Shipboard meteorology

In this section we will discuss underway meteorology measurements made during WHP cruises. The Voluntary Observing Ship (VOS) programme is beyond the scope of this chapter.

WOCE planners recognized that meteorological data from research ships would have a number of uses; these included: initialization of atmospheric models (especially in data-sparse regions); provision of accurate estimates of basic meteorological variables for comparison with ships of opportunity; comparison with model output; comparison with satellite-derived quantities (Liu and Katsaros, Chapter 3.4); validation of climatologies and model-derived fluxes. The relevant chapter in the WOCE operations and methods manual (Taylor and Weller, 1991) gives further explanation.

The greatest requirement was for meteorological measurements that would enable the definition of surface fluxes of heat, water and momentum (Large and Nurser, Chapter 5.1). The objective (WCRP, 1988b) was to obtain estimates (averaged over monthly or longer time scales) of the four components of heat flux to an uncertainty of 10 Wm~2, of evaporation and precipitation to 1 mm per day and wind stress to within 10%.

The basic observables are sea surface temperature, air temperature, wind velocity, barometric pressure, incoming short- and long-wave radiation and humidity. Ships in the WOCE programme were valuable platforms from which to make accurate in-situ measurements. The chief advantages of WOCE ships were: they travelled through data-sparse areas; they were manned by crews and scientists with an interest in obtaining good meteorological data; and their operating schedules permitted sensors and electronics to be returned to laboratories periodically for calibration.

3.1.4.1 Developments for and during WOCE

Prior to WOCE, acquisition of meteorological data was somewhat uneven, even on research ships. Many, indeed most, of the research ships that would take part in the WHP did not have automated systems, so the only meteorological data returned were the manual observations made by the bridge officers and transmitted by radio or in delayed mode from bridge Meteorological Logbooks. In the late 1980s, several countries were developing automated systems to ensure that their research ships routinely acquired and reported high-quality meteorological data. In the UK, for example, the Institute of Oceanographic Sciences Deacon Laboratory (IOSDL) was developing a system called MultiMet, and in the USA the IMET (Improved Meteorological Measurements) system was being developed at WHOI. Other countries were undertaking similar developments. A careful test of the ability of different systems and platforms to make the measurements was carried out as part of the TOGA-COARE experiment in November 1992-March 1993 (Bradley and Weller, 1995a,b, 1997).

The minimum suite of measurements required on an automated system included wind velocity, air temperature, air humidity, sea surface temperature, downward radiative fluxes and air pressure. Other parameters of value included wind stress measured using the dissipation technique, ocean skin temperature (by downward-looking radiometer) and other radiative flux measurements. The latter quantities are required specifically for satellite data validation.

The main challenge to automated systems was the selection, siting around the ship and maintenance of the sensor suite. In order to achieve good exposure for any relative wind direction, two or three sensors for each variable are required. Calibration and maintenance of the sensors is required at frequent intervals.

The introduction of high-quality automated data systems meant that many ships were now acquiring and reporting meteorological data for the first time: IMET systems were installed on numerous (six to eight) US research vessels, and are now being installed on some US Voluntary Observing Ships. A comprehensive account of the status of shipboard meteorology measurements and the associated surface flux calculations at the end of the 1990s can be found in the Final Report of the Joint WCRP/SCOR Working Group on Air-Sea Fluxes (WGASF, 2000).

3.1.4.2 Data assembly and quality control

The Data Assembly Centre for surface meteorology is at the Center for Ocean-Atmosphere Prediction Studies (COAPS) in Florida State University. Data are assembled, reviewed for quality using an automatic system, have quality flags attached, and are made available to the user community (Smith et al., 1996). (Note in passing that while QC flags are set, data are not changed. Thus the end user is responsible for determining the action taken with flagged data.) Data assembled via this route are particularly valuable because of the quality of the instrumentation, continuous data collection and documentation efforts. At the time of writing, the DAC has data from 454 WOCE cruises, which they estimate to be 74% of the completed total. The distribution of data held at the DAC is shown in Fig. 3.1.8 (see Plate 3.1.8, p. 172).

3.1.4.3 Uses for shipboard meteorological data

In addition to the high quality, there are a number of other characteristics that make these data particularly valuable:

• Coverage is obtained from data-sparse regions, particularly the Southern Ocean, which is rarely visited by ships of opportunity.

• Many of the automated data are not reported via the GTS. This means they are not included in Numerical Weather Prediction products and can be used as an independent check.

• Some ships report standard bridge observations at synoptic hours, but also have independent automated systems. This allows an intercompar-ison between the two data streams.

The high quality of research vessel data also makes them suitable for evaluating flux products. While this can be done on short time scales, i.e. synoptic rather than climatological or other longer-averaged time scales, spatial and temporal coverage is limited. Cruises are not typically repeated to a particular region, and coincidence of ships with, for example, satellite overpasses may not be exact.

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