W. R. Seguin and M. Garstang a comparison ol meteorological Department of Meteorology sensors used on the Florida State University, Tallahassee USCGSS Olscoverer during me 1000 Barbados Experiment Abstract station by alternatively steaming slow ahead and drifting near a point determined by anchored marker buoys. Large scale experiments planned for the future as part The meteorological measurements taken on board the of the Global Atmospheric Research Project (GARP) Discoverer can be divided into three categories: will call upon measurements taken from all categories of ocean vessels. Type 1. "Standard" ship observations: These observa- During the 1968 Barbados Experiment meteorological tions include routine hourly sling psychrometer measurements were made aboard the USCGSS Discoverer wet and dry bulb temperatures, sea surface using a variety of sensors and methods typical of many (bucket) temperature, and wind speed and direc- ocean vessels. A comparison of the Discoverer data sys- tion (from masthead anemometers), which are tems provides an estimate of the inherent error in some typical of measurements made by all ships at sea. shipboard measurements. It is demonstrated that under Type 2. "Routine" research vessels observations: These unstable stratification, wind speed differences from observations are all typical of a research vessel anemometers mounted at separate locations and heights conducting meteorological investigations, and are small while under neutral conditions large differ- include continuous hygrothermograph measure- ences appear. Unless special precautions are taken, heat- ments, radiosondes and radiometersondes, pilot ing of sensors due to solar radiation and ship effects may balloons, expendable and conventional bathy- be as large as 2C. Such an error in dry bulb and wet thermographs. 1 bulb temperatures with a mean wind speed of 6 m sec" Type 3. "Special" measurements: A boom on the bow of yields a 100% error in the sensible heat flux and a the Discoverer carried an array of sensors from 50% error in the latent heat flux calculations. Nomo- which continuous measurements were made of grams of sensible and latent heat flux are constructed to sea surface temperature, wet and dry bulb tem- demonstrate the manner in which such errors influence peratures, wind speed and direction, short and these quantities. long wave radiation and the albedo of the sea surface. Introduction Thirty-seven days of measurements were made over In 1968 the Florida State University (FSU) with the two periods of approximately equal length in July and support of and in cooperation with a number of other August 1968. The total set of data from each of the 1 organizations, conducted a meteorological field experi- above categories provides an opportunity for compari- ment on and in the vicinity of the island of Barbados son and assessment between each category. Such a study (13N 59.5W). The..experiment has been described in is motivated by the difficulties encountered when at- some detail by Garstang and La Seur (1968) and Garstang tempting ship board meteorological measurements. et al. (1970). Emphasis was placed upon the study of the These difficulties have been extensively discussed in the interaction of the atmosphere, the ocean and the land. literature and are summarized by Roll (1965). Further- Systems were deployed such that information could be more, the large scale experiments planned for the gathered on a range of atmospheric scales of motion. future as part of the Global Atmospheric Research The United States Coast and Geodetic Survey Ship Project (GARP) will call upon measurements taken from (USCGSS) Discoverer, was stationed, as part of this ex- all categories of ocean vessels. A comparison of the periment, 100 km due east of Barbados. The ship kept Discoverer data systems can provide an estimate of the inherent error in some shipboard measurements, the 1 Environmental Science Services Administration (ESSA effect of the error upon such computed quantities as the now NOAA), under Grant E-22-148-68-(G). The U. S. Army flux of water vapor and sensible heat and a measure Electronics Command (THEMIS) Contract DAAB07-69-C- of the way in which atmospheric stratification influences 0062, and the National Center for Atmospheric Research (NCAR). such errors. Bulletin American Meteorological Society 1071 Unauthenticated | Downloaded 10/05/21 12:13 PM UTC Vol. 52, No. 11, November 1971 tions made in the vicinity of the Crawford show that the computed values may be exceeded by an order of magnitude on the downstream side of the ship. Such observations were felt to be consistent with the compu- tations if heat was concentrated in specific parts of the volume instead of mixed uniformly throughout the volume. Amot (1955) found similar results. Yet another effect was discussed by Stevenson (1965). He concluded that in regions with shallow thermoclines, upwelling caused by the ship can seriously influence the observa- tions. It is even more difficult to make adequate wind mea- surements at sea than temperature. Any results obtained for one ship do not necessarily apply to another with differing hull, superstructure and power plant. To com- pound the difficulties, those studies that have been con- ducted draw conflicting conclusions. Soo and Elder FIG. 1. United States Coast and Geodetic Survey Ship Dis- (1967) working in the Great Lakes conducted a study of coverer. Type 1 and Type 2 winds from the aerovanes marked T1 and T2, Type 2 wet and dry bulb temperatures winds measured on three research vessels and a research from hygrothermometer marked T2 and Type 3 tem- tower and were able to draw some qualitative con- peratures and winds at the end of the horizontal boom clusions. They concluded that the influence of the ship marked T3. Meteorological data collected on the vertical is small when anemometers are mounted on a well mast on the boom were not used in this study. exposed mast and that the estimated error in wind speed is about 5%. They also concluded that bow ane- Instrumentation mometers cannot be used to measure accurately the wind profile. Bogorodskii (1966), as did Deacon et al. (1956), Type 1: This instrumentation consisted of the usual found that profiles could be representative provided hand held meteorological sensors such as the sling they were determined while the wind direction was psychrometer and mercury-in-glass bucket thermometer. constant relative to the ship and that corrections were Winds were read off the forward mast anemometer made for roll, drift speed and relative wind direction. (Fig. 1) dials rather than estimated from the state of Bogorodskii's study included profiles made from a spar the sea. These measurements were recorded once every hour. Type 2: Measurements of interest in this category fol- low in detail: Temperature and dew point observations were made by the hygrothermometer mounted over the bow of the vessel as shown in Fig. 1. Both of these sensors were aspirated. Wind speed and direction were measured by the two anemometers mounted on the forward mast. Sea surface temperature was measured by a thermistor protected by a metal housing and protruding from the bow observation port about 5 m below mean sea level. Output from all of these sensors were recorded on analog chart recorders with the exception of the sea surface temperature which was read visually every 3 hr and recorded with the surface observations. The hull and superstructure distorts the ambient air flow over a ship. Similarly air temperature and humidity and sea surface temperature are affected by radiative heating and cooling and by the heat gen- erated due to the combustion of fuel. Garstang (1964) considered the effect of a relatively small vessel: (R. V. Crawford, 125 ft, 250 gross tons as compared with the Discoverer which is 300 ft, 2580 to 3800 long tons). He estimated temperature rises at times of maximum in- solation in a volume of sea and air advected past the ship. If heat is uniformly distributed through the whole volume then temperature increases for the sea and air were computed as 0.01C and 0.1C, respectively. Observa- FIG. 2. Radiation shield for dry and wet bulb thermistor. 1072 Unauthenticated | Downloaded 10/05/21 12:13 PM UTC Bulletin American Meteorological Society buoy for comparison with profiles made on the bow of the ship. The latter study also included wind measure- ments on the windward side of the bridge and found them to average 11.5% higher than the true wind. Based upon these findings and influenced by practical considerations, a retractable boom was mounted on the bow of the Discoverer. The boom in the extended oper- ating position is shown in Fig. 1. In this position the sensors at the end of the boom were 10 m forward of the bow of the ship and 10 m above mean water level. This boom provided the platform for the following Type 3 measurements. Type 3: Dry and wet bulb temperatures were mea- sured by thermistors mounted in radiation shields of the type illustrated in Fig. 2. The radiation shield, FIG. 3. Comparison of the sea surface, air and wet bulb developed at FSU, was designed to minimize the effects temperatures as measured by instrumentation Type 1 (---), of short- and long-wave radiational heating of the tem- Type 2 (—) and Type 3 ( ) for a five day period. perature sensors, as well as to avoid errors due to wetting by rain or sea spray. The sensors were wind aspirated. tures were converted to wet bulb temperatures using Tests (to be published in a separate article) show that the hygrothermometer temperatures. Wind speeds were 1 wind speeds above 2 m sec" provide adequate ventila- read from the strip charts every 5 min, corrected for tion and in general wind speeds in the trades exceed ship's speed and heading, and then hourly averages this value by a factor of three.