374 Background 

Accurate readings of Sea Surface Temperature (SST) are crucial to modeling water-energy cycles, which  are used for weather prediction and understanding c limate change. Satellite measurements cover broad  areas but must be calibrated with actual surface m easurements of the water temperature. Current NASA  satellites such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High  Resolution Radiometer (AVHRR) operated by NO AA, provide SST measurements with a spatial  resolution of one kilometer and a precision of 0.1 °C. These satellite SST sensors are calibrated with shipborne SST instruments that provide a few single po int measurements. The ship-board instruments are  very expensive to build, operate, and deploy.    The BESST modular radiometer offers an alternativ e for calibration and vali dation of on-orbit SST  instruments. This alternative uses a small, lightweight radiometer integrated to an Unmanned Aerial Vehicle or conventional aircraft to provide highly accurate, high-resolution SST data for frequent, lowercost validation of satellite dat a. The BESST radiometer system has a precision of 0.1 °C and can be flown  quickly over a large area providing a more complete  calibration/validation of image data from the orbiting  satellite than is available from ship-borne instruments.    In addition to calibration and validation of satellite  measurements, the BESST radiomet er can perform studies not  previously possible with current SST instruments. BESST can perform high spatial resolution mapping of single satellite pixels to look for small-scale variability in infrared SST patterns. This is  particularly important near coastal areas and where rivers empty into coastal waters and form es tuaries critical to near-shore  activities. Understanding small-scale variability will provide insight and may help close the air-sea heat budget. Figure 2 shows the small-scale variability present within satellite pixel  data. Data from the MODIS satellite, large square pixels, is shown with 2 strips of data from a BESST airborne measurement overlaid. The BESST plot clearly shows temperature variability within that region that can significantly affect ocean and climate models.  Another possible application for the BESST radiometer is  the study of diurnal warming. Variability of SST  on sub-day time scales is often dominated by the diurnal solar warm layer phenomenon. This warm layer is stronger in lighter winds and solar forcing. In light  winds (<2 m/s) it can exc eed 2 percent. The depth is  inversely proportional to the strength.   There are a number of ways to account for the warm layer in relating SST to the actual subsurface temperature structure. Fairall et al. (1996a) present a simple 1-D model integrated into the COARE bulk  flux model (Fairall et al., 1996b, 2003). More rece ntly, Zeng and Beljaars (2005) developed a similar  scheme that has been implemented at European  Centre for Medium-Range Weather Forecasts  (ECMWF) (Brunke et al., 2008). The Fairall scheme was developed and optimized for tropical applications  and does not include physics to deal with turning of the near-surface currents by the Coriolis force. Thus,  the timing and magnitude of this warm layer at mid-latitudes may not be well-described by the model.  To date, studies of diurnal warming in the SST measurements have been limited to multi-day  measurements indicating amplitude change over ti me. The BESST radiometer can be deployed over a  region for full 2D spatial mapping of the diurnal signal at a specific time making it a unique tool for the SST community. Diurnal warming has been recently i dentified by a NASA study group on SST as the  primary uncertainty in satellite SST measurements, and this unique capability of the BESST to study this Figure 2:  BESST data showing fine  structure within MODIS single pixels Single  MODIS  Pixel  BESST  NASA/CP-2010-216272