In order to produce high-resolution atmospheric temperature and water vapor retrievals, a sounding instrument must achieve near continuous spectral coverage throughout the 600 - 2600 cm-1 region with a spectral resolution of 0.1% (Smith et al. 1979, 1983) in order to avoid smearing of absorption and transparent regions of the spectra.
For example, in the CO2 region (600 - 700 cm-1), spectral resolution of 0.7 cm-1 is required since the line spacing is approximately 1.5 cm-1. HIS radiance spectra obtained since 1986 have a resolution of 0.35 cm-1 in the 600 - 1000 cm-1 region (Smith, 1990), far superior to the 15.0 cm-1 resolution of the current GOES I/M filter wheel radiometer.
Vertical resolution of atmospheric sounders are dependent upon the width and number of weighting functions observable with the instrument. Temperature profile weighting functions, the vertical derivative of atmospheric transmittance with respect to the natural logarithm of pressure (Smith, 1990), indicate from which layer the observed radiation originates, and the relative strength of the radiation. The spectral smearing mentioned above causes unwanted absorption contamination in atmospheric "windows" used for sensing the earth's surface temperature, and it greatly limits the vertical resolution of temperature and water vapor profiles because it broadens the atmospheric weighting functions.
As can be clearly seen by the above figures, HIS temperature profile weighting functions are far more numerous and have much smaller half-widths than current GOES-VAS weighting functions, enabling finer scale analysis of atmospheric temperature and water vapor retrievals. HIS aircraft vertical resolution is on the order of 1.0 km near the surface (3.0 near 100 mb), as compared to the relatively poor 5.0 km resolution of the GOES-VAS. Horizontal resolution of the HIS is also far superior, with a resolution of about 2.0 km as compared to the 7.0 km of the GOES-VAS.
For the retrieval of atmospheric profiles from the HIS, it is desirable to utilize all spectral observations in a simultaneous solution for temperature, water vapor, and other desired absorbing constituents (e.g., ozone, methane, etc.). Such a general solution has been formulated for treating the HIS spectra (Smith et al. 1988, 1990, 1991, Huang, 1989). Currently at the Cooperative Institute of Meteorological Satellite Studies (CIMSS) , a "physical/statistical simultaneous" retrieval algorithm is being used to transform the HIS radiance spectra into atmospheric temperature and water vapor profiles. The algorithm is based upon a linear form of the Radiative Transfer Equation (RTE), making full use of the high spectral coverage and resolution of the HIS instrument.
The HIS aircraft instrument has flown more than 100 times since its initial flight back in Spring of 1986, producing near 100,000 individual radiance spectra of a wide variety of atmospheric conditions worldwide. The HIS has participated in such field experiments as ASHOE, STORM-FEST, COHMEX, FIRE I and II, and CaPE, producing atmospheric temperature, water vapor, and cooling rate profiles in both northern and southern hemispheres over land and water at low and high latitudes.
HIS temperature and water vapor retrievals have been compared with hundreds of atmospheric profiles collected by National Weather Service and special "project related" radiosondes in order to show the accuracy of the retrievals produced by the HIS.
In order to further demonstrate the power of the HIS for meteorological analysis, case studies of mesoscale atmospheric structures have been performed using HIS retrievals (Bradshaw and Fuelberg, 1993, Olander, 1993). The following figures are of the Florida Sea-Breeze Front on August 8, 1991, as analyzed by the HIS flown aboard a NASA/ER2, during the CaPE field experiment.
Also, HIS temperature retrievals have been compared with various atmospheric models and retrievals, such as the Nested Grid Model (NGM), the European Center for Medium-range Weather Forecasts (ECMWF) model, the United Kingdom Meteorological Office (UKMO) model, TIROS Operational Vertical Sounder (TOVS) retrievals, and the Australian Bureau of Meteorology Global Assimilation and Prediction System (GASP) model.
Another highly important product of the HIS are measurements of ocean skin surface temperatures. Direct measurements of ocean skin surface temperatures have been performed utilizing the instrument's ability to resolve the fine scale structure of the spectra in the water vapor window region of Band I (800 - 1000 cm-1). Direct comparisons with ocean buoy water temperature measurements have shown the accuracy of the HIS in determining the skin surface temperature from 20 km (Nalli 1995).
Finally, the HIS has demonstrated its ability to retrieve atmospheric ozone profiles and column densities from upwelling radiance observations (Knuteson et al., 1993). Comparisons with ozonesonde profiles clearly indicate the ability of the HIS instrument to retrieve ozone profiles, derived utilizing the inversion algorithm of the linearized radiative transfer equation (Lee, 1992), within the upper troposphere and lower stratosphere.