods. Improved understanding of air-sea interaction in the warm pool was, in effect, to be achieved by better measurement methods, using direct eddy-correlation measurement of energy and momentum fluxes to calibrate the exchange coefficients used in appropriate bulk flux formulae. Sampling of the broad range of space and time scales was accomplished with a continuous data set over the 4-month IOP with a continuous presence of flux-measuring platforms, achieved through staggered scheduling of ships and aircraft. Thus, accurate point measurement of the fluxes would lead to improved bulk algorithms, available for use with high quality bulk meteorological observations over larger space and longer time scales.

The most intensive measurement programs were conducted in the Intensive Flux Array (IFA) (Fig. 1). Land-based meteorological stations at Kavieng and at Kapingamarangi atoll defined the western and northern corners of the perimeter of the IFA, while the research vessels Kexue 1 and Shiyan 3 were stationed at the eastern and southern corners. Two vessels, RV Vickers and Xiangyanghong 5, operated stabilized, Doppler radars for the duration of the IOP to measure and map rainfall (Rutledge et al. 1993; Short et al. 1997). RV Keifu Maru also operated a Doppler radar during November 1992. The TOGA Tropical Atmosphere and Ocean (TAO) (Hayes et al. 1991) array of surface buoys provided a large scale and longer term context and was expanded during COARE to improve zonal resolution of westerly wind bursts and ocean response, and to measure shortwave radiation and rainfall in the warm pool. Shipboard, ground-based, aircraft, and satellite sampling in the COARE large-scale domain (LSD) and outer sounding array (OSA) also provided large scale coverage for the air-sea interface work. Three surface moorings provided time series at fixed points within the IFA. The TAO mooring at 0°, 156° E measured wind velocity, sea (1 m depth) and air temperature, humidity, rainfall, and shortwave radiation. Those at 2° S, 156° E and at 0°, 154° E measured wind velocity, air temperature and humidity, incoming, rainfall, and sea temperature. The Woods Hole Oceanographic Institution (WHOI) surface mooring, identified as the IMET (Improved METeorological) mooring, at 1.75° S, 156° E measured wind velocity, incoming shortwave and longwave radiation, air temperature and relative humidity, barometric pressure and ocean temperature near the sea surface (0.45 m).

Time series from the IMET mooring (Fig. 2a) provide an overview of the temporal variability of the surface meteorology within the IFA (for more detail see Weller and Anderson 1996, and for the large-scale ocean-atmosphere context during COARE, see Lukas et al. 1995). From late October to early November 1992 a series of 3 to 7-day long southwesterly wind events with speeds between 3 and 8 m s-1 was observed. Low wind speeds, typically 2 m s-1, persisted from November 14 through December 12. The following period until about January 4, 1993 was marked by a sequence of moderately strong wind events, with flow toward the southeast associated with the now celebrated December westerly wind burst (WWB). Instantaneous winds greater than 30 m s-1 were observed on RV Wecoma, and a peak wind of 17.2 m s-1 was recorded by the IMET mooring on December 23. The WWB was followed by another period of very low wind speeds from January 4 to 15, and then a period of sustained westerlies and squalls.

These time series can be interpreted as a record of three cycles of the intraseasonal oscillation (ISO; Madden and Julian, 1972) that passed through the IFA (Kiladis et al. 1994). The active phases of these ISOs passed the site of the mooring in the first week of November 1992, around December 20, 1992, and at the end of January into early February 1993. The periods characterized by low winds and clear skies indicate the suppressed phase of the ISO. According to Chen et al. (1996) the phase of the second ISO where convection is suppressed lasts from November 13 to December 8, 1992 in the IFA; the active phase is from December 9, 1992 through early January 1993, though there is a break in the convective activity during December 17 to 19. The suppressed phase of the third ISO lasts from January 6 to 26 and as with ISO2 was marked by very little high cloud cover. During the active phase of the ISO3, which continued through the end of the IOP, moderate winds with short-lived (several hours to one day in duration), higher speed westerly wind events were seen in the IFA rather than sustained westerlies. The ISO during the COARE IOP was unusually strong, related to the resurgence of ENSO warm event conditions that had started during 1991, but which had relaxed to near normal prior to the IOP (Lukas et al. 1995).

Research vessels Alis, Franklin, Hakuho Maru, Malaita, Moana Wave, Natsushima, Noroit and Wecoma worked within the IFA for various periods during the IOP, as indicated in Figure 3. The three periods when both the oceanographic ships worked simultaneously at the center of the IFA are called Legs 1-O, 2-O and 3-O. The shipboard observations (Table 1) included direct turbulent flux measurements discussed below and surface meteorology. Ships that made near-surface oceanographic measurements provided information on both the local forcing and oceanic response. For example, Figure 4 shows time series obtained aboard RV Franklin during the passage of two small convective systems with accompanying changes in air temperature, wind and rain. The shipboard sampling program also provided information about spatial variability within and across the IFA. Figure 5 shows one rainfall map from the time series over the center of the IFA observed by the two Doppler radar-equipped ships. The two radar ships were both on station for three periods: Legs 1-M, 2-M and 3-M.

Five of the research aircraft participating in COARE carried out “boundary-layer” flights between 30 and 100 m above the sea surface as part of the air-sea interface component. They were the Electra from the National Center for Atmospheric Research (NCAR), two P-3s from the National Oceanic and Atmospheric Administration (NOAA), the C-130 from the UK Meteorological Office, and the Cessna (C340) from Flinders University, Australia. Boundary layer missions are summarized in Table 2. During these flights, the aircraft measured bulk meteorological parameters and turbulent fluxes, and frequently overflew ships and moorings to make comparison with surface observations. On some occasions they flew side by side to inter-compare their own sensors. These intercomparisons are discussed in Burns et al. (1999, 2000). Whereas the surface platforms provide long records of temporal variability over a limited area, the aircraft data complements this strikingly with “snapshots” of spatial variability over hundreds of km. Figure 6, for example, shows the contrast between considerable spatial variability of SST on a low-wind day and one when SST was substantially more uniform due to strong wind mixing. Figure 7 is an example of mesoscale variability in the marine atmospheric surface layer observed during a Cessna flight. Different air masses were encountered, distinguished by different mean temperature and humidity, and quite dissimilar behavior of the turbulent fluxes.

At higher altitudes, the National Aeronautic and Space Administration (NASA) DC-8 and ER-2 collected information about clouds, lightning, radiation fields, atmospheric temperature and humidity, and tested methods for remote sensing of rain. During the IOP, surface meteorological and flux fields from satellites and from the analysis cycle of the operational numerical weather prediction models were collected and archived for later use by COARE investigators. GMS, NOAA, DMSP, ERS-1, and TOPEX/POSEIDON satellites provided data, including sea surface temperature, sea surface topography, radiation, wind speed, and rainfall. Model output was also obtained from the Bureau of Meteorology Research Centre (BMRC, Melbourne, Australia), the National Centers for Environmental Prediction (NCEP, Washington DC, USA), the European Centre for Medium Range Weather Forecasting (ECMWF, Reading, UK), and the Japanese Meteorological Agency (JMA, Tokyo, Japan).

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