Steven P. Anderson

Surface Meteorology and Air-Sea Fluxes in the Western Equatorial Pacific Warm Pool during the TOGA Coupled Ocean-Atmosphere Response Experiment

Abstract A major goal of the Coupled Ocean-Atmosphere Response Experiment (COARE) was to achieve significantly more accurate and complete descriptions of the surface meteorology and air-sea fluxes in the western equatorial warm pool region. Time series of near-surface meteorology from a buoy moored near the center of the COARE Intensive Flux Array (IFA) are described here. The accuracies of the measurements and the derived fluxes are quantified; agreement between average net heat fluxes at the buoy and two nearby research ships is better than 10 W m−2 during three intercomparisons. Variability in the surface meteorology and fluxes associated with westerly wind bursts, periods of low winds, and short-lived, deep convective events characteristic of the region was large compared to the 4-month means. The ECMWF (European Centre for Medium-Range Weather Forecasts) analysis and prediction fields differed most from the buoy data during periods of short-lived, deep convective events, when several day averages of ...

Surface Buoyancy Forcing and the Mixed Layer of the Western Pacific Warm Pool: Observations and 1D Model Results

Abstract The broad, shallow body of warm (>29°C) water found in the western tropical Pacific Ocean plays an important role in the coupled ocean-atmosphere dynamics and thermodynamics associated with the El Nino-Southern Oscillation phenomenon. Thus, it is important to understand the processes that maintain and perturb that warm pool. Measurements from a buoy moored in the center of the warm pool during the TOGA Coupled Ocean-Atmosphere Response Experiment show that the exchange of mass between the ocean and atmosphere is as important as the exchange of heat. Rain forms a shallow, buoyant layer that does not mix with the water below except during infrequent strong wind events. Using a one-dimensional mixed layer model, it is demonstrated that the rate of local precipitation governs the mixed layer depth and can thus alter the rates of change in sea surface temperature during both warming and cooling periods. The observed mixed layer depth in the warm pool is at a depth that allows for maximum warming by ca...

A New Look at Calibration and Use of Eppley Precision Infrared Radiometers. Part I: Theory and Application

Abstract The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the ...

Radiative Heating Errors in Naturally Ventilated Air Temperature Measurements Made from Buoys

Abstract Solar radiative heating errors in buoy-mounted, naturally ventilated air temperature sensors are examined. Data from sensors with multiplate radiation shields and collocated, fan-aspirated air temperature sensors from three buoy deployments during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment) and the Arabian Sea Mixed Layer Dynamics Experiment are used to describe the errors in the naturally ventilated measurements. The naturally ventilated sensors have mean daytime errors of 0.27°C and maximum instantaneous errors of 3.4°C. The errors are at times larger than the difference between the air and sea surface temperatures. These errors lead to mean daytime biases in sensible and latent heat fluxes of 1–4 W m−2 and instantaneous errors up to 22 W m−2. The heating errors increase with increasing shortwave radiation and diminish with increasing wind speed. The radiative heating is also found to be a function of sun elevation with maximum heating errors occur...

An estimate of the surface shortwave cloud forcing over the western Pacific during TOGA COARE

Estimates of the shortwave cloud forcing at the surface have been computed for the TOGA COARE Intensive Observation Period (IOP) using the IMET buoy surface insolation measurements. Two different methods have been employed to calculate the clear-sky surface insolation. The first is based on an empirical approximation using the buoy observations. The second incorporates both modeling and empirical procedures. The net surface shortwave cloud forcing values derived from these two methods are -103 and -107 Wm -2 , respectively. These values indicate that for the COARE IOP, the surface cooling effect from shortwave cloud forcing is comparable to that from latent heat flux (-108.5 Wm -2 ). Comparing IOP values of OLR and MSU rain rate to their climatological values indicates that the mean cloudiness during the IOP corresponds closely to climatology, suggesting that these estimates of shortwave cloud forcing may be fairly representative of the climatological value.

Upper-ocean heat and salt balances in the Western Equatorial Pacific in response to the Intraseasonal Oscillation during TOGA COARE

During the TOGA COARE Intensive Observing Period (IOP) from November 1992 through February 1993, temperature, salinity, and velocity profiles were repeatedly obtained within a 130 km 3 130 km region near the center of the Intensive Flux Array (IFA) in the western equatorial Pacific warm pool. Together with high quality measurements of air‐sea heat flux, rain rate, upper-ocean microstructure, and penetrating solar radiation, they make up a unique dataset for upper-ocean heat and freshwater budget studies. Three survey cruises sampled different phases of the Intraseasonal Oscillation (ISO) during the IOP. Temporal evolution and advective terms in the heat and salt balance equations, on timescales of 3 days and longer, are estimated using the survey data. The upper-ocean (0‐50 m) heat and salt budgets at the center of the IFA were estimated and are closed to within 10 Wm 22 of observed air‐sea heat fluxes and to within approximately 20% of observed rain rates during each of the three cruises. Generally, advection in the upper ocean cannot be neglected during the IOP. Zonal advection alternates sign but had a net warming and freshening tendency. Meridional advection decreased temperature and increased salinity in the surface layer, while vertical advection warmed and freshened the surface layer because of the general downwelling trend. Heat advection is as important as the net air‐sea flux during the westerly wind burst time periods. The sub-ISO timescale upper-ocean dynamics, such as the strong meridional advection caused by inertial motions, are found to have important contributions to the upper-ocean heat and freshwater balances.

A New Look at Calibration and Use of Eppley Precision Infrared Radiometers. Part II: Calibration and Use of the Woods Hole Oceanographic Institution Improved Meteorology Precision Infrared Radiometer*

Abstract For some years, investigators have made measurements of downwelling longwave irradiance with the Eppley Precision Infrared Radiometer (PIR), recording the values of thermopile voltage and body and dome thermistor resistances and combining them in data processing. Part I of this paper reviews previous work on the processing equation and presents an improved equation. It establishes that the standard single-output Eppley has an inherent uncertainty of 5%. By measuring the three possible outputs separately and comparing them in the improved equation, the inherent accuracy can be improved to 1.5%. Part II presents a method of calibrating the Eppley PIR for the three-output equation using an easily constructed blackbody cavity in a temperature bath capable of a 0°–50°C temperature range. Calibration of PIR thermistors is recommended since occasionally one is found out of specifications. An outdoor comparison of 15 PIRs calibrated with the technique was carried out in groups of four, with one PIR used ...

Shear, Strain, and Richardson Number Variations in the Thermocline. Part I: Statistical Description

Abstract Quasi-continuous depth–time observations of shear (5.5-m, 6-min resolution) and strain, (∂η/∂z, (2-m, 2.1- min resolution) obtained from the R/P FLIP are applied to a study of Richardson number (Ri) statistics. Data were collected off the coast of central California in the 1990 Surface Waves Processes Experiment. Observations are presented in Eulerian and in isopycnal-following frames. In both frames, shear variance is found to scale as N2 in the thermocline, in agreement with previous findings of Gargett et al. The probability density function for squared shear magnitude is very nearly exponential. Strain variance is approximately uniform with depth. The magnitude of the fluctuations is sufficient to influence the Ri field significantly at finescale. To model the Richardson number, the detailed interrelationship between shear and strain must be specified. Two contrasting hypotheses are considered: One (H I) holds that fluctuations in the cross-isopycnal shear are independent of isopycnal separat...

A Modified Beam-to-Earth Transformation to Measure Short-Wavelength Internal Waves with an Acoustic Doppler Current Profiler

Abstract The algorithm used to transform velocity signals from beam coordinates to earth coordinates in an acoustic Doppler current profiler (ADCP) relies on the assumption that the currents are uniform over the horizontal distance separating the beams. This condition may be violated by (nonlinear) internal waves, which can have wavelengths as small as 100–200 m. In this case, the standard algorithm combines velocities measured at different phases of a wave and produces horizontal velocities that increasingly differ from true velocities with distance from the ADCP. Observations made in Massachusetts Bay show that currents measured with a bottom-mounted upward-looking ADCP during periods when short-wavelength internal waves are present differ significantly from currents measured by point current meters, except very close to the instrument. These periods are flagged with high error velocities by the standard ADCP algorithm. In this paper measurements from the four spatially diverging beams and the backscatt...

Moored Observations of Precipitation Temperature

Abstract Direct observations of precipitation temperature were made from a surface buoy deployed for four months in the western Pacific warm pool. The observed rain droplet temperatures are equal to the wet-bulb temperature to within the measured wet-bulb temperature uncertainty of ±0.4°C. The rain droplet temperatures are 4.8°–5.8°C cooler than the ocean surface temperature. The sensible heat flux associated with the rain is found to be a significant component for the net surface heat while it is raining, ranging from −65.0 to −204 W m−2 (ocean cooling) and accounting for 15%–60% of the net heat flux for any single rain event. The rain heat flux is also important on longer timescales in the warm pool, where there is a close balance between surface heating and cooling and high precipitation rates. During the 4-month deployment period, the rain heat flux is 2.8 W m−2 (ocean cooling) and 15% of the net surface heat flux.