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L2P Dynamic Flags

Introduction

GHRSST-PP L2P dynamic flays are designed to provide a user with full control over which SST data are useful for a given application. When a data provider makes decisions to reject SST observations (e.g., data are assumed to be corrupt by atmospheric aerosols) these decisions are made to provide the most accurate SST data sets possible. However, a large proportion of acceptable SST's derived from infrared satellite instruments may also be flagged as bad due to AOD impacts. Dynamic flags allow users to flag data according to the best available information describing the environmental conditions at the time of SST measurement. In this way, users can choose what data to flag above that already removed by a data provider.

The following L2P dynamic flags are provided in version 1.7 of the GHRSST-PP GDS:

DT_analysis

The difference between the observational data and the previous SST analysis provided by US GODAE (FNMOC analysis). This field is useful as it can reveal signkificant deviations in SST (assuming the analysis is correct) helpoing to identify suspect data. Problems of cloud clearing, aerosol contamination and diurnal variability can be considered. DT_analysis provides a simple sanity check on the SST data provided in the L2P file. It can be used dynamically by setting a threshold deviation beyond which data are rejected from your application.

Surface Wind Speed

The surface wind speed provided either from an NWP system or contemperaneous satellite measurement (as in the case of the AMSR-E). The purpse of providing a co-located wind speed estimate is to provide some information as to the environmental conditions at the air-sea interface. If there is a moderate or steong wind speed, then the relationship between SSTskin and SSTdepth is likely to be be good. However, for microwave SST retrievals this may indicate a reduced quality data set as surface foam can affect the microwave emissivity of seawater.

Surface Solar Irradiance(SSI)

An NWP or satellite derived measure of SSI is provided to consider the potential for significant thermal stratification of the sea surface (diurnal warming) and can be used to parameterise diurnal effects. Typically, SSI is used together with the surface wind speed.

Aerosol Optical Depth (AOD)

A model (e.g. NAAPS)or satellite derived estimate of AOD is extremely useful to asess the quality of infrared SST retrievals. Aerosols in the atmosphere are cooler than the SST and can make a significant contribution to the measured signal in space. Significant errors are associatyed with aeosol effects from volcaneoes and desert dust.

Sea Ice Concentration

Satellite derived sea ice concentration is extremely important if accurate and reliable SST is to be successfully retrieved from space in the marginal ice zone. Knowing the concentration of sea ice within an area is useful to consider the quality of the SST retrieval.

Solar Zenith Angle(SZA)

Satellite specific SZA is extremely important in the context of wide swath infrared satellite sensors. At the edge of the satellite swath, the atmopsphereic pathlength increases so that atmospheric correction algorithnms are less reliable. In fact, in certain circumstances and very wide zenith angles, the retrieved SST may be reliant on a strong coupling to the air temperature rather which is where the majority of the measured signal will originate from.