PUBLICATIONS Earth and Space Science RESEARCH ARTICLE The use of hyperspectral sounding information 10.1002/2015EA000122-T to monitor atmospheric tendencies leading Key Points: to severe local storms • Hyperspectral sounders add independent information to Elisabeth Weisz1, Nadia Smith1, and William L. Smith Sr.1 existing data sources • Time series of retrievals provides 1Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison, Madison, Wisconsin, USA valuable details to storm analysis • Forecasters are encouraged to utilize hyperspectral data Abstract Operational space-based hyperspectral sounders like the Atmospheric Infrared Sounder, the Infrared Atmospheric Sounding Interferometer, and the Cross-track Infrared Sounder on polar-orbiting satellites provide radiance measurements from which profiles of atmospheric temperature and moisture can Correspondence to: be retrieved. These retrieval products are provided on a global scale with the spatial and temporal resolution E. Weisz, [email protected] needed to complement traditional profile data sources like radiosondes and model fields. The goal of this paper is to demonstrate how existing efforts in real-time weather and environmental monitoring can benefit from this new generation of satellite hyperspectral data products. We investigate how retrievals from all four Citation: Weisz, E., N. Smith, and W. L. Smith Sr. operational sounders can be used in time series to monitor the preconvective environment leading up to the (2015), The use of hyperspectral sounding outbreak of a severe local storm. Our results suggest thepotentialbenefit of independent, consistent, and information to monitor atmospheric fi tendencies leading to severe local high-quality hyperspectral pro le information to real-time monitoring applications. storms, Earth and Space Science, 2, doi:10.1002/2015EA000122-T. 1. Introduction Received 25 JUN 2015 Accepted 14 AUG 2015 The ability to understand, interpret, and ultimately accurately forecast an event relies on the real-time accu- Accepted article online 19 AUG 2015 mulation of high-quality measurements. Complex weather systems, like severe local storms, are difficult to measure in their entirety and are instead parameterized into basic physical parameters (e.g., temperature, humidity, pressure, and wind) and measured with instruments at a range of scales (e.g., at point source on the Earth’s surface or aloft from radiosondes, aircraft, and satellites). Uncertainty in the measurements and an incomplete knowledge of the physics of weather systems can contribute to the overall uncertainty about weather systems, in particular their prediction. This is especially an issue in the case of severe weather events when reliable monitoring and forecast accuracy is a matter of life and death. We determine here how recent advances in the quality of measurements and products from hyperspectral sounders may contribute to improvements in severe weather monitoring operations. While high spectral resolution (or hyperspectral) satellite sounder data have been available for more than a decade, the information available is not yet entirely understood to adequately be used in data assimilation, numerical prediction models, and in particular in satellite-based monitoring and forecasting applications. Challenges include complex radiative transfer computations and the computational cost due to the large number of channels as well as establishing adequate error characteristics. Furthermore, determining accurate atmospheric profiles, surface temperatures, trace gas concentrations, and cloud parameters from clear as well as cloud-contaminated radiances is still a difficult task, as will be discussed in section 2. Another problem has been the fact that these hyperspectral sounders are on polar-orbiting satellites, each of which gives near- global coverage over the course of a day. At low latitudes, any given location will have an overpass twice per day. Severe weather forecasting relies on timely information and therefore benefits most from measure- ments made by instruments on geostationary satellites with a regional view and high temporal frequency (e.g., every 15 min). However, the temporal frequency of satellite soundings from polar-orbiting platforms ©2015. The Authors. can be improved by using the four hyperspectral instruments now in operational orbit. The Atmospheric This is an open access article under the Infrared Sounder (AIRS), launched in 2002 on the EOS Aqua platform with an equatorial crossing time of terms of the Creative Commons 1:30 P.M. local time, was the first of this new generation of advanced sounders. The Infrared Atmospheric Attribution-NonCommercial-NoDerivs License, which permits use and distri- Sounding Interferometer (IASI) is flown on MetOp-A and MetOp-B, launched in 2006 and 2012, respectively, bution in any medium, provided the with equatorial crossing times of 9:30 and 10:40 A.M. Another innovative sounder, the Cross-track Infrared original work is properly cited, the use is non-commercial and no modifications Sounder (CrIS), has been launched in 2011 on the Suomi NPP (S-NPP) satellite. Suomi NPP is in the same or adaptations are made. orbital plane as Aqua; i.e., it crosses the equator at 1:30 P.M. but is positioned in slightly higher altitude so that WEISZ ET AL. HYPERSPECTRAL SOUNDERS TO MONITOR STORMS 1 Earth and Space Science 10.1002/2015EA000122-T the local overpass time differs from that of Aqua by as much as 45 min. Hence, since early 2013, there are at least eight measurements globally per day at low latitudes and up to an order of magnitude more per day at high-latitude locations. Hyperspectral infrared sounders report at a spatial resolution of ~13 km, which is lower than multispectral imagers, like MODIS (Moderate Resolution Imaging Spectroradiometer) and VIIRS (Visible Infrared Imaging Radiometer Suite), with 1 km or higher spatial resolution. But due to their high spectral resolution, hyperspectral sounders provide more detailed information about the vertical atmospheric structure and composition than the imagers and first-generation broadband multispectral sounders such as the High-resolution Infrared Radiation Sounder. Hence, hyperspectral sounders provide independent measurements that complement existing space-based imagers and have the potential to enhance the performance of information systems focused on weather or environmental monitoring and prediction. Several recent studies demonstrate how hyperspectral measurements improve the assimilation of the atmospheric state for numerical weather prediction [e.g., Le Marshall et al., 2009; Li and Liu, 2009; Guidard et al., 2011; Thepaut et al., 2011; Singh et al., 2012] by adding independent and detailed space-time information about the initial three-dimensional structure of the atmosphere. In this paper we do not focus on data assimilation but instead aim to describe the type and quality of infor- mation that hyperspectral sounders add to weather monitoring and short-term forecasting systems, which typically rely on combining measurements from myriad sources to characterize atmospheric and convective conditions. With this, we highlight the consistency and continuity that can be achieved in real-time among sounders on different platforms by applying the same atmospheric profile retrieval algorithm to all radiance measurements. In section 2 we give an overview of the hyperspectral sounder retrieval method, and in section 3 we explain the quality of information from these retrievals with a specific case study of the tornado outbreak that occurred in central Oklahoma on 20 May 2013. A summary is given in section 4. 2. Atmospheric Parameter Retrieval From Hyperspectral Sounders Hyperspectral infrared sounders, such as AIRS (Atmospheric Infrared Sounder) on the EOS Aqua satellite, IASI (Infrared Atmospheric Sounding Interferometer) on the MetOp-A and MetOp-B satellites, and CrIS (Cross-track Infrared Sounder) on the Suomi NPP (S-NPP) satellite, measure the top-of-atmosphere radiance emitted by the Earth system with very high spectral resolution in several thousand channels at À wavenumbers from roughly 650 to 2700 cm 1. The capability of sounders to detect atmospheric structure is determined by the vertical width of the weighting functions, the noise level, and the number of indepen- dent spectral channels associated with the radiance measurements. A weighting function, defined as the vertical derivative of the atmospheric transmittance profile, represents that fraction of the Planck radiance, associated with the temperature at each atmospheric level, which contributes to the measured radiance; it therefore determines the height region that is sensed from space by the radiance measurement at each spectral channel. In contrast to first-generation broadband sounders, which have only a few very broad weighting functions, instruments with several thousands of channels have sharper weighting functions because their spectral resolution is smaller than the spectral spacing of atmospheric absorption lines. Given a spectrum of radiance measurements, temperature and humidity profiles and other parameters can be derived using an inverse solution of the radiative transfer equation as part of the so-called inversion problem (i.e., the retrieval of the atmospheric state from atmospheric spectral radiance measurements). Theoretically, a spectrum of infrared radiances at high spectral resolution can be inverted to produce tem- perature and humidity profiles with a vertical resolution