TITLE:
TOVS-Derived Downwelling Surface Longwave Radiation in the Arctic 1979-2005

AUTHOR:
Dr. Jennifer Francis
Institute of Marine and Coastal Sciences, Rutgers University
-Email: francis@imcs.rutgers.edu
-Mailing address: 107 Cove Circle, Marion, MA 02738 USA
Phone: (732) 688-2382

FUNDING SOURCE AND GRANT NUMBER:
NSF/OPP-0240791, Atmospheric Radiation Measurement (ARM) project 0007366.

DATA SET OVERVIEW:
-Abstract: from Francis (1997) The dominant component of the polar surface energy budget during half the year is the downwelling flux of longwave radiation (DLF), yet little is known about its spatial and temporal variability except on monthly timescales. As surface measurements will always be sparse, the most promising opportunity for diagnosing the DLF is provided by satellite data. Estimating this flux from space, however, presents challenges over all surface types and particularly in polar environments where cloud detection and cloud fraction estimation are less certain. A new method is presented to estimate DLF from measurements by the TIROS-N operational vertical sounder (TOVS). Temperature profiles, humidity estimates, and cloud cover are retrieved from TOVS radiances using the improved initialization inversion algorithm, which has been modified to produce more accurate results over snow and sea ice. This information is combined with brightness temperature differences from pairs of infrared and near-infrared TOVS channels. These differences are used to infer cloud phase and geometric thickness. Longwave fluxes are then calculated using a forward radiative transfer model. Results during winter 1988 and spring 1992 are compared with hourly radiation measurements form the Coordinated Eastern Arctic Experiment in the eastern Arctic basin and from the Lead Experiment in the Beaufort Sea. Error analyses yield a bias of approximately 3 W m-2, a standard deviation of 23 W m-2, and a correlation coefficient of about 0.75. These errors are comparable to results from similar studies over midlatitude land and ocean areas where clouds are more easily identified.
-Time period covered by the data: daily, 1979-2005
-Area coverage, grid: north of 70oN, 100km x 100km EASE grid
-Data source: TOVS radiances obtained from NOAA NESDIS

INSTRUMENT DESCRIPTION: The TIROS Operational Vertical Sounder (TOVS) system consisted of three instruments: the High Resolution Infrared Radiation Sounder Version 2 (HIRS/2), the Microwave Sounding Unit (MSU) and the Stratospheric Sounding Unit (SSU). This system was onboard the NOAA-6 through NOAA-14 and TIROS-N satellites. See Francis (1994) and Scott et al (1999) for further detail.

DATA COLLECTION and PROCESSING: See Francis (1997). -Data intercomparisons, if applicable: Comparison with surface downwelling longwave radiation measurements from the Surface Heat Balance of the Arctic (SHEBA) experiment (1997/98) can be viewed here: http://marine.rutgers.edu/~francis/graphs/francis13.jpg and comparisons with measurements at the Barrow, AK, ARM site.

DATA FORMAT:
The data is in HDF4 SDS file format, one file per day. The general naming convention is:

nflx.(YEAR)(JULIAN).hdf, example - nflx.1995180.hdf

It is arranged on a 67 by 67 grid, with each grid point 100km apart.
Each file contains:

X - coordinates in X direction, km
Y - coordinates in Y direction, km
NLWD - Longwave Down, Wm^-2
NLWU - Longwave Up, Wm^-2
NSWD - Shortwave Down, Wm^-2
NSWU - Shortwave Up, Wm^-2
CLDBOT - Cloud Bottom Height, m
CLDTOP - Cloud Top Height, m
CLDWC - Cloud Water Content, gm^-3
CLDRE - Cloud Droplet Radius, um

Generally, each variable will contain the following attributes: valid_range, long_name, units, coordsys, _FillValue, scale_factor, scale_factor_err, add_offset, add_offset_err, calibrated_nt
Missing/fill data is represented by -9999, and the valid range of each can be found in the valid_range attribute attached to each HDF file. All values are daily averages. All files are considered 1.0, created 2001

DATA REMARKS:
-Missing Dates: Files missing for multiple days throughout the data set. Please be advised when reading for yearly plots.
-- Shortwave data were obtained from the AVHRR Polar Pathfinder data set. Please see their website for further information: http://stratus.ssec.wisc.edu/products/appx/appx.html
-Software compatibility: Libraries exist for reading into C, Fortran, IDL, and MATLAB. Viewers exist at the HDF Group's website, http://www.hdfgroup.org/, as well as conversion tools to convert from HDF4 to HDF5.

---REFERENCES:
Francis, J. A. (1994), Improvements to TOVS retrievals over sea ice and applications to estimating Arctic energy fluxes, J. Geophys. Res., 99, 10,395-10,408.
Francis, J. A. (1997), A method to derive longwave fluxes at the Arctic surface from TIROS operational vertical sounder data, J. Geophys. Res., 102, 1795-1806.
Scott, N. A., A. Chedin, R. Armante, J. Francis, C. Stubenrauch, J.-P. Chaboureau, F. Chevallier, C. Claud, and F. Cheruy (1999), Characteristics of the TOVS pathfinder path-b dataset. Bull. Am. Meteor. Soc. 80 2679-701.