Air-Sea surface heat flux, solar irradiance and surface temperature measurement using an in-situ sensor

A light, wave-following buoy measuring ocean surface heat flux, solar irradiance and temperature is described. This sensor float is designed such that surface tension balances against buoyancy thereby positioning two, thin flux plates within the aqueous conductive sublayer. Measurement of net heat flux and surface temperature is obtained. Each flux plate is designed with different solar absorption properties so solar irradiance can be distinguished from net heat flux.

Initial development of the sensor float included laboratory characterization as well as freshwater and ocean deployments. Testing and analysis using an improved flux plate design continues, including response to solar radiation and overall performance assessment under a variety of meteorological conditions during a series of field tests on a lake in Wisconsin.

This lake field program was undertaken using a small, instrumented catamaran. Measurements of apparent wind speed and direction, air temperature and humidity, bulk water temperature, long and short wavelength downwelling radiation were made. Surface fluxes derived from these measurements are compared with results from the in-situ sensor. The new technique demonstrates the ability to measure, with reasonable accuracy, day and nighttime net heat flux in low wind conditions. During higher wind speeds, when a significant fraction of waves are breaking, the current design sensor float has difficulty maintaining flux plate position within the conductive sublayer. Measurements of solar irradiance compare favorably with commercially available pyranometers in clear sky, partly cloudy, and overcast conditions, also only during light winds. In the absence of infrared radiometric measurements, the accuracy of sensor float surface temperature could not be determined. However, three point measurements of vertical temperature profiles in the upper two meters are compatable with observed heat fluxes.