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Warm Organized Rain Systems over the Tropical Eastern Pacific

JOURNAL OF CLIMATE 2016年第9期29卷 3403-3422页

作者： Chen, Baohua Liu, Chuntao Texas A&M Univ Dept Phys & Environm Sci 6300 Ocean DrNRC Room 3508 Corpus Christi TX 78412 USA

This study uses 16-yr Tropical Rainfall Measuring Mission (TRMM) radar precipitation feature (RPF) data to characterize warm rain systems in the tropics with large horizontal extensions, referred to as warm organized rain systems. The systems are selected by specifying the RPFs with minimum infrared brightness temperature warmer than 0 degrees C and rain area greater than 500 km(2). ERA-Interim atmospheric fields and SST from NOAA are analyzed to highlight the environmental characteristics of warm organized rain systems. Warm organized systems occur over specific oceanic regions, including the eastern Pacific ITCZ, the eastern part of the SPCZ, and coastal regions. In contrast with ubiquitous warm isolated RPFs, warm organized systems have greater near-surface radar reflectivity. The rainfall amounts generated by warm organized systems are greater in winter than in summer. Composite analyses indicate that warm organized RPFs prefer to coexist with a dry midtroposphere associated with a strong upper-level descent, an enhanced near-surface moisture convergence, and a strong low-level large-scale ascent. The shallow meridional circulation in the eastern Pacific is significantly stronger for warm organized RPFs compared to the circulation for warm isolated RPFs. Warm organized systems over the tropical eastern Pacific occur at warm SSTs with mean value of about 27 degrees C and a strong SST meridional gradient. The warm organized RPFs in the tropical eastern Pacific are found to be at the southern edge of deep ITCZ cores. This is probably related to the meridional asymmetrical thermodynamic structure over the eastern Pacific ITCZ with a higher low-level humidity to the south. Similar favorable large-scale environments for the warm organized RPFs are also found over the SPCZ and other regions.

关键词： Clouds Convective parameterization Cumulus clouds models and modeling Physical Meteorology and Climatology

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Calibrating Climate Change Time-Slice Projections with Estimates of Seasonal Forecast Reliability

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JOURNAL OF CLIMATE 2016年第10期29卷 3831-3840页

作者： Matsueda, M. Weisheimer, A. Palmer, T. N. Univ Tsukuba Ctr Computat Sci 1-1-1 Tennodai Tsukuba Ibaraki 3058577 Japan Univ Oxford Dept Phys Atmospher Ocean & Planetary Phys Oxford England Univ Oxford Dept Phys Natl Ctr Atmospher Sci Oxford England European Ctr Medium Range Weather Forecasts Shinfield Pk Reading RG2 9AX Berks England

In earlier work, it was proposed that the reliability of climate change projections, particularly of regional rainfall, could be improved if such projections were calibrated using quantitative measures of reliability obtained by running the same model in seasonal forecast mode. This proposal is tested for fast atmospheric processes (such as clouds and convection) by considering output from versions of the same atmospheric general circulation model run at two different resolutions and forced with prescribed sea surface temperatures and sea ice. Here output from the high-resolution version of the model is treated as a proxy for truth. The reason for using this approach is simply that the twenty-first-century climate change signal is not yet known and, hence, no climate change projections can be verified using observations. Quantitative assessments of reliability of the low-resolution model, run in seasonal hindcast mode, are used to calibrate climate change time-slice projections made with the same low-resolution model. Results show that the calibrated climate change probabilities are closer to the proxy truth than the uncalibrated probabilities. Given that seasonal forecasts are performed operationally already at several centers around the world, in a seamless forecast system they provide a resource that can be used without cost to help calibrate climate change projections and make them more reliable for users.

关键词： Forecasting Climate prediction Forecast verification/skill Hindcasts Probability forecasts/models/distribution Seasonal forecasting models and modeling Climate models

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OSSEs for an Ensemble 3DVAR Data Assimilation System with Radar Observations of Convective Storms

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第6期73卷 2403-2426页

作者： Gao, Jidong Fu, Chenghao Stensrud, David J. Kain, John S. NOAA Natl Severe Storms Lab Norman OK 73069 USA Hunan Meteorol Bur Changsha Hunan Peoples R China Univ Oklahoma Cooperate Inst Mesoscale Meteorol Studies Norman OK 73019 USA Penn State Univ Dept Meteorol 503 Walker Bldg University Pk PA 16802 USA

An ensemble of the three-dimensional variational data assimilation (En3DA) method for convective-scale weather has been developed. It consists of an ensemble of three-dimensional variational data assimilations and forecasts in which member differences are introduced by perturbing initial conditions and/or observations, and it uses flow-dependent error covariances generated by the ensemble forecasts. The method is applied to the assimilation of simulated radar data for a supercell storm. Results indicate that the flow-dependent ensemble covariances are effective in enabling convective-scale analyses, as the most important features of the simulated storm, including the low-level cold pool and midlevel mesocyclone, are well analyzed. Several groups of sensitivity experiments are conducted to test the robustness of the method. The first group demonstrates that incorporating a mass continuity equation as a weak constraint into the En3DA algorithm can improve the quality of the analyses when radial velocity observations contain large errors. In the second group of experiments, the sensitivity of analyses to the microphysical parameterization scheme is explored. Results indicate that the En3DA analyses are quite sensitive to differences in the microphysics scheme, suggesting that ensemble forecasts with multiple microphysics schemes could reduce uncertainty related to model physics errors. Experiments also show that assimilating reflectivity observations can reduce spinup time and that it has a small positive impact on the quality of the wind field analysis. Of the threshold values tested for assimilating reflectivity observations, 15 dBZ provides the best analysis. The final group of experiments demonstrates that it is not necessary to perturb radial velocity observations for every ensemble number in order to improve the quality of the analysis.

关键词： Circulation Dynamics Convective storms Observational techniques and algorithms Radars Radar observations models and modeling Data assimilation Ensembles

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Microwave Sounder Cloud Detection Using a Collocated High-Resolution Imager and Its Impact on Radiance Assimilation in Tropical Cyclone Forecasts

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MONTHLY WEATHER REVIEW 2016年第10期144卷 3937-3959页

作者： Han, Hyojin Li, Jun Goldberg, Mitch Wang, Pei Li, Jinlong Li, Zhenglong Sohn, B. -J. Li, Juan Univ Wisconsin Cooperat Inst Mesoscale Meteorol Studies 1225 West Dayton St Madison WI 53706 USA Korea Meteorol Adm Natl Inst Meteorol Sci Jeju Do South Korea NOAA NESDIS Joint Polar Satellite Syst Lanham MD USA Univ Wisconsin Dept Atmospher & Ocean Sci Madison WI USA Seoul Natl Univ Sch Earth & Environm Sci Seoul South Korea China Meteorol Adm Numer Weather Predict Ctr Beijing Peoples R China

Accurate cloud detection is one of the most important factors in satellite data assimilation due to the uncertainties associated with cloud properties and their impacts on satellite-simulated radiances. To enhance the accuracy of cloud detection and improve radiance assimilation for tropical cyclone (TC) forecasts, measurements from the Advanced Microwave Sounding Unit-A (AMSU-A) on board the Aqua satellite and the Advanced Technology Microwave Sounder (ATMS) are collocated with high spatial resolution cloud products from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board Aqua and the Visible Infrared Imager Radiometer Suite (VIIRS) on board the Suomi-National Polar-Orbiting Partnership (Suomi-NPP) satellite. The cloud-screened microwave radiance measurements are assimilated for Hurricane Sandy (2012) and Typhoon Haiyan (2013) forecasts using the Weather Research and Forecasting (WRF) Model and the three-dimensional variational (3DVAR)-based Gridpoint Statistical Interpolation (GSI) data assimilation system. Experiments are carried out to determine the optimal thresholds of cloud fraction (CF) for minimizing track and intensity forecast errors. The results indicate that the use of high spatial resolution cloud products can improve the accuracy of TC forecasts by better eliminating cloud-contaminated microwave sounder field-of-views (FOVs). In conclusion, the combination of advanced microwave sounders and collocated high spatial resolution imagers is able to improve the radiance assimilation and TC forecasts. The methodology used in this study can be applied to process data from other pairs of microwave sounders and imagers on board the same platform.

关键词： Physical Meteorology and Climatology Clouds Observational techniques and algorithms Satellite observations models and modeling Data assimilation

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Cross-Seasonal Relationship between the Boreal Autumn SAM and Winter Precipitation in the Northern Hemisphere in CMIP5

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JOURNAL OF CLIMATE 2016年第18期29卷 6617-6636页

作者： Liu, Ting Li, Jianping Feng, Juan Wang, Xiaofan Li, Yang Second Inst Oceanog State Key Lab Satellite Ocean Environm Dynam Hangzhou Peoples R China Chinese Acad Sci Inst Atmospher Phys State Key Lab Numer Modeling Atmospher Sci & Geop Beijing Peoples R China Univ Chinese Acad Sci Beijing Peoples R China Beijing Normal Univ Coll Global Change & Earth Syst Sci 19 XinJieKouWai St Beijing 100875 Peoples R China Joint Ctr Global Change Studies Beijing Peoples R China Chengdu Univ Informat Technol Coll Atmospher Sci Chengdu Peoples R China

Recent work suggests that the boreal autumn Southern Hemisphere annular mode (SAM) favors a tripole pattern of winter precipitation anomalies in the Northern Hemisphere. This study focuses on the abilities of climate models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to reproduce the physical processes involved in this observed cross-seasonal connection. A systematic evaluation suggested that 16 out of 25 models were essentially capable of reproducing this cross-seasonal connection. Two categories of models were selected to explore the underlying reasons for these successful simulations. models that successfully simulated the cross-seasonal relationship were placed in the type-I category, and these performed well in reproducing the related physical mechanism, known as the "coupled ocean atmosphere bridge," in terms of the SST variability associated with the SAM and response of the meridional circulation to these SST anomalies. In contrast, the type-II category of models showed poor performance in representing the related processes and associated feedbacks, and the model biases compromised the performance of the simulated cross seasonal relationship. These results demonstrate that the capability of the CMIP5 models to reproduce SST variability associated with the boreal autumn SAM and related coupled ocean atmosphere bridge process plays a decisive role in the successful simulation of the cross-seasonal relationship.

关键词： Circulation/ Dynamics Annular mode models and modeling Model evaluation/performance

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The Linear Response Function of an Idealized Atmosphere. Part I: Construction Using Green's Functions and Applications

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第9期73卷 3423-3439页

作者： Hassanzadeh, Pedram Kuang, Zhiming Harvard Univ Dept Earth & Planetary Sci 20 Oxford St Cambridge MA 02138 USA Harvard Univ Ctr Environm 24 Oxford St Cambridge MA 02138 USA Harvard Univ John A Paulson Sch Engn & Appl Sci Cambridge MA 02138 USA

A linear response function (LRF) determines the mean response of a nonlinear climate system to weak imposed forcings, and an eddy flux matrix (EFM) determines the eddy momentum and heat flux responses to mean-flow changes. Neither LRF nor EFM can be calculated from first principles owing to the lack of a complete theory for turbulent eddies. Here the LRF and EFM for an idealized dry atmosphere are computed by applying numerous localized weak forcings, one at a time, to a GCM with Held Suarez physics and calculating the mean responses. The LRF and EFM for zonally averaged responses are then constructed using these forcings and responses through matrix inversion. Tests demonstrate that LRF and EFM are fairly accurate. Spectral analysis of the LRF shows that the most excitable dynamical mode, the neutral vector, strongly resembles the model's annular mode. The framework described here can be employed to compute the LRF and EFM for zonally asymmetric responses and more complex GCMs. The potential applications of the LRF and EFM constructed here are (i) forcing a specified mean flow for hypothesis testing, (ii) isolating/quantifying the eddy feedbacks in complex eddy mean flow interaction problems, and (iii) evaluating/improving more generally applicable methods currently used to construct LRFs or diagnose eddy feedbacks in comprehensive GCMs or observations. As an example for (iii), in Part II, the LRF is also computed using the fluctuation dissipation theorem (FDT), and the previously calculated LRF is exploited to investigate why FDT performs poorly in some cases. It is shown that dimension reduction using leading EOFs, which is commonly used to construct LRFs from the FDT, can significantly degrade the accuracy owing to the non normality of the operator.

关键词： Circulation/ Dynamics Annular mode Atmospheric circulation Eddies Nonlinear dynamics Mathematical and statistical techniques Singular vectors models and modeling Primitive equations model

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The Role of Ocean Heat Transport in the Global Climate Response to Projected Arctic Sea Ice Loss

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JOURNAL OF CLIMATE 2016年第19期29卷 6841-6859页

作者： Tomas, Robert A. Deser, Clara Sun, Lantao Natl Ctr Atmospher Res Climate & Global Dynam Div POB 3000 Boulder CO 80307 USA Univ Colorado Boulder Cooperat Inst Res Environm Sci Boulder CO USA NOAA Earth Syst Res Lab Boulder CO USA

The purpose of this study is to elucidate the individual and combined roles of thermodynamic and dynamic ocean-atmosphere coupling in the equilibrium global climate response to projected Arctic sea ice loss using a suite of experiments conducted with Community Climate System Model, version 4, at 18 latitude-longitude spatial resolution. The results highlight the contrasting spatial structures and partially compensating effects of thermodynamic and dynamic coupling. In combination, thermodynamic and dynamic coupling produce a response pattern that is largely symmetric about the equator, whereas thermodynamic coupling alone yields an antisymmetric response. The latter is characterized by an interhemispheric sea surface temperature (SST) gradient, with maximum warming at high northern latitudes decreasing toward the equator, which displaces the intertropical convergence zone (ITCZ) and Hadley circulation northward. In contrast, the fully coupled response shows enhanced warming at high latitudes of both hemispheres and along the equator;the equatorial warming is driven by anomalous ocean heat transport convergence and is accompanied by a narrow equatorward intensification of the northern and southern branches of the ITCZ. In both cases, the tropical precipitation response to Arctic sea ice loss feeds back onto the atmospheric circulation at midlatitudes via Rossby wave dynamics, highlighting the global interconnectivity of the coupled climate system. This study demonstrates the importance of ocean dynamics in mediating the equilibrium global climate response to Arctic sea ice loss.

关键词： Circulation/ Dynamics Atmosphere-ocean interaction Atmospheric circulation Feedback Large-scale motions Teleconnections models and modeling Climate models

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Past and Projected Changes in Western North Pacific Tropical Cyclone Exposure

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JOURNAL OF CLIMATE 2016年第16期29卷 5725-5739页

作者： Kossin, James P. Emanuel, Kerry A. Camargo, Suzana J. NOAA Natl Ctr Environm Informat Asheville NC USA MIT 77 Massachusetts Ave Cambridge MA 02139 USA Columbia Univ Lamont Doherty Earth Observ Palisades NY USA

The average latitude where tropical cyclones (TCs) reach their peak intensity has been observed to be shifting poleward in some regions over the past 30 years, apparently in concert with the independently observed expansion of the tropical belt. This poleward migration is particularly well observed and robust in the western North Pacific Ocean (WNP). Such a migration is expected to cause systematic changes, both increases and decreases, in regional hazard exposure and risk, particularly if it persists through the present century. Here, it is shown that the past poleward migration in the WNP has coincided with decreased TC exposure in the region of the Philippine and South China Seas, including the Marianas, the Philippines, Vietnam, and southern China, and increased exposure in the region of the East China Sea, including Japan and its Ryukyu Islands, the Korea Peninsula, and parts of eastern China. Additionally, it is shown that projections of WNP TCs simulated by, and downscaled from, an ensemble of numerical models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) demonstrate a continuing poleward migration into the present century following the emissions projections of the representative concentration pathway 8.5 (RCP8.5). The projected migration causes a shift in regional TC exposure that is very similar in pattern and relative amplitude to the past observed shift. In terms of regional differences in vulnerability and resilience based on past TC exposure, the potential ramifications of these future changes are significant. Questions of attribution for the changes are discussed in terms of tropical belt expansion and Pacific decadal sea surface temperature variability.

关键词： Geographic location/entity North Pacific Ocean Atm/Ocean Structure/ Phenomena Hurricanes/typhoons models and modeling Climate models Variability Multidecadal variability Pacific decadal oscillation Trends

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The Impact of GCM Dynamical Cores on Idealized Sudden Stratospheric Warmings and Their QBO Interactions

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第9期73卷 3397-3421页

作者： Yao, Weiye Jablonowski, Christiane Princeton Univ Program Atmospher & Ocean Sci 300 Forrestal Rd Princeton NJ 08544 USA Univ Michigan Dept Climate & Space Sci & Engn Ann Arbor MI 48109 USA

The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land-sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held-Suarez-Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR's Community Atmosphere Model, version 5 (CAMS), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models. Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave-mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD's more vigorous wave-mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)-like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO-SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton-Tan effect.

关键词： Circulation/ Dynamics Atmospheric circulation Planetary waves Stratospheric circulation Waves atmospheric models and modeling Numerical analysis/modeling

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The Effects of Downscaling Method on the Variability of Simulated Watershed Response to Climate Change in Five US Basins

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EARTH INTERACTIONS 2016年第11期20卷 1-27页

作者： Nover, D. M. Witt, J. W. Butcher, J. B. Johnson, T. E. Weaver, C. P. US Agcy Int Dev Amer Assoc Adv Sci Accra Ghana US EPA Off Res & Dev Washington DC 20460 USA Tetra Tech Inc Res Triangle Pk NC USA US EPA Off Res & Dev Res Triangle Pk NC 27711 USA

Simulations of future climate change impacts on water resources are subject to multiple and cascading uncertainties associated with different modeling and methodological choices. A key facet of this uncertainty is the coarse spatial resolution of GCM output compared to the finer-resolution information needed by water managers. To address this issue, it is now common practice to apply spatial downscaling techniques, using either higher-resolution regional climate models or statistical approaches applied to GCM output, to develop finer-resolution information. Downscaling, however, can also introduce its own uncertainties into water resources' impact assessments. This study uses watershed simulations in five U.S. basins to quantify the sources of variability in streamflow, nitrogen, phosphorus, and sediment loads associated with the underlying GCM compared to the choice of downscaling method (both statistically and dynamically downscaled GCM output). This study also assesses the specific, incremental effects of downscaling by comparing watershed simulations based on downscaled and nondownscaled GCM model output. Results show that the underlying GCM and the downscaling method each contribute to the variability of simulated watershed responses. The relative contribution of GCM and downscaling method to the variability of simulated responses varies by watershed and season of the year. Results illustrate the potential implications of one key methodological choice in conducting climate change impact assessments for water-the selection of downscaled climate change information.

关键词： Climate prediction Ensembles models and modeling Coupled models Hydrologic models

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