Jennifer Delamere

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.

Development and recent evaluation of the MT_CKD model of continuum absorption

Water vapour continuum absorption is an important contributor to the Earths radiative cooling and energy balance. Here, we describe the development and status of the MT_CKD (MlawerTobinCloughKneizysDavies) water vapour continuum absorption model. The perspective adopted in developing the MT_CKD model has been to constrain the model so that it is consistent with quality analyses of spectral atmospheric and laboratory measurements of the foreign and self continuum. For field measurements, only cases for which the characterization of the atmospheric state has been highly scrutinized have been used. Continuum coefficients in spectral regions that have not been subject to compelling analyses are determined by a mathematical formulation of the spectral shape associated with each water vapour monomer line. This formulation, which is based on continuum values in spectral regions in which the coefficients are well constrained by measurements, is applied consistently to all water vapour monomer lines from the microwave to the visible. The results are summed-up (separately for the foreign and self) to obtain continuum coefficients from 0 to 20 000 cm−1. For each water vapour line, the MT_CKD line shape formulation consists of two components: exponentially decaying far wings of the line plus a contribution from a water vapour molecule undergoing a weak interaction with a second molecule. In the MT_CKD model, the first component is the primary agent for the continuum between water vapour bands, while the second component is responsible for the majority of the continuum within water vapour bands. The MT_CKD model should be regarded as a semi-empirical model with strong constraints provided by the known physics. Keeping the MT_CKD continuum consistent with current observational studies necessitates periodic updates to the water vapour continuum coefficients. In addition to providing details on the MT_CKD line shape formulation, we describe the most recent update to the model, MT_CKD_2.5, which is based on an analysis of satellite- and ground-based observations from 2385 to 2600 cm−1 (approx. 4 μm).

Assessing 1D Atmospheric Solar Radiative Transfer Models: Interpretation and Handling of Unresolved Clouds

Abstract The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport ...

The Continual Intercomparison of Radiation Codes: Results from Phase I

[1] We present results from Phase I of the Continual Intercomparison of Radiation Codes (CIRC), intended as an evolving and regularly updated reference source for evaluation of radiative transfer (RT) codes used in global climate models and other atmospheric applications. CIRC differs from previous intercomparisons in that it relies on an observationally validated catalog of cases. The seven CIRC Phase I baseline cases, five cloud free and two with overcast liquid clouds, are built around observations by the Atmospheric Radiation Measurements program that satisfy the goals of Phase I, namely, to examine RT model performance in realistic, yet not overly complex, atmospheric conditions. Besides the seven baseline cases, additional idealized “subcases” are also employed to facilitate interpretation of model errors. In addition to quantifying individual model performance with respect to reference line-by-line calculations, we also highlight RT code behavior for conditions of doubled CO2, issues arising from spectral specification of surface albedo, and the impact of cloud scattering in the thermal infrared. Our analysis suggests that improvements in the calculation of diffuse shortwave flux, shortwave absorption, and shortwave CO2 forcing as well as in the treatment of spectral surface albedo should be considered for many RT codes. On the other hand, longwave calculations are generally in agreement with the reference results. By expanding the range of conditions under which participating codes are tested, future CIRC phases will hopefully allow even more rigorous examination of RT codes.

Air-Broadened Half-Widths of the 22- and 183-GHz Water-Vapor Lines

Air-broadened half-widths of the 22- and 183-GHz water-vapor lines and associated uncertainties have been determined using comparisons between ground-based radiometric measurements from Atmospheric Radiation Measurement sites in Oklahoma and Alaska, and MonoRTM, a radiative transfer model. Values of the widths obtained using the measurements are 0.0900 cm-1/atm with 1.6% uncertainty for the 22-GHz line and 0.0992 cm-1/atm with 2.4% uncertainty for the 183-GHz line. Also presented are spectroscopic parameters for these lines from new calculations performed using the complex implementation of the Robert-Bonamy theory (CRB). The CRB values of the air-broadened widths are 0.0913 cm-1/atm with 3% uncertainty and a temperature exponent of 0.755 for the 22-GHz line and 0.0997 cm-1/atm with 3% uncertainty and a temperature exponent of 0.769 for the 183-GHz line. The values for the air-broadened half-widths derived from the measurement/model comparisons show good agreement with the new CRB calculations. For future versions of MonoRTM, width values of 0.0900 and 0.0997 cm-1/atm are to be adopted with temperature dependences of 0.76 and 0.77 for the 22- and 183-GHz lines, respectively.

Ground-based high spectral resolution observations of the entire terrestrial spectrum under extremely dry conditions

[1] A field experiment was conducted in northern Chile at an altitude of 5.3 km to evaluate the accuracy of line-by-line radiative transfer models in regions of the spectrum that are typically opaque at sea level due to strong water vapor absorption. A suite of spectrally resolved radiance instruments collected simultaneous observations that, for the first time ever, spanned the entire terrestrial thermal spectrum (i.e., from 10 to 3000 cm 1 , or 1000 to 3.3 mm). These radiance observations, together with collocated water vapor and temperature profiles, are used to provide an initial evaluation of the accuracy of water vapor absorption in the farinfrared of two line-by-line radiative transfer models. These initial results suggest that the more recent of the two models is more accurate in the strongly absorbing water vapor pure rotation band. This result supports the validity of the Turner et al. (2012) study that demonstrated that the use of the more recent water vapor absorption model in climate simulations resulted in significant radiative and dynamical changes in the simulation relative to the older water vapor model. Citation: Turner, D. D., et al. (2012), Ground-based high spectral resolution observations of the entire terrestrial spectrum under extremely dry conditions, Geophys. Res. Lett., 39, L10801,

Radiative Transfer Modeling for Hyperspectral Applications: Status and Validation of LBLRTM

The Line-By-Line Radiative Transfer Model, its associated spectroscopic databases and continua are subject to ongoing validation against measurements spanning submillimeter to visible wavelengths. Here we present examples of recent updates in the far- and mid-infrared.

The Continual Intercomparison of Radiation Codes (CIRC): A New Standard for Evaluating GCM Radiation Codes

The Continual Intercomparison of Radiation Codes (CIRC) is intended as an evolving and regularly updated permanent reference source for GCM‐type radiative transfer (RT) code evaluation that will help in the improvement of radiation parameterizations. CIRC seeks to establish itself as the standard against which code performance is documented in scientific publications and coordinated joint modeling activities such as GCM intercomparisons. A feature that distinguishes CIRC from previous intercomparisons is that its pool of cases is largely based on observations. Atmospheric and surface input, as well as radiative fluxes used for consistency checks with the reference line‐by‐line calculations come primarily from the Atmospheric Radiation Measurement (ARM) Climate Research Facility measurements and satellite observations compiled in the Broadband Heating Rate Profile (BBHRP) product. Additional datasets beyond BBHRP such as measurements from ARM field campaigns and spectral radiances from the AERI instrument are also used to complete the set of desired cases and to ensure the quality of the input. For Phase I, launched in June, CIRC aims to assess the baseline errors of GCM RT codes and therefore provides test cases that evaluate performance under the least challenging conditions, i.e, well‐understood clear‐sky and homogeneous, single‐layer overcast liquid cloud cases.The Continual Intercomparison of Radiation Codes (CIRC) is intended as an evolving and regularly updated permanent reference source for GCM‐type radiative transfer (RT) code evaluation that will help in the improvement of radiation parameterizations. CIRC seeks to establish itself as the standard against which code performance is documented in scientific publications and coordinated joint modeling activities such as GCM intercomparisons. A feature that distinguishes CIRC from previous intercomparisons is that its pool of cases is largely based on observations. Atmospheric and surface input, as well as radiative fluxes used for consistency checks with the reference line‐by‐line calculations come primarily from the Atmospheric Radiation Measurement (ARM) Climate Research Facility measurements and satellite observations compiled in the Broadband Heating Rate Profile (BBHRP) product. Additional datasets beyond BBHRP such as measurements from ARM field campaigns and spectral radiances from the AERI instrument ...