An Overview of the Naval Research Laboratory ocean surface flux (NFLUX) system

The net heat flux at the ocean interface is an important aspect of ocean, atmospheric, and coupled air-sea forecast modelling. The four components of the net heat flux are insolation, net longwave radiation, latent heat flux, and sensible heat flux. Operational forecast models use numerical weather prediction (NWP) models to provide the heat flux fields because the existing satellite-based products are not produced operationally and individual in situ and satellite observations lack adequate temporal and spatial coverage. The Naval Research Laboratory (NRL) ocean surface flux (NFLUX) system offers an alternative to the near real time NWP heat flux fields by providing operational satellite-based heat flux fields over the global ocean. The NFLUX system also provides a mechanism for using satellite and in situ observations of the air-sea interface to assess and monitor NWP products.

NFLUX is an end-to-end data processing and assimilation system consisting of three primary components: satellite swath-level data processing, automated quality control, and two-dimensional variational analysis. Swath-level air temperature, specific humidity, and wind speed estimates are produced utilizing satellite sensor data records (SDR) from the Special Sensor Microwave Imager/Sounder, the Advanced Microwave Sounding Unit-A, the Advanced Technology Microwave Sounder, and the Advanced Microwave Scanning Radiometer-2 sensors. The satellite SDRs are translated into estimates of the surface state parameters using multiple polynomial regression algorithms based on a series expansion of the hyperbolic tangent function. Swath-level wind speed estimates are also retrieved from satellite environmental data records from WindSat, the Advanced scatterometers, and the Oceansat scatterometer.

Swath-level solar and longwave radiative flux estimates are produced utilizing the Rapid Radiative Transfer Model for Global Circulation Models (RRTMG). The primary inputs to the RRTMG, swath-level atmospheric temperature / moisture profiles, come from the Microwave Integrated Retrieval System (MIRS). MIRS also provides corresponding swath-level cloud liquid / ice water paths. Other inputs to the RRTMG include: aerosol optical depths from the NRL Aerosol Analysis and Prediction System, ozone from the Stratosphere Monitoring Ozone Blended Analysis, sea surface temperature from the Hybrid Coordinate Ocean Model, and trace gas amounts from the World Meteorological Organization.

All swath-level satellite estimates and in situ observations undergo an automated quality control process. The quality controlled observations are then assimilated with atmospheric model forecasts to produce gridded analysis fields of air temperature, specific humidity, wind speed, solar flux, and longwave radiative flux. The turbulent heat flux fields, latent and sensible heat flux, are determined from the Coupled Ocean-Atmosphere Response Experiment (COARE) 3.0 bulk algorithms using inputs of air temperature, specific humidity, wind speed, and a sea surface temperature model field.

The NFLUX system produces 3-hourly global gridded analysis fields. Quality-controlled in situ observations over a one-year time period from May 2013 through April 2014 form the reference for validating estimated ocean surface state parameter and heat flux fields. The NFLUX fields are evaluated alongside the Navy's operational global atmospheric model, the Navy Global Environmental Model (NAVGEM). NFLUX is shown to have smaller biases and lower or similar root mean square errors compared to NAVGEM.