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Climate Change over the Extratropical Southern Hemisphere: The Tale from an Ensemble of Reanalysis Datasets

JOURNAL OF CLIMATE 2016年第5期29卷 1673-1687页

作者： Solman, Silvina A. Orlanski, Isidoro Univ Buenos Aires Ctr Invest Mar & Atmosfera Consejo Nacl Invest Cient & Tecn Fac Ciencias Exactas & Nat Buenos Aires DF Argentina Univ Buenos Aires Fac Ciencias Exactas & Nat Dept Ciencias Atmosfera & Oceanos Buenos Aires DF Argentina Princeton Univ Atmospher & Ocean Sci Program Princeton NJ 08544 USA

In this study, a set of five reanalysis datasets [ERA-Interim, NCEP-DOE AMIP-II reanalysis (R2), MERRA, the Twentieth Century Reanalysis (20CR), and the CFS Reanalysis (CFSR)] is used to provide a robust estimation of precipitation change in the middle-to-high latitudes of the Southern Hemisphere during the last three decades. Based on several metrics accounting for the eddy activity and moisture availability, an attempt is also made to identify the dynamical mechanisms triggering these changes during extended summer and winter seasons. To that aim, a weighted reanalysis ensemble is built using the inverse of the variance as weighting factors for each variable. Results showed that the weighted reanalysis ensemble reproduced the observed precipitation changes at high and middle latitudes during the two seasons, as depicted by the GPCP dataset. For the extended summer season, precipitation changes were dynamically consistent with changes in the eddy activity, attributed mostly to ozone depletion. For the extended winter season, the eddy activity and moisture availability both contributed to the precipitation changes, with the increased concentration of greenhouse gases being the main driver of the climate change signal. In addition, output from a five-member ensemble of the high-resolution GFDL CM2.5 for the period 1979-2010 was used in order to explore the capability of the model in reproducing both the observed precipitation change and the underlying dynamical mechanisms. The model was able to capture the rainfall change signal. However, the increased availability of moisture from the lower levels controls the precipitation change during both summer and winter.

关键词： Variability models and modeling Extratropical cyclones Circulation/ Dynamics Precipitation Large-scale motions Reanalysis data Observational techniques and algorithms Atm/Ocean Structure/ Phenomena Trends Databases

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Impact of Thermally Driven Turbulence on the Bottom Melting of Ice

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第4期46卷 1171-1187页

作者： Keitzl, T. Mellado, J. P. Notz, D. Max Planck Inst Meteorol Bundesstr 53 D-20146 Hamburg Germany

Direct numerical simulation and laboratory experiments are used to investigate turbulent convection beneath a horizontal ice-water interface. Scaling laws are derived that quantify the dependence of the melt rate of the ice on the far-field temperature of the water under purely thermally driven conditions. The scaling laws, the simulations, and the laboratory experiments consistently yield that the melt rate increases by two orders of magnitude, from ?10(1) to ?10(3) mm day(-1), as the far-field temperature increases from 4 degrees to 8 degrees C. The strong temperature dependence of the melt rate is explained by analyzing the vertical structure of the flow: For far-field temperatures below 8 degrees C, the flow features a stably stratified, diffusive layer next to the ice that shields it from the warmer, turbulent outer layer. The stratification in the diffusive layer diminishes as the far-field temperature increases and vanishes for far-field temperatures far above 8 degrees C. Possible implications of these results for ice-ocean interfaces are discussed. The drastic melt-rate increase implies that turbulence needs to be considered in the analysis of ice-water interfaces even in shear-free conditions.

关键词： Circulation Dynamics Convection Turbulence Atm Ocean Structure Phenomena Snowmelt icemelt models and modeling Laboratory physical models

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Coordinated Global and Regional Climate modeling

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JOURNAL OF CLIMATE 2016年第1期29卷 17-35页

作者： Scinocca, J. F. Kharin, V. V. Jiao, Y. Qian, M. W. Lazare, M. Solheim, L. Flato, G. M. Biner, S. Desgagne, M. Dugas, B. Environm Canada Canadian Ctr Climate Modelling & Anal Victoria BC V8W 3V6 Canada Consortium Ouranos Montreal PQ Canada Environm Canada Div Rech Previs Numer Montreal PQ Canada

A new approach of coordinated global and regional climate modeling is presented. It is applied to the Canadian Centre for Climate Modelling and Analysis Regional Climate Model (CanRCM4) and its parent global climate model CanESM2. CanRCM4 was developed specifically to downscale climate predictions and climate projections made by its parent global model. The close association of a regional climate model (RCM) with a parent global climate model (GCM) offers novel avenues of model development and application that are not typically available to independent regional climate modeling centers. For example, when CanRCM4 is driven by its parent model, driving information for all of its prognostic variables is available (including aerosols and chemical species), significantly improving the quality of their simulation. Additionally, CanRCM4 can be driven by its parent model for all downscaling applications by employing a spectral nudging procedure in CanESM2 designed to constrain its evolution to follow any large-scale driving data. Coordination offers benefit to the development of physical parameterizations and provides an objective means to evaluate the scalability of such parameterizations across a range of spatial resolutions. Finally, coordinating regional and global modeling efforts helps to highlight the importance of assessing RCMs' value added relative to their driving global models. As a first step in this direction, a framework for identifying appreciable differences in RCM versus GCM climate change results is proposed and applied to CanRCM4 and CanESM2.

关键词： Physical Meteorology and Climatology Climate change models and modeling Climate models Regional models

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A Physically Based Subgrid Parameterization for the Production and Maintenance of Mixed-Phase Clouds in a General Circulation Model

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第1期73卷 279-291页

作者： Furtado, K. Field, P. R. Boutle, I. A. Morcrette, C. J. Wilkinson, J. M. Met Off Exeter EX1 3PB Devon England

A physically based method for parameterizing the role of subgrid-scale turbulence in the production and maintenance of supercooled liquid water and mixed-phase clouds is presented. The approach used is to simplify the dynamics of supersaturation fluctuations to a stochastic differential equation that can be solved analytically, giving increments to the prognostic liquid cloud fraction and liquid water content fields in a general circulation model (GCM). Elsewhere, it has been demonstrated that the approach captures the properties of decameter-resolution large-eddy simulations of a turbulent mixed-phase environment. In this paper, it is shown that it can be implemented in a GCM, and the effects that this has on Southern Ocean biases and on Arctic stratus are investigated.

关键词： Physical Meteorology and Climatology Cloud water phase Condensation models and modeling Cloud parameterizations Subgrid-scale processes

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Evaluation of PBL Parameterizations for modeling Surface Wind Speed during Storms in the Northeast United States

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WEATHER AND FORECASTING 2016年第5期31卷 1511-1528页

作者： Frediani, Maria E. B. Hacker, Joshua P. Anagnostou, Emmanouil N. Hopson, Thomas Univ Connecticut Dept Civil & Environm Engn Storrs CT 06269 USA Natl Ctr Atmospher Res POB 3000 Boulder CO 80307 USA

This study identifies conditions that determine errors in numerical simulations of 10-m wind speed over moderately complex terrain, emphasizing winds that lead to overhead power-line damage over a subregion of the northeast United States. Simulations with the Mellor-Yamada-Janjic (MYJ) scheme, the Yonsei University (YSU) scheme, and a subgrid-scale topographic drag correction (Topo) applied to YSU are used to investigate error components. The wind speed distribution is dominated by low speeds, which are well depicted by Topo, but are underestimated by the MYJ and YSU schemes. Conversely, moderate and high speeds are underestimated by Topo, and MYJ and YSU perform better across specific ranges. Verification samples are conditioned by season, diurnal cycle, topography, and spatial patterns obtained with a clustering analysis. The systematic error is characterized by a positive bias in low speeds, and as speed increases the biases become more negative. Quantile comparisons, along with systematic and random errors, indicate that beyond the dependence on wind speed itself, errors also depend on seasonal characteristics, indirectly defined by scheme stability profiles. The positive relationship between absolute bias and speed originates in the friction velocity parameterization, and the correction for drag in the Topo scheme exacerbates the effect. The Topo scheme adjusts the total bias and sharpens the bias spread but penalizes moderate and high winds. Clusters reveal that in Topo the bias is primarily driven by wind direction. Excessive correction occurs on terrain-interacting flows, and oceanic flow modulates the adjustment, enhancing the scheme's performance.

关键词： Forecasting Forecast verification/skill Numerical weather prediction/forecasting models and modeling Model evaluation/performance

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Impact of Strong ENSO on Regional Tropical Cyclone Activity in a High-Resolution Climate Model in the North Pacific and North Atlantic Oceans

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JOURNAL OF CLIMATE 2016年第7期29卷 2375-2394页

作者： Krishnamurthy, Lakshmi Vecchi, Gabriel A. Msadek, Rym Murakami, Hiroyuki Wittenberg, Andrew Zeng, Fanrong NOAA Geophys Fluid Dynam Lab Princeton Univ Forrestal Campus201 Forrestal Rd Princeton NJ 08540 USA Univ Corp Atmospher Res Boulder CO USA Princeton Univ Atmospher & Ocean Sci Program Princeton NJ 08544 USA

Tropical cyclone (TC) activity in the North Pacific and North Atlantic Oceans is known to be affected by the El Nino-Southern Oscillation (ENSO). This study uses the GFDL Forecast Oriented Low Ocean Resolution Model (FLOR), which has relatively high resolution in the atmosphere, as a tool to investigate the sensitivity of TC activity to the strength of ENSO events. This study shows that TCs exhibit a nonlinear response to the strength of ENSO in the tropical eastern North Pacific (ENP) but a quasi-linear response in the tropical western North Pacific (WNP) and tropical North Atlantic. Specifically, a stronger El Nino results in disproportionate inhibition of TCs in the ENP and North Atlantic, and leads to an eastward shift in the location of TCs in the southeast of the WNP. However, the character of the response of TCs in the Pacific is insensitive to the amplitude of La Nina events. The eastward shift of TCs in the southeast of the WNP in response to a strong El Nino is due to an eastward shift of the convection and of the associated environmental conditions favorable for TCs. The inhibition of TC activity in the ENP and Atlantic during El Nino is attributed to the increase in the number of days with strong vertical wind shear during stronger El Nino events. These results are further substantiated with coupled model experiments. Understanding of the impact of strong ENSO on TC activity is important for present and future climate as the frequency of occurrence of extreme ENSO events is projected to increase in the future.

关键词： Climate models ENSO Atm/Ocean Structure/ Phenomena models and modeling Tropical cyclones Circulation/ Dynamics

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Time Scales of Southern Ocean Eddy Equilibration

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第9期46卷 2785-2805页

作者： Sinha, Anirban Abernathey, Ryan P. Columbia Univ Appl Phys & Appl Math Dept 200 SW Mudd BldgMC 4701500 W 120th St New York NY 10027 USA Columbia Univ Dept Earth & Environm Sci New York NY USA

Stratification in the Southern Ocean is determined primarily by a competition between westerly wind-driven upwelling and baroclinic eddy transport. This study investigates the time scales of equilibration of the Southern Ocean in response to changing winds through an idealized channel model. An analytical framework describing the energetic pathways between wind input, available potential energy (APE), eddy kinetic energy (EKE), and dissipation provides a simple theory of the phase and amplitude response to oscillating wind stress. The transient ocean response to variable winds lies between the two limits of Ekman response (high frequency), characterized by the isopycnal slope responding directly to wind stress, and "eddy saturation'' (low frequency), wherein a large fraction of the anomalous wind work goes into mesoscale eddies. The crossover time scale is the time scale of meridional eddy diffusive transport across the Antarctic Circumpolar Current (ACC) front. For wind variability with a period of 3 months (high-frequency forcing), the relative conversion of wind work to APE/EKE is 11, while for a period of 16 years (low-frequency forcing), the relative conversion to APE/EKE reduces to 3. The system's frequency response is characterized by a complex transfer function. Both the phase and amplitude response of EKE and APE predicted by the linear analytic framework are verified using multiple ensemble experiments in an eddy-resolving (4-km horizontal resolution) isopycnal coordinate model. The results from the numerical experiments show agreement with the linear theory and can be used to explain certain features observed in previous modeling studies and observations.

关键词： Circulation/ Dynamics Ekman pumping/transport Energy transport Mesoscale processes Mixing Atm/Ocean Structure/ Phenomena Eddies models and modeling Isopycnal coordinates

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The Mechanical Energy Budget of a Regional Ocean Model

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第9期46卷 2719-2733页

作者： MacCready, Parker Giddings, Sarah N. Univ Washington POB 355351 Seattle WA 98195 USA Univ Calif San Diego La Jolla CA 92093 USA

A method is presented for calculating a complete, numerically closed, mechanical energy budget in a realistic simulation of circulation in a coastal-estuarine domain. The budget is formulated in terms of the "local'' available potential energy (APE;Holliday and McIntyre 1981). The APE may be split up into two parts based on whether a water parcel has been displaced up or down relative to its rest depth. This decomposition clearly shows the different APE signatures of coastal upwelling (particles displaced up by wind) and the estuary (particles displaced down by mixing). Because the definition of APE is local in almost the same sense that kinetic energy is, this study may form meaningful integrals of reservoir and budget terms even over regions that have open boundaries. However, the choice of volume to use for calculation of the rest state is not unique and may influence the results. Complete volume-integrated energy budgets over shelf and estuary volumes in a realistic model of the northeast Pacific and Salish Sea give a new way to quantify the state of these systems and the physical forces that influence that state. On the continental shelf, upwelling may be quantified using APE, which is found to have order-one seasonal variation with an increase due to winds and decrease due to mixing. In the Salish Sea estuarine system, the APE has much less seasonal variation, and the magnitude of the most important forcing terms would take over 7 months to fully drain this energy.

关键词： Circulation/ Dynamics Coastal flows Ocean dynamics models and modeling Diagnostics Numerical analysis/modeling Ocean models Regional models

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Effects of Irrigation on Summer Precipitation over the United States

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JOURNAL OF CLIMATE 2016年第10期29卷 3541-3558页

作者： Pei, Lisi Moore, Nathan Zhong, Shiyuan Kendall, Anthony D. Gao, Zhiqiu Hyndman, David W. Chinese Acad Sci Inst Atmospher Phys State Key Lab Atmospher Boundary Layer Phys & Atm Beijing Peoples R China Univ Chinese Acad Sci Beijing Peoples R China Michigan State Univ Dept Geog E Lansing MI 48824 USA Michigan State Univ Ctr Global Change & Earth Observat E Lansing MI 48824 USA Michigan State Univ Dept Geol Sci E Lansing MI 48824 USA Univ Corp Atmospher Res Boulder CO USA NOAA Great Lakes Environm Res Lab 4840 S State Rd Ann Arbor MI 48108 USA

Irrigation's effects on precipitation during an exceptionally dry summer (June-August 2012) in the United States were quantified by incorporating a novel dynamic irrigation scheme into the Weather Research and Forecasting (WRF) Model. The scheme is designed to represent a typical application strategy for farmlands across the conterminous United States (CONUS) and a satellite-derived irrigation map was incorporated into the WRF-Noah-Mosaic module to realistically trigger the irrigation. Results show that this new irrigation approach can dynamically generate irrigation water amounts that are in close agreement with the actual irrigation water amounts across the high plains (HP), where the prescribed scheme best matches real-world irrigation practices. Surface energy and water budgets have been substantially altered by irrigation, leading to modified large-scale atmospheric circulations. In the studied dry summer, irrigation was found to strengthen the dominant interior high pressure system over the southern and central United States and deepen the trough over the upper Midwest. For the HP and central United States, the rainfall amount is slightly reduced over irrigated areas, likely as a result of a reduction in both local convection and large-scale moisture convergence resulting from interactions and feedbacks between the land surface and atmosphere. In areas downwind of heavily irrigated regions, precipitation is enhanced, resulting in a 20%-100% reduction in the dry biases (relative to the observations) simulated over a large portion of the downwind areas without irrigation in the model. The introduction of irrigation reduces the overall mean biases and root-mean-square errors in the simulated daily precipitation over the CONUS.

关键词： Physical Meteorology and Climatology Atmosphere-land interaction models and modeling Climate models Land surface model Model evaluation/performance Regional models Applications Land use

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Variations of the Global Net Air-Sea Heat Flux during the "Hiatus'' Period (2001-10)

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JOURNAL OF CLIMATE 2016年第10期29卷 3647-3660页

作者： Liang, Xinfeng Yu, Lisan Univ S Florida Coll Marine Sci 140 7th Ave South St Petersburg FL 33701 USA Woods Hole Oceanog Inst Dept Phys Oceanog Woods Hole MA 02543 USA

An assessment is made of the mean and variability of the net air-sea heat flux, Q(net), from four products (ECCO, OAFlux-CERES, ERA-Interim, and NCEP1) over the global ice-free ocean from January 2001 to December 2010. For the 10-yr "hiatus'' period, all products agree on an overall net heat gain over the global ice-free ocean, but the magnitude varies from 1.7 to 9.5 Wm(-2). The differences among products are particularly large in the Southern Ocean, where they cannot even agree on whether the region gains or loses heat on the annual mean basis. Decadal trends of Qnet differ significantly between products. ECCO and OAFlux-CERES show almost no trend, whereas ERA-Interim suggests a downward trend and NCEP1 shows an upward trend. Therefore, numerical simulations utilizing different surface flux forcing products will likely produce diverged trends of the ocean heat content during this period. The downward trend in ERA-Interim started from 2006, driven by a peculiar pattern change in the tropical regions. ECCO, which used ERA-Interim as initial surface forcings and is constrained by ocean dynamics and ocean observations, corrected the pattern. Among the four products, ECCO and OAFlux-CERES show great similarities in the examined spatial and temporal patterns. Given that the two estimates were obtained using different approaches and based on largely independent observations, these similarities are encouraging and instructive. It is more likely that the global net air-sea heat flux does not change much during the so-called hiatus period.

关键词： Physical Meteorology and Climatology Heat budgets/fluxes Surface fluxes models and modeling Reanalysis data Variability Climate variability Interannual variability Seasonal variability

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