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J. Sirutis and K. Miyakoda

Abstract

Four packages of subgrid scale (SGS) physics parameterization are tested by including them in a general circulation model and by applying the four models to 1-month forecasts. The four models are formulated by accumulatively increasing the elaboration and the sophistication of the physics. The first is the reference model (the A-physics); the second model (the E-physics) uses the Monin–Obukhov similarity theory for the fluxes of surface boundary layer, the turbulence closure scheme for the fluxes in the entire atmosphere, and subsurface soil heat conduction; the third model (the F-physics) replaces the cumulus parameterization by the Arakawa–Schubert method; and the fourth model (the FM-physics) enhances the SGS orography. One-month integrations are performed for eight January cases, with each case consisting of three different forecasts. Originally the forecast performance was expected to be a stepwise improvement with the elaboration of the SGS physics from the A to the FM, but the forecast results do not show up in such a simple way. The impact of these processes on the 1-month integration is subtle and yet significant. The superiority of the F-model over the A- and the E-models is evident in the last 10 days of the 1-month forecasts, though the performance of the E-model is consistently good, in comparison with the other models, in terms of root-mean-square (rms) error of geopotential height. It is likely that 80% condensation criterion in the E (instead of 100%) is at least partly responsible for the forecast deterioration in the last 10 days, compared with the F. The FM-model gives the lowest rms error, but the predicted transient eddies are extremely low, probably due to the excessively enhanced orography. The simulated global precipitation patterns are presented for the different models, and the drawbacks are discussed. The F- and the FM-models produce spatially smooth distribution of tropical rainfall. The 30-day forecast performance appears to be more sensitive to the initial conditions, rather than the SGS physics. The systematic errors in all of the models are substantial in magnitude, though they vary with the SGS physics.

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K. Miyakoda and J. Sirutis

Abstract

The capability of blocking prediction is investigated with respect to four models of different subgrid scale parameterization packages, which were described in Part I. In order to assess the capability, blocking indices are defined, and threat and bias scores are set up for the predicted blocking index against the observation. Applying this evaluation scheme to the dataset of one-month forecasts for eight January cases, we conduct a study on the performance of blocking simulation.

First, it is immediately disclosed that the systematic biases in this forecast set are overwhelmingly large, so that the blocking index has to be adjusted to this bias. One, of the major issues, suggested by Tibaldi and Molteni, is whether the systematic bias is generated by the failure of blocking forecasts. Overall, this study supports this assertion, despite the different definitions of blocking. The study also reveals that the A-model is inferior to the other three models, such as the E-model, with regard to blocking forecasts. The reason for this is that the E-model, for example, which includes turbulence closure parameterization, appears to provide an adequate conversion of low-frequency eddy potential to kinetic energy, and thereby produces a more reasonable amount of standing eddies related to the persistent ridges. It is also pointed out that the blocking activity in the winter Northern Hemisphere is manifested by a distinct subpolar peak in the meridional distribution of standing eddy kinetic energy. The E-model tends to generate a well-defined peak of this energy distribution. All models are deficient in expanding the zonal mean westerlies to higher latitudes, particularly the A-model. In this connection, a hypothesis is postulated on a precondition for blocking: the upstream westerlies prior to the onset have to be displaced relatively at lower latitude. In the successful cases of blocking forecast, the upstream westerlies at 40°–60°N are relatively weaker than those in the unsuccessful cases.

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K. Miyakoda, J. Sirutis, and J. Ploshay

Abstract

A series of one-month forecasts were carried out for eight January cases, using a particular prediction model and prescribing climatological sea-surface temperature as the boundary condition. Each forecast is a stochastic prediction that consists of three individual integrations. These forecasts start with observed initial conditions derived from datasets of three meteorological centers. The forecast skill was assessed with respect to time means of variables based on the ensemble average of three forecasts. The time or space filter is essential to suppress unpredictable components of atmospheric variabilities and thereby to make an attempt at extending the limit of predictability. The circulation patterns of the three individual integrations tend to be similar to each other on the one-month time scale, implying that forecasts for the 10 day (or 20 day) means are not fully stochastic. The overall results indicate that the 10-day mean height prognoses resemble observations very well in the first ten days, and then start to lose similarity to real states, and yet there is some recognizable skill in the last ten days of the month. The main interests in this study are the feasibility of one-month forecasts, the adequacy of initial conditions produced by a particular data assimilation, and the growth of stochastic uncertainty. An outstanding problem turns out to be a considerable degree of systematic error included in the prediction model, which is now known to be “climate drift.” Forecast errors are largely due to the model's systematic bias. Thus, forecast skill scores are substantially raised if the final prognoses are adjusted for the model's known climatic drift.

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K. Miyakoda, J. Sheldon, and J. Sirutis

Abstract

The GATE analysis was repeated utilizing the full GATE data set in the delayed mode and a revised four-dimensional analysis procedure. The resulting maps wore compared with maps of other author. Based on the new analysis, macroscale circulation features for the tropical African continent and Atlantic Ocean region were calculated, and other characteristic phenomena of this area were investigated. The easterly waves, in particular, were studied with respect to their formation, propagation, associated condensation, and possible conversion to hurricanes. It was possible to trace nine distinct easterly waves throughout their entire life history, and the analyzed tracks of thew easterly waves agreed quite well with the subjective analyses of Sadler and Oda (1978). The time sequences of precipitation over the GATE A/B-array obtained by the present analysis and by satellite estimates were compared with some success.

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Robert E. Tuleya, Morris Bender, Thomas R. Knutson, Joseph J. Sirutis, Biju Thomas, and Isaac Ginis

Abstract

The GFDL hurricane modeling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution will be briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range of three environmental factors that are often identified as key factors in tropical cyclone (TC) evolution: SST, atmospheric stability (upper-air thermal anomalies), and vertical wind shear (westerly through easterly). A wide range of minimum central pressure intensities resulted (905–980 hPa). The results confirm that a scenario (e.g., global warming) in which the upper troposphere warms relative to the surface will have less TC intensification than one with a uniform warming with height. The TC rainfall is also investigated for the SST–stability parameter space. Rainfall increases for combinations of SST increase and increasing stability similar to global warming scenarios, consistent with climate change TC downscaling studies with the GFDL model. The forecast system’s sensitivity to vertical shear was also investigated. The idealized model simulations showed weak disturbances dissipating under strong easterly and westerly shear of 10 m s−1. A small bias for greater intensity under easterly sheared versus westerly sheared environments was found at lower values of SST. The impact of vertical shear on intensity was different when a strong vortex was used in the simulations. In this case, none of the initial disturbances weakened, and most intensified to some extent.

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K. Miyakoda, L. Umscheid, D. H. Lee, J. Sirutis, R. Lusen, and F. Pratte

Abstract

Global upper air and surface data for the entire GATE period from 15 June to 24 September 1974, were collected by the Data Assimilation Branch of NMC and mailed to GFDL. After processing these data, a four-dimensional analysis technique was applied for the entire GATE period, using a global numerical model. For a selected period, several different versions of the data processing scheme were tested. The resulting analyses were compared with each other and with the objective analysis of NMC in Washington D.C., and ANMRC in Melbourne. Overall, the analyses for the extratropics were satisfactory for the Northern Hemisphere, and to a lesser extent, for the Southern Hemisphere, though flow patterns are somewhat excessively smoothed. The analyses for the tropics were not of the same quality as those for the extratropics, and yet they were much improved compared with those of several years ago. A noteworthy point is that tropical cyclones were successfully represented in several cases.

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K. Miyakoda, T. Gordon, R. Caverly, W. Stern, J. Sirutis, and W. Bourke

Abstract

January 1977 was a month noted for its extraordinary weather over North America. The winter was dominated by two persistent large amplitude ridges positioned over the west coast of North America and the Icelandic region of the Atlantic Ocean. A very intense trough reached deep into the eastern United States and caused one of the coldest Januaries on record. One-month integrations of various GCM's were conducted in order to test their ability to simulate this blocking event. Reasonably high resolution finite difference and spectral models available at GFDL were used. Each GCM was integrated from three different analyses of the initial conditions. For some models, a fairly accurate forecast was obtained and considerable skill was recognized in the simulation of the 30-day evolution in terms of the 5-day or 10-day mean flow fields, including the period of record breaking coldness over the eastern United States. The main conclusion is that proper treatment of the subgrid-scale processes as well as sufficient spatial resolution are essential for the simulations of this phenomenon as an initial value problem. Weak zonal wind poleward of about 40°N and upstream of the blocking ridge appears to be crucial for the successful simulation of the sustained blocking ridge.

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Thomas R. Knutson, Joseph J. Sirutis, Stephen T. Garner, Isaac M. Held, and Robert E. Tuleya

In this study, a new modeling framework for simulating Atlantic hurricane activity is introduced. The model is an 18-km-grid nonhydrostatic regional model, run over observed specified SSTs and nudged toward observed time-varying large-scale atmospheric conditions (Atlantic domain wavenumbers 0–2) derived from the National Centers for Environmental Prediction (NCEP) reanalyses. Using this “perfect large-scale model” approach for 27 recent August–October seasons (1980–2006), it is found that the model successfully reproduces the observed multidecadal increase in numbers of Atlantic hurricanes and several other tropical cyclone (TC) indices over this period. The correlation of simulated versus observed hurricane activity by year varies from 0.87 for basinwide hurricane counts to 0.41 for U.S. landfalling hurricanes. For tropical storm count, accumulated cyclone energy, and TC power dissipation indices the correlation is ~0.75, for major hurricanes the correlation is 0.69, and for U.S. landfalling tropical storms, the correlation is 0.57. The model occasionally simulates hurricanes intensities of up to category 4 (~942 mb) in terms of central pressure, although the surface winds (< 47 m s−1) do not exceed category-2 intensity. On interannual time scales, the model reproduces the observed ENSO-Atlantic hurricane covariation reasonably well. Some notable aspects of the highly contrasting 2005 and 2006 seasons are well reproduced, although the simulated activity during the 2006 core season was excessive. The authors conclude that the model appears to be a useful tool for exploring mechanisms of hurricane variability in the Atlantic (e.g., shear versus potential intensity contributions). The model may be capable of making useful simulations/projections of pre-1980 or twentieth-century Atlantic hurricane activity. However, the reliability of these projections will depend on obtaining reliable large-scale atmospheric and SST conditions from sources external to the model.

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Thomas R. Knutson, Joseph J. Sirutis, Ming Zhao, Robert E. Tuleya, Morris Bender, Gabriel A. Vecchi, Gabriele Villarini, and Daniel Chavas

Abstract

Global projections of intense tropical cyclone activity are derived from the Geophysical Fluid Dynamics Laboratory (GFDL) High Resolution Atmospheric Model (HiRAM; 50-km grid) and the GFDL hurricane model using a two-stage downscaling procedure. First, tropical cyclone genesis is simulated globally using HiRAM. Each storm is then downscaled into the GFDL hurricane model, with horizontal grid spacing near the storm of 6 km, including ocean coupling (e.g., “cold wake” generation). Simulations are performed using observed sea surface temperatures (SSTs) (1980–2008) for a “control run” with 20 repeating seasonal cycles and for a late-twenty-first-century projection using an altered SST seasonal cycle obtained from a phase 5 of CMIP (CMIP5)/representative concentration pathway 4.5 (RCP4.5) multimodel ensemble. In general agreement with most previous studies, projections with this framework indicate fewer tropical cyclones globally in a warmer late-twenty-first-century climate, but also an increase in average cyclone intensity, precipitation rates, and the number and occurrence days of very intense category 4 and 5 storms. While these changes are apparent in the globally averaged tropical cyclone statistics, they are not necessarily present in each individual basin. The interbasin variation of changes in most of the tropical cyclone metrics examined is directly correlated to the variation in magnitude of SST increases between the basins. Finally, the framework is shown to be capable of reproducing both the observed global distribution of outer storm size—albeit with a slight high bias—and its interbasin variability. Projected median size is found to remain nearly constant globally, with increases in most basins offset by decreases in the northwest Pacific.

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Thomas R. Knutson, Joseph J. Sirutis, Gabriel A. Vecchi, Stephen Garner, Ming Zhao, Hyeong-Seog Kim, Morris Bender, Robert E. Tuleya, Isaac M. Held, and Gabriele Villarini

Abstract

Twenty-first-century projections of Atlantic climate change are downscaled to explore the robustness of potential changes in hurricane activity. Multimodel ensembles using the phase 3 of the Coupled Model Intercomparison Project (CMIP3)/Special Report on Emissions Scenarios A1B (SRES A1B; late-twenty-first century) and phase 5 of the Coupled Model Intercomparison Project (CMIP5)/representative concentration pathway 4.5 (RCP4.5; early- and late-twenty-first century) scenarios are examined. Ten individual CMIP3 models are downscaled to assess the spread of results among the CMIP3 (but not the CMIP5) models. Downscaling simulations are compared for 18-km grid regional and 50-km grid global models. Storm cases from the regional model are further downscaled into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model (9-km inner grid spacing, with ocean coupling) to simulate intense hurricanes at a finer resolution.

A significant reduction in tropical storm frequency is projected for the CMIP3 (−27%), CMIP5-early (−20%) and CMIP5-late (−23%) ensembles and for 5 of the 10 individual CMIP3 models. Lifetime maximum hurricane intensity increases significantly in the high-resolution experiments—by 4%–6% for CMIP3 and CMIP5 ensembles. A significant increase (+87%) in the frequency of very intense (categories 4 and 5) hurricanes (winds ≥ 59 m s−1) is projected using CMIP3, but smaller, only marginally significant increases are projected (+45% and +39%) for the CMIP5-early and CMIP5-late scenarios. Hurricane rainfall rates increase robustly for the CMIP3 and CMIP5 scenarios. For the late-twenty-first century, this increase amounts to +20% to +30% in the model hurricane’s inner core, with a smaller increase (~10%) for averaging radii of 200 km or larger. The fractional increase in precipitation at large radii (200–400 km) approximates that expected from environmental water vapor content scaling, while increases for the inner core exceed this level.

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