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- Teixeira, J., et al.
(författare)
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Tropical and Subtropical Cloud Transitions in Weather and Climate Prediction Models : The GCSS/WGNE Pacific Cross-Section Intercomparison (GPCI)
- 2011
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 24:20, s. 5223-5256
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Tidskriftsartikel (refereegranskat)abstract
- A model evaluation approach is proposed in which weather and climate prediction models are analyzed along a Pacific Ocean cross section, from the stratocumulus regions off the coast of California, across the shallow convection dominated trade winds, to the deep convection regions of the ITCZ-the Global Energy and Water Cycle Experiment Cloud System Study/Working Group on Numerical Experimentation (GCSS/WGNE) Pacific Cross-Section Intercomparison (GPCI). The main goal of GPCI is to evaluate and help understand and improve the representation of tropical and subtropical cloud processes in weather and climate prediction models. In this paper, a detailed analysis of cloud regime transitions along the cross section from the subtropics to the tropics for the season June-July-August of 1998 is presented. This GPCI study confirms many of the typical weather and climate prediction model problems in the representation of clouds: underestimation of clouds in the stratocumulus regime by most models with the corresponding consequences in terms of shortwave radiation biases; overestimation of clouds by the 40-yr ECMWF Re-Analysis (ERA-40) in the deep tropics (in particular) with the corresponding impact in the outgoing longwave radiation; large spread between the different models in terms of cloud cover, liquid water path and shortwave radiation; significant differences between the models in terms of vertical cross sections of cloud properties (in particular), vertical velocity, and relative humidity. An alternative analysis of cloud cover mean statistics is proposed where sharp gradients in cloud cover along the GPCI transect are taken into account. This analysis shows that the negative cloud bias of some models and ERA-40 in the stratocumulus regions [as compared to the first International Satellite Cloud Climatology Project (ISCCP)] is associated not only with lower values of cloud cover in these regimes, but also with a stratocumulus-to-cumulus transition that occurs too early along the trade wind Lagrangian trajectory. Histograms of cloud cover along the cross section differ significantly between models. Some models exhibit a quasi-bimodal structure with cloud cover being either very large (close to 100%) or very small, while other models show a more continuous transition. The ISCCP observations suggest that reality is in-between these two extreme examples. These different patterns reflect the diverse nature of the cloud, boundary layer, and convection parameterizations in the participating weather and climate prediction models.
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| 4. |
- Buehler, Stefan A., et al.
(författare)
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ARTS, the atmospheric radiative transfer simulator
- 2005
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 91:1, s. 65-93
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Tidskriftsartikel (refereegranskat)abstract
- RTS is a modular program that simulates atmospheric radiative transfer. The paper describes ARTS version 1.0, which is applicable in the absence of scattering. An overview over all major parts of the model is given: calculation of absorption coefficients, the radiative transfer itself, and the calculation of Jacobians. ARTS can be freely used under a GNU general public license. Unique features of the program are its scalability and modularity, the ability to work with different sources of spectroscopic parameters, the availability of several self-consistent water continuum and line absorption models, and the analytical calculation of Jacobians.
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| 5. |
- Buehler, Stefan, et al.
(författare)
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Recent developments in the line-by-line modeling of outgoing longwave radiation
- 2006
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 98:3, s. 446-457
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Tidskriftsartikel (refereegranskat)abstract
- High frequency resolution radiative transfer model calculations with the Atmospheric Radiative Transfer Simulator (ARTS) were used to simulate the clear-sky outgoing longwave radiative flux (OLR) at the top of the atmosphere. Compared to earlier calculations by Clough and coworkers the model used a spherical atmosphere instead of a plane parallel atmosphere, updated spectroscopic parameters from HITRAN, and updated continuum parameterizations from Mlawer and coworkers. These modifications lead to a reduction in simulated OLR by approximately 4.1%, the largest part, approximately 2.5%, being due to the absence of the plane parallel approximation. As a simple application of the new model, the sensitivity of OLR to changes in humidity, carbon dioxide concentration, and temperature were investigated for different cloud-free atmospheric scenarios. It was found that for the tropical scenario a 20% change in humidity has a larger impact than a doubling of the carbon dioxide concentration. The sensitive altitude region for temperature and humidity changes is the entire free troposphere, including the upper troposphere where humidity data quality is poor.
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| 6. |
- John, Viju Oommen, et al.
(författare)
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Understanding the variability of clear-sky outgoing long-wave radiation based on ship-based temperature and water vapour measurements
- 2006
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 132:621, s. 2675-2691
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution radiative transfer model calculations with the Atmospheric Radiative Transfer Simulator (ARTS) were used to simulate the clear-sky outgoing long-wave radiative flux (OLR) at the top of the atmosphere. The unique set of radiosonde data collected by the research vessel Polarstern of the Alfred Wegener Institute for Polar and Marine Research during 27 expeditions in the years 1982 to 2003 was used to investigate the sources of clear-sky OLR variability for ocean areas in different climate zones and seasons. For this dataset, tropospheric temperature variations contribute approximately 33 W m(-2) OLR variability. tropospheric relative humidity variations 8.5 W m(-2), and vertical structure 2.3-3.4 W m(-2). Of these, 0.3-1.0 W m(-2) are due to structures on a vertical scale smaller than 4 km, which cannot be resolved by conventional remote-sensing instruments. It was also found that the poor absolute accuracy of current humidity data in the upper troposphere, approximately 40% relative error in relative humidity, leads to a significant uncertainty in OLR of about 3.8 W m(-2) (for a midlatitude summer atmosphere), which should be put in the context of the double CO2 effect of only 2.6 W m(-2) (for the same atmosphere).
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| 7. |
- Verdes, C. L., et al.
(författare)
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Partition function data and impact on retrieval quality for an mm/sub-mm limb sounder
- 2005
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Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 90:2, s. 217-238
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Tidskriftsartikel (refereegranskat)abstract
- The partition function plays a major role as it mainly governs the dependence of the spectral line strength (or line intensity) on the temperature. A wrong partition function will lead to an incorrect line strength, which will lead to a systematic error in the retrieval. Therefore, a careful investigation of the available partition function data and the sensitivity of the retrieval to this is required. Two partition function data sets, the HITRAN and the JPL, are considered in this paper. In a first step, we investigate the spread of the partition function ratios from the simple approximation values given by theory. We find that the HITRAN partition functions are usually higher than the values quoted by the simple approximation. The JPL partition functions are much closer to the values given by the approximation. Comparing directly the two datasets, with some exceptions, a good agreement is found. Only for 16 molecules (from 66 considered molecules), the deviations in the two data sets are larger than 2%. The retrieval error analysis shows that, for the case of the molecular species with strong signatures, an uncertainty in the partition function is directly translated into a retrieval error of the species in question. However, the uncertainty in the partition function of this species can have a high impact on the quality of weak species retrieval.
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| 8. |
- Verdes, C., et al.
(författare)
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Pointing and temperature retrieval from millimeter-submillimeter limb soundings
- 2002
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:16, s. 4299-
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Tidskriftsartikel (refereegranskat)abstract
- Passive microwave limb sounding instruments like the Millimeter-Wave Atmospheric Sounder (MAS) or the Microwave Limb Sounder (MLS) observe dedicated oxygen lines for the derivation of temperature and pointing information, since these quantities are essential for the quality of the retrieval of the trace gas mixing ratio. Emission lines of oxygen are chosen because the volume mixing ratio (VMR) profile is known. In this paper, we demonstrate the capabilities of a new and innovative method by means of which accurate temperature and pointing information can be gathered from other strong spectral features like ozone lines, without including accurate knowledge of the VMR profile of these species. For this purpose, retrievals from two observation bands with a bandwidth of about 10 GHz each, one including an oxygen line, have been compared. A full error analysis was performed with respect to critical instrument and model parameters, such as uncertainties in the antenna pattern, calibration uncertainties, random pointing error, baseline ripples, baseline discontinuities, and spectroscopic parameters. The applied inversion algorithm was the optimal estimation method. For the selected scenario and instrumental specifications we find that the retrieval of a pointing offset and the atmospheric temperature profile can be achieved with a good accuracy. The retrieval precision of the pointing offset is better than 24 m. The retrieval precision of the temperature profile is better than 2 K for altitudes ranging from 10 to 40 km. Systematic errors (due to model parameter uncertainties) are somewhat larger than these purely statistical errors. Investigations carried out for different atmospheric states or different instrumental specifications show similar results.
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