D. C. Tobin

The Mixed-Phase Arctic Cloud Experiment

The Mixed-Phase Arctic Cloud Experiment (M-PACE) was conducted from 27 September through 22 October 2004 over the Department of Energys Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) on the North Slope of Alaska. The primary objectives were to collect a dataset suitable to study interactions between microphysics, dynamics, and radiative transfer in mixed-phase Arctic clouds, and to develop/evaluate cloud property retrievals from surface-and satellite-based remote sensing instruments. Observations taken during the 1977/98 Surface Heat and Energy Budget of the Arctic (SHEBA) experiment revealed that Arctic clouds frequently consist of one (or more) liquid layers precipitating ice. M-PACE sought to investigate the physical processes of these clouds by utilizing two aircraft (an in situ aircraft to characterize the microphysical properties of the clouds and a remote sensing aircraft to constraint the upwelling radiation) over the ACRF site on the North Slope of Alaska. The measureme...

An overview of the AIRS radiative transfer model

The two main elements of the Atmospheric Infrared Sounder radiative transfer algorithm (AIRS-RTA) are described in this paper: 1) the fast parameterization of the atmospheric transmittances that are used to perform the AIRS physical retrievals and 2) the spectroscopy used to generate the parameterized transmittances. We concentrate on those aspects of the spectroscopy that are especially relevant for temperature and water vapor retrievals. The AIRS-RTA is a hybrid model in that it parameterizes most gases on a fixed grid of pressures, while the water optical depths are parameterized on a fixed grid of water amounts. Water vapor, ozone, carbon monoxide, and methane profiles can be varied, in addition to the column abundance of carbon dioxide.

Dry Bias and Variability in Vaisala RS80-H Radiosondes: The ARM Experience

Thousands of comparisons between total precipitable water vapor (PWV) obtained from radiosonde (Vaisala RS80-H) profiles and PWV retrieved from a collocated microwave radiometer (MWR) were made at the Atmospheric Radiation Measurement (ARM) Program’s Southern Great Plains Cloud and Radiation Testbed (SGP CART) site in northern Oklahoma from 1994 to 2000. These comparisons show that the RS80-H radiosonde has an approximate 5% dry bias compared to the MWR. This observation is consistent with interpretations of Vaisala RS80 radiosonde data obtained during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). In addition to the dry bias, analysis of the PWV comparisons as well as of data obtained from dual-sonde soundings done at the SGP show that the calibration of the radiosonde humidity measurements varies considerably both when the radiosondes come from different calibration batches and when the radiosondes come from the same calibration batch. This variability can result in peak-to-peak differences between radiosondes of greater than 25% in PWV. Because accurate representation of the vertical profile of water vapor is critical for ARM’s science objectives, an empirical method for correcting the radiosonde humidity profiles is developed based on a constant scaling factor. By using an independent set of observations and radiative transfer models to test the correction, it is shown that the constant humidity scaling method appears both to improve the accuracy and reduce the uncertainty of the radiosonde data. The ARM data are also used to examine a different, physically based, correction scheme that was developed recently by scientists from Vaisala and the National Center for Atmospheric Research (NCAR). This scheme, which addresses the dry bias problem as well as other calibration-related problems with the RS80-H sensor, results in excellent agreement between the PWV retrieved from the MWR and integrated from the corrected radiosonde. However, because the physically based correction scheme does not address the apparently random calibration variations observed, it does not reduce the variability either between radiosonde calibration batches or within individual calibration batches.

Atmospheric Emitted Radiance Interferometer. Part I: Instrument Design

Abstract A ground-based Fourier transform spectrometer has been developed to measure the atmospheric downwelling infrared radiance spectrum at the earths surface with high absolute accuracy. The Atmospheric Emitted Radiance Interferometer (AERI) instrument was designed and fabricated by the University of Wisconsin Space Science and Engineering Center (UW-SSEC) for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program. This paper emphasizes the key features of the UW-SSEC instrument design that contribute to meeting the AERI instrument requirements for the ARM Program. These features include a highly accurate radiometric calibration system, an instrument controller that provides continuous and autonomous operation, an extensive data acquisition system for monitoring calibration temperatures and instrument health, and a real-time data processing system. In particular, focus is placed on design issues crucial to meeting the ARM requirements for radiometric calibration, spectral cali...

Atmospheric Emitted Radiance Interferometer. Part II: Instrument Performance

The Atmospheric Emitted Radiance Interferometer (AERI) instrument was developed for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program by the University of Wisconsin Space Science and Engineering Center (UW-SSEC). The infrared emission spectra measured by the instrument have the sensitivity and absolute accuracy needed for atmospheric remote sensing and climate studies. The instrument design is described in a companion paper. This paper describes in detail the measured performance characteristics of the AERI instruments built for the ARM Program. In particular, the AERI systems achieve an absolute radiometric calibration of better than 1% (3s) of ambient radiance, with a reproducibility of better than 0.2%. The knowledge of the AERI spectral calibration is better than 1.5 ppm (1s) in the wavenumber range 400‐ 3000 cm21.

Downwelling spectral radiance observations at the SHEBA ice station: Water vapor continuum measurements from 17 to 26μm

Earth loses energy to space in the form of longwave (or infrared) radiation. Much of this energy is radiated through the transparent portion of the water vapor rotational band from 17 to 33 μm (300 to 600 cm−1). Very few measurements have been made in this spectral region to characterize how water vapor absorbs and emits longwave radiation. An Atmospheric Emitted Radiance Interferometer (AERI) with extended longwave spectral coverage has been deployed at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station 300 miles north of the Alaskan coast to measure downwelling radiances at wavelengths of 3 to 26 μm (380 to 3000 cm−1). The spectral and radiometric performance of the instrument, installation at the ice station, and initial observations are shown. Comparisons to line-by-line radiative transfer calculations for selected clear-sky cases are presented, and air-broadened water vapor continuum absorption coefficients are determined in the wing of the pure rotational band from 17 to 26 μm (380 to 600 cm−1). Comparisons of the coefficients with the widely used Clough Kneizys Davies (CKD) water vapor continuum model suggest empirical modifications to this model are necessary. Comparisons to laboratory measurements of Burch et al. [1974] made at room temperature suggests little or no temperature dependence of the continuum from 400 to 550 cm−1. Implications of these modifications on top-of-atmosphere and surface fluxes, as well as atmospheric cooling rates, are discussed.

The ARM program's water vapor intensive observation periods - Overview, initial accomplishments, and future challenges

A series of water vapor intensive observation periods (WVIOPs) were conducted at the Atmospheric Radiation Measurement (ARM) site in Oklahoma between 1996 and 2000. The goals of these WVIOPs are to characterize the accuracy of the operational water vapor observations and to develop techniques to improve the accuracy of these measurements. The initial focus of these experiments was on the lower atmosphere, for which the goal is an absolute accuracy of better than 2% in total column water vapor, corresponding to ~1 W m−2 of infrared radiation at the surface. To complement the operational water vapor instruments during the WVIOPs, additional instrumentation including a scanning Raman lidar, microwave radiometers, chilled-mirror hygrometers, a differential absorption lidar, and ground-based solar radiometers were deployed at the ARM site. The unique datasets from the 1996, 1997, and 1999 experiments have led to many results, including the discovery and characterization of a large (> 25%) sonde-to-sonde variab...

The QME AERI LBLRTM: A Closure Experiment for Downwelling High Spectral Resolution Infrared Radiance

Abstract Research funded by the U.S. Department of Energys Atmospheric Radiation Measurement (ARM) program has led to significant improvements in longwave radiative transfer modeling over the last decade. These improvements, which have generally come in small incremental changes, were made primarily in the water vapor self- and foreign-broadened continuum and the water vapor absorption line parameters. These changes, when taken as a whole, result in up to a 6 W m−2 improvement in the modeled clear-sky downwelling longwave radiative flux at the surface and significantly better agreement with spectral observations. This paper provides an overview of the history of ARM with regard to clear-sky longwave radiative transfer, and analyzes remaining related uncertainties in the ARM state-of-the-art Line-by-Line Radiative Transfer Model (LBLRTM). A quality measurement experiment (QME) for the downwelling infrared radiance at the ARM Southern Great Plains site has been ongoing since 1994. This experiment has three...

Characterization of Upper-Troposphere Water Vapor Measurements during AFWEX Using LASE

Water vapor profiles from NASAs Lidar Atmospheric Sensing Experiment (LASE) system acquired during the ARM/FIRE Water Vapor Experiment (AFWEX) are used to characterize upper troposphere (UT) water vapor measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors. Initial comparisons showed the average Vaisala radiosonde measurements to be 5-15% drier than the average LASE, Raman lidar, and DC-8 in situ diode laser hygrometer measurements. They show that corrections to the Raman lidar and Vaisala measurements significantly reduce these differences. Precipitable water vapor (PWV) derived from the LASE water vapor profiles agrees within 3% on average with PWV derived from the ARM ground-based microwave radiometer (MWR). The agreement among the LASE, Raman lidar, and MWR measurements demonstrates how the LASE measurements can be used to characterize both profile and column water vapor measurements and that ARM Raman lidar, when calibrated using the MWR PWV, can provide accurate UT water vapor measurements.

An intercomparison of radiation codes for retrieving upper-tropospheric humidity in the 6.3-μm band: A report from the first GVaP workshop

Abstract An intercomparison of radiation codes used in retrieving upper–tropospheric humidity (UTH) from observations in the n2 (6.3 mm) water vapor absorption band was performed. This intercomparison is one part of a coordinated effort within the Global Energy and Water Cycle Experiment Water Vapor Project to assess our ability to monitor the distribution and variations of upper–tropospheric moisture from spaceborne sensors. A total of 23 different codes, ranging from detailed line–by–line (LBL) models, to coarser–resolution narrowband (NB) models, to highly parameterized single–band (SB) models participated in the study. Forward calculations were performed using a carefully selected set of temperature and moisture profiles chosen to be representative of a wide range of atmospheric conditions. The LBL model calculations exhibited the greatest consistency with each other, typically agreeing to within 0.5 K in terms of the equivalent blackbody brightness temperature(Tb). The majority of NB and SB models ag...