Longwave Radiative Transfer In The Atmosphere: Model Development And Applications

Thesis (Ph.D.) University of Alaska Fairbanks, 2003A FLexible Radiative Transfer Tool (FLRTT) has been developed to facilitate the construction of longwave, correlated k-distribution, radiative transfer models. The correlated k-distribution method is a technique which accelerates calculations of radiances, fluxes, and cooling rates in inhomogeneous atmospheres; therefore, correlated k-distribution models are appropriate for simulations of satellite radiances and inclusion into general circulation models. FLRTT was used to build two new rapid radiative transfer models, RRTM_HIRS and RRTM_v3.0, which maintain accuracy comparable to the line-by-line radiative transfer model LBLRTM. Iacono et al. [2003] evaluated upper tropospheric water vapor (UTWV) simulated by the National Center for Atmospheric Research Community Climate Model, CCM3, by comparing modeled, clear-sky brightness-temperatures to those observed from space by the High-resolution Radiation Sounder (HIRS). CCM3 was modified to utilize the rapid radiative transfer model RRTM and the separate satellite-radiance module, RRTM_HIRS, which calculates brightness temperatures in two HIRS channels. By incorporating these accurate radiative transfer models into CCM3, the longwave radiative transfer calculations have been removed as a significant source of error in the simulations. An important result of this study is that CCM3 exhibits moist and dry discrepancies in UTWV of 50% in particular climatic regions, which may be attributed to errors in the CCM3 dynamical schemes. RRTM_v3.0, an update of RRTM, is a rapid longwave radiative transfer appropriate for use in general circulation models. Fluxes calculated by RRTM_v3.0 agree with those computed by the LBLRTM to within 1.0 W/m2 at all levels, and the computed cooling rates agree to within 0.1 K/day and 0.3 K/day in the troposphere and stratosphere, respectively. This thesis also assessed and improved the modeling of clear-sky, longwave radiative fluxes at the Atmospheric Radiation Measurement Program North Slope of Alaska site by simultaneously addressing the specification of the atmosphere, radiometric measurements, and radiative transfer modeling. Consistent with findings from other field sites, the specification of the atmospheric water vapor is found to be a large source of uncertainty in modeled radiances and fluxes. Improvements in the specification of carbon dioxide optical depths within LBLRTM resulted, in part, from this analysis

The Continual Intercomparison of Radiation Codes: Results from Phase I

The computer codes that calculate the energy budget of solar and thermal radiation in Global Climate Models (GCMs), our most advanced tools for predicting climate change, have to be computationally efficient in order to not impose undue computational burden to climate simulations. By using approximations to gain execution speed, these codes sacrifice accuracy compared to more accurate, but also much slower, alternatives. International efforts to evaluate the approximate schemes have taken place in the past, but they have suffered from the drawback that the accurate standards were not validated themselves for performance. The manuscript summarizes the main results of the first phase of an effort called "Continual Intercomparison of Radiation Codes" (CIRC) where the cases chosen to evaluate the approximate models are based on observations and where we have ensured that the accurate models perform well when compared to solar and thermal radiation measurements. The effort is endorsed by international organizations such as the GEWEX Radiation Panel and the International Radiation Commission and has a dedicated website (i.e., http://circ.gsfc.nasa.gov) where interested scientists can freely download data and obtain more information about the effort's modus operandi and objectives. In a paper published in the March 2010 issue of the Bulletin of the American Meteorological Society only a brief overview of CIRC was provided with some sample results. In this paper the analysis of submissions of 11 solar and 13 thermal infrared codes relative to accurate reference calculations obtained by so-called "line-by-line" radiation codes is much more detailed. We demonstrate that, while performance of the approximate codes continues to improve, significant issues still remain to be addressed for satisfactory performance within GCMs. We hope that by identifying and quantifying shortcomings, the paper will help establish performance standards to objectively assess radiation code quality, and will guide the development of future phases of CIR

Radiative Forcing by Long-Lived Greenhouse Gases: Calculations with the AER Radiative Transfer Models

A primary component of the observed, recent climate change is the radiative forcing from increased concentrations of long-lived greenhouse gases (LLGHGs). Effective simulation of anthropogenic climate change by general circulation models (GCMs) is strongly dependent on the accurate representation of radiative processes associated with water vapor, ozone and LLGHGs. In the context of the increasing application of the Atmospheric and Environmental Research, Inc. (AER) radiation models within the GCM community, their capability to calculate longwave and shortwave radiative forcing for clear sky scenarios previously examined by the radiative transfer model intercomparison project (RTMIP) is presented. Forcing calculations with the AER line-by-line (LBL) models are very consistent with the RTMIP line-by-line results in the longwave and shortwave. The AER broadband models, in all but one case, calculate longwave forcings within a range of -0.20 to 0.23 W m{sup -2} of LBL calculations and shortwave forcings within a range of -0.16 to 0.38 W m{sup -2} of LBL results. These models also perform well at the surface, which RTMIP identified as a level at which GCM radiation models have particular difficulty reproducing LBL fluxes. Heating profile perturbations calculated by the broadband models generally reproduce high-resolution calculations within a few hundredths K d{sup -1} in the troposphere and within 0.15 K d{sup -1} in the peak stratospheric heating near 1 hPa. In most cases, the AER broadband models provide radiative forcing results that are in closer agreement with high 20 resolution calculations than the GCM radiation codes examined by RTMIP, which supports the application of the AER models to climate change research

Balancing Accuracy, Efficiency, and Flexibility in Radiation Calculations for Dynamical Models

This paper describes the initial implementation of a new toolbox that seeks to balance accuracy, efficiency, and flexibility in radiation calculations for dynamical models. The toolbox consists of two related code bases: Radiative Transfer for Energetics (RTE), which computes fluxes given a radiative transfer problem defined in terms of optical properties, boundary conditions, and source functions; and RRTM for General circulation model applications&mdash;Parallel (RRTMGP), which combines data and algorithms to map a physical description of the gaseous atmosphere into such a radiative transfer problem. The toolbox is an implementation of well‐established ideas, including the use of a k‐distribution to represent the spectral variation of absorption by gases and the use of two‐stream, plane‐parallel methods for solving the radiative transfer equation. The focus is instead on accuracy, by basing the k‐distribution on state‐of‐the‐art spectroscopy and on the sometimes‐conflicting goals of flexibility and efficiency. Flexibility is facilitated by making extensive use of computational objects encompassing code and data, the latter provisioned at runtime and potentially tailored to specific problems. The computational objects provide robust access to a set of high‐efficiency computational kernels that can be adapted to new computational environments. Accuracy is obtained by careful choice of algorithms and through tuning and validation of the k‐distribution against benchmark calculations. Flexibility with respect to the host model implies user responsibility for maps between clouds and aerosols and the radiative transfer problem, although comprehensive examples are provided for clouds. </div

Day–night monitoring of volcanic so2 and ash clouds for aviation avoidance at northern polar latitudes

We describe NASA’s Applied Sciences Disasters Program, which is a collaborative project between the Direct Readout Laboratory (DRL), ozone processing team, Jet Propulsion Laboratory, Geographic Information Network of Alaska (GINA), and Finnish Meteorological Institute (FMI), to expedite the processing and delivery of direct readout (DR) volcanic ash and sulfur dioxide (SO2) satellite data. We developed low-latency quantitative retrievals of SO2 column density from the solar backscattered ultraviolet (UV) measurements using the Ozone Mapping and Profiler Suite (OMPS) spectrometers as well as the thermal infrared (TIR) SO2 and ash indices using Visible Infrared Imaging Radiometer Suite (VIIRS) instruments, all flying aboard US polar-orbiting meteorological satellites. The VIIRS TIR indices were developed to address the critical need for nighttime coverage over northern polar regions. Our UV and TIR SO2 and ash software packages were designed for the DRL’s International Planetary Observation Processing Package (IPOPP); IPOPP runs operationally at GINA and FMI stations in Fairbanks, Alaska, and Sodankylä, Finland. The data are produced within 30 min of satellite overpasses and are distributed to the Alaska Volcano Observatory and Anchorage Volcanic Ash Advisory Center. FMI receives DR data from GINA and posts composite Arctic maps for ozone, volcanic SO2, and UV aerosol index (UVAI, proxy for ash or smoke) on its public website and provides DR data to EUMETCast users. The IPOPP-based software packages are available through DRL to a broad DR user community worldwide