Timothy R. Shippert

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...

Atmospheric longwave irradiance uncertainty: Pyrgeometers compared to an absolute sky‐scanning radiometer, atmospheric emitted radiance interferometer, and radiative transfer model calculations

Because atmospheric longwave radiation is one of the most fundamental elements of an expected climate change, there has been a strong interest in improving measurements and model calculations in recent years. Important questions are how reliable and consistent are atmospheric longwave radiation measurements and calculations and what are the uncertainties? The First International Pyrgeometer and Absolute Sky-scanning Radiometer Comparison, which was held at the Atmospheric Radiation Measurement programs Southern Great Plains site in Oklahoma, answers these questions at least for midlatitude summer conditions and reflects the state of the art for atmospheric longwave radiation measurements and calculations. The 15 participating pyrgeometers were all calibration-traced standard instruments chosen from a broad international community. Two new chopped pyrgeometers also took part in the comparison. An absolute sky-scanning radiometer (ASR), which includes a pyroelectric detector and a reference blackbody source, was used for the first time as a reference standard instrument to field calibrate pyrgeometers during clear-sky nighttime measurements. Owner-provided and uniformly determined blackbody calibration factors were compared. Remarkable improvements and higher pyrgeometer precision were achieved with field calibration factors. Results of nighttime and daytime pyrgeometer precision and absolute uncertainty are presented for eight consecutive days of measurements, during which period downward longwave irradiance varied between 260 and 420 W m−2. Comparisons between pyrgeometers and the absolute ASR, the atmospheric emitted radiance interferometer, and radiative transfer models LBLRTM and MODTRAN show a surprisingly good agreement of <2 W m−2 for nighttime atmospheric longwave irradiance measurements and calculations.

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.

Physically Based Global Downscaling: Regional Evaluation

Abstract The climate simulated by a global atmosphere–land model with a physically based subgrid orography scheme is evaluated in 10 selected regions. Climate variables simulated for each of multiple elevation classes within each grid cell are mapped according to a high-resolution distribution of surface elevation in each region. Comparison of the simulated annual mean climate with gridded observations leads to the following conclusions. At low to moderate elevations the downscaling scheme correctly simulates increasing precipitation, decreasing temperature, and increasing snow with increasing elevation across distances smaller than 100 km. At high elevations the downscaling scheme correctly simulates decreasing precipitation with increasing elevation. The rain shadow of many mountain ranges is poorly resolved, with too little precipitation simulated on the windward side of mountain ranges and too much on the lee side. The simulated sensitivity of surface air temperature to surface elevation is too strong...

Deriving Arctic Cloud Microphysics at Barrow, Alaska: Algorithms, Results, and Radiative Closure

AbstractCloud phase and microphysical properties control the radiative effects of clouds in the climate system and are therefore crucial to characterize in a variety of conditions and locations. An Arctic-specific, ground-based, multisensor cloud retrieval system is described here and applied to 2 yr of observations from Barrow, Alaska. Over these 2 yr, clouds occurred 75% of the time, with cloud ice and liquid each occurring nearly 60% of the time. Liquid water occurred at least 25% of the time, even in winter, and existed up to heights of 8 km. The vertically integrated mass of liquid was typically larger than that of ice. While it is generally difficult to evaluate the overall uncertainty of a comprehensive cloud retrieval system of this type, radiative flux closure analyses were performed in which flux calculations using the derived microphysical properties were compared with measurements at the surface and the top of the atmosphere. Radiative closure biases were generally smaller for cloudy scenes re...

Load Balancing and Scalability of a Subgrid Orography Scheme in a Global Climate Model

A subgrid orography scheme has been applied to the National Center for Atmospheric Research Community Atmosphere Model. The scheme applies all of the model column physics to each of up to 11 elevation classes within each grid cell. The distribution of the number of elevation classes in each grid cell is highly inhomogeneous. This could produce a serious load imbalance if the domain decomposition distributes grid cells evenly across processors. However, since the distribution of classes is static, static load balancing can be used to distribute the elevation classes uniformly across processors. The load balancing is accomplished first by distributing the number of classes evenly within each process. The number of chunks on processes is distributed uniformly across processes and the dynamics-physics transpose cost is minimized by assigning chunks to processes with the most dynamics grid cells from that chunk. Parallel efficiency with the subgrid scheme and load balancing exceeds parallel efficiency without the subgrid scheme for up to 128 processors. The load balancing across processes decreases run-time by 10-30% depending on configuration.

A scientific data processing framework for time series NetCDF data

The Atmospheric Radiation Measurement (ARM) Data Integrator (ADI) is a framework designed to streamline the development of scientific algorithms that analyze, and models that use time-series NetCDF data. ADI automates the process of retrieving and preparing data for analysis, provides a modular, flexible framework that simplifies software development, and supports a data integration workflow. Algorithm and model input data, preprocessing, and output data specifications are defined through a graphical interface. ADI includes a library of software modules to support the workflow, and a source code generator that produces C, IDL^(R), and Python(TM) templates to jump start development. While developed for processing climate data, ADI can be applied to any time-series data. This paper discusses the ADI framework, and how ADIs capabilities can decrease the time and cost of implementing scientific algorithms allowing modelers and scientists to focus their efforts on their research rather than preparing and packaging data.

Accelerating numerical calculation on the Cray XMT

The Cray XMT provides hardware support for parallel algorithms that would be communication- or memory-bound on other machines. Unfortunately, even if an algorithm meets these criteria, performance suffers if the algorithm is too numerically intensive. We present a lookup-based approach that achieves a significant performance advantage over explicit calculation. We describe an approach to balancing memory bandwidth against on-chip floating point capabilities, leading to further speedup. Finally, we provide table lookup algorithms for a number of common functions.

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 ...

Physically Based Global Downscaling: Climate Change Projections for a Full Century

A global atmosphere/land model with an embedded subgrid orography scheme is used to simulate the period 1977-2100 using ocean surface conditions and radiative constituent concentrations for a climate change scenario. Climate variables simulated for multiple elevation classes are mapping according to a high-resolution elevation dataset in ten regions with complex terrain. Analysis of changes in the simulated climate leads to the following conclusions. Changes in precipitation vary widely, with precipitation increasing more with increasing altitude in some region, decreasing more with altitude in others, and changing little in still others. In some regions the sign of the precipitation change depends on surface elevation. Changes in surface air temperature are rather uniform, with at most a two-fold difference between the largest and smallest changes within a region; in most cases the warming increases with altitude. Changes in snow water are highly dependent on altitude. Absolute changes usually increase with altitude, while relative changes decrease. In places where snow accumulates, an artificial upper bound on snow water limits the sensitivity of snow water to climate change considerably. The simulated impact of climate change on regional mean snow water varies widely, with little impact in regions in which the upper bound on snow water is the dominant snow water sink, moderate impact in regions with a mixture of seasonal and permanent snow, and profound impacts on regions with little permanent snow.