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Dennis P. Robinson and Robert X. Black

Abstract

Comparative diagnostic analyses of developing synoptic-scale baroclinic disturbances in NCEP–NCAR reanalyses and the NASA–NCAR (NASCAR) and Aries [NASA’s Seasonal-to-Interannual Prediction Project (NSIPP)] general circulation model simulations are performed. In particular, lag composite analyses of wintertime cyclonic and anticyclonic events occurring in the North Pacific and North Atlantic storm tracks are constructed to pursue a synoptic and dynamic characterization of eddy development. The data are also seasonally stratified to study aspects of the North Pacific midwinter suppression phenomenon.

Winter-averaged results indicate that the model-simulated events are generally too weak in amplitude, particularly in the upper troposphere. For the North Pacific storm track, model-simulated events are also anomalously distended in the meridional direction. The existing model biases in eddy structure and magnitude lead to anomalously weak baroclinic energy conversions for both cyclonic and anticyclonic events over the North Pacific. For the North Atlantic storm track the NASCAR model provides a very good representation of the structure of developing cyclonic events. However, growing North Atlantic cyclones in the NSIPP model are anomalously weak and horizontally too isotropic (meridionally retracted). These latter two characteristics are also observed in both models for developing anticyclonic flow anomalies over the North Atlantic. The relative weakness of NSIPP synoptic events over the North Atlantic region is largely responsible for the 50% deficiency in areal-averaged baroclinic energy conversions. Conversely, the NASCAR model climatology features anomalously strong temperature gradients over the western North Atlantic that provide local enhancements to the baroclinic energy conversion field.

A seasonally stratified diagnostic analysis reveals that the simulated climatological storm tracks over the North Pacific undergo larger spatial migrations during the cool season compared to observations. It is further determined that the suppression of synoptic eddy activity observed in the Pacific storm track is associated with a relative midwinter weakness in the magnitude of the growing cyclonic anomalies. Specifically, during midwinter the cyclonic perturbations entering the Pacific storm track are deficient in magnitude compared to their early and late winter counterparts. It is also discovered that the midwinter suppression pattern over the North Pacific region has a clear organized extension upstream into Siberia, the region from which incipient upper-tropospheric short-wave features emanate. This behavior is found in both observations and the model simulations. The results herein support the idea that the North Pacific midwinter suppression phenomenon is linked to a midwinter weakness in the upstream formation of upper-level short waves, leading to anomalously weak “seeding” of baroclinic disturbances in the Pacific storm track.

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Dennis P. Robinson and Robert X. Black

Abstract

A comprehensive analysis of midlatitude intraseasonal variability in extended integrations of NASA GSFC general circulation models (GCMs) is conducted. This is approached by performing detailed intercomparisons of the representation of the storm tracks and anomalous weather regimes occurring during wintertime in the Atmospheric Model Intercomparison Project (AMIP)-type simulations of both the NASA–NCAR and a version of the Aries model used in NASA’s Seasonal-to-Interannual Prediction Project (NSIPP) model. The model-simulated statistics, three-dimensional structure, and dynamical characteristics of these phenomena are diagnosed and directly compared to parallel observational analyses derived from NCEP–NCAR reanalyses.

A qualitatively good representation of the vertical structure of intraseasonal eddy kinetic energy (EKE) is provided by both models with maximum values of EKE occurring near 300 hPa. The main model shortcoming is an underestimation of EKE in the upper troposphere, especially for synoptic eddies in the NSIPP model. Nonetheless, both models provide a reasonable representation of the three-dimensional structure and dynamical characteristics of synoptic eddies. Discrepancies in the storm-track structures simulated by the models include an anomalous local minimum over the eastern Pacific basin. However, both GCMs faithfully reproduce the observed Pacific midwinter storm-track suppression. Interestingly, the NSIPP model also produces a midwinter suppression feature over the Atlantic storm track in association with the anomalously strong upper-level jet stream simulated by NSIPP in this region.

The regional distribution of anomalous weather regime events is well simulated by the models. However, substantial structural differences exist between observed and simulated events over the North Pacific region. In comparison to observations, model events are horizontally more isotropic, have stronger westward vertical tilts, and are more strongly driven by baroclinic dynamics. The structure and dynamics of anomalous weather regimes occurring over the North Atlantic region are qualitatively better represented by the models. The authors suggest that model deficiencies in representing the zonally asymmetric climatological-mean flow field (particularly the magnitude and structure of the Pacific and Atlantic jet streams) help contribute to model shortcomings in (i) the strength and seasonal variability of the storm tracks and (ii) dynamical distinctions in the maintenance of large-scale weather regimes.

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H. E. Willoughby and P. G. Black

Four meteorological factors aggravated the devastation when Hurricane Andrew struck South Florida: completed replacement of the original eyewall by an outer, concentric eyewall while Andrew was still at sea; storm translation so fast that the eye crossed the populated coastline before the influence of land could weaken it appreciably; extreme wind speed, 82 m s− 1 winds measured by aircraft flying at 2.5 km; and formation of an intense, but nontornadic, convective vortex in the eyewall at the time of landfall. Although Andrew weakened for 12 h during the eyewall replacement, it contained vigorous convection and was reintensifying rapidly as it passed onshore. The Gulf Stream just offshore was warm enough to support a sea level pressure 20–30 hPa lower than the 922 hPa attained, but Andrew hit land before it could reach this potential. The difficult-to-predict mesoscale and vortex-scale phenomena determined the course of events on that windy morning, not a long-term trend toward worse hurricanes.

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K. R. Bryan, K. P. Black, and R. M. Gorman

Abstract

Three-dimensional point velocity measurements from within and near the surf zone are used to examine changes in turbulent dissipation rate with location relative to the breakpoint and wave conditions. To separate turbulence from wave orbital currents, dissipation rate is derived using the inertial subrange of the wavenumber spectrum. The measured frequency spectrum is transformed into a wavenumber spectrum by generalizing Taylor's hypothesis to advection of turbulence past a sensor by monochromatic water waves in arbitrary water depth. The resulting dissipation rate is compared with that obtained by averaging the dissipation rates calculated from frequency spectra of very short segments of the time series, over which Taylor's hypothesis for steady currents can be used. The dissipation rate calculated using the latter compares well, in the averaged sense, to the dissipation rate calculated using the whole time series, even though the whole time series includes segments that violate Taylor's assumptions. Measured dissipation rates inside and very near the breakpoint show large increases shoreward and lesser increases with deep-water significant wave height. Farther outside the surf zone, dissipation rate depended on frequency, significant wave height, and depth. Simple models show that the surf-zone patterns are explained by shoreward increases in the probability of wave breaking, although measured turbulence levels are significantly less than needed to dissipate the measured wave energy, suggesting that most of the dissipation occurs above trough level or very near the bed.

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Joseph P. Gerrity Jr., Thomas L. Black, and Russell E. Treadon

Abstract

A new method is presented for obtaining the numerical solution of the production-dissipation component of the turbulent kinetic energy equation that arises in the Mellor-Yamada level 2.5 turbulent closure model. The development of this new method was motivated by the occasional appearance of large temporal oscillations in the solution provided by a previously used method. Analysis of the equation revealed that the solution should tend toward a stationary asymptotic value, which is the equilibrium value of turbulent kinetic energy for the level 2, Mellor-Yamada model. Failure to identify the correct asymptotic value in the formalism underlying the numerical solution of the equation allows the solution to overshoot the equilibrium. Repeated overshooting gives rise to an oscillation in the solution from one time step to the next. The new method prevents this from happening.

Idealized cases are used to demonstrate the performance of the new method. It has been incorporated into the eta coordinate, numerical weather prediction model being used by the National Meteorological Center.

Although the new method corrects the particular deficiency of the previous method, the integration of the equation for turbulent kinetic energy remains subject to oscillatory solutions associated with rapid variations of the Richardson number. An example of this is provided.

Additionally, even with the new method, it is sometimes necessary to revert to the level 2 model when the numerical integration of the full system of equations yields a value of turbulent kinetic energy that falls below a value associated with a singularity of the level 2.5 model. In future work, modifications of the Mellor-Yamada turbulent closure system that avoid this limitation will be investigated.

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Caroline M. Dunning, Emily Black, and Richard P. Allan

Abstract

Changes in the seasonality of precipitation over Africa have high potential for detrimental socioeconomic impacts due to high societal dependence upon seasonal rainfall. Here, for the first time we conduct a continental-scale analysis of changes in wet season characteristics under the RCP4.5 and RCP8.5 climate projection scenarios across an ensemble of CMIP5 models using an objective methodology to determine the onset and cessation of the wet season. A delay in the wet season over West Africa and the Sahel of over 5–10 days on average, and later onset of the wet season over southern Africa, is identified and associated with increasing strength of the Saharan heat low in late boreal summer and a northward shift in the position of the tropical rain belt over August–December. Over the Horn of Africa rainfall during the “short rains” season is projected to increase by over 100 mm on average by the end of the twenty-first century under the RCP8.5 scenario. Average rainfall per rainy day is projected to increase, while the number of rainy days in the wet season declines in regions of stable or declining rainfall (western and southern Africa) and remains constant in central Africa, where rainfall is projected to increase. Adaptation strategies should account for shorter wet seasons, increasing rainfall intensity, and decreasing rainfall frequency, which will have implications for crop yields and surface water supplies.

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Caroline M. Wainwright, Emily Black, and Richard P. Allan

Abstract

Climate change will result in more dry days and longer dry spells; however, the resulting impacts on crop growth depend on the timing of these longer dry spells in the annual cycle. Using an ensemble of Coupled Model Intercomparison Project phase 5 and phase 6 (CMIP5 and CMIP6) simulations, and a range of emission scenarios, here we examine changes in wet and dry spell characteristics under future climate change across the extended tropics in wet and dry seasons separately. Delays in the wet seasons by up to 2 weeks are projected by 2070–99 across South America, southern Africa, West Africa, and the Sahel. An increase in both mean and maximum dry spell length during the dry season is found across Central and South America, southern Africa, and Australia, with a reduction in dry season rainfall also found in these regions. Mean dry season dry spell lengths increase by 5–10 days over northeast South America and southwest Africa. However, changes in dry spell length during the wet season are much smaller across the tropics with limited model consensus. Mean dry season maximum temperature increases are found to be up to 3°C higher than mean wet season maximum temperature increases over South America, southern Africa, and parts of Asia. Longer dry spells, fewer wet days, and higher temperatures during the dry season may lead to increasing dry season aridity and have detrimental consequences for perennial crops.

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Mark D. Powell, Peter P. Dodge, and Michael L. Black

Abstract

Hurricane Hugo struck Charleston, South Carolina, on 22 September 1989 as the most intense hurricane to affect the United States since Camille in 1969. The northeastern eyewall, which contained the maximum winds measured by reconnaissance aircraft shortly before landfall, moved inland over a relatively unpopulated area and there were few fatalities. However, no observations were available to document the surface wind distribution in this part of the storm as it continued inland.

To improve specification of surface winds in Hugo, empirically adjusted aircraft winds were combined with coastal, offshore, and inland surface observations and were input to the Ooyama objective analysis algorithm. The wind analysis at landfall was then compared with subsequent analyses at 3 and 6 h after landfall. Reconstruction of the surface wind field at landfall suggests that the maximum (∼13 min mean) surface wind at the coast was 50 m s−1 in the Bulls Bay region, ∼40 km northeast of Charleston. Surface roughness over land caused wind speeds to drop off rapidly just inland of the coast to only 50% of values measured by reconnaissance aircraft at the same location relative to the storm over water. Despite relatively rapid increases in the central sea-level pressure and decreases in the mean circulation as Hugo progressed inland, hurricane-force wind gusts extended Hugo's damage pattern well past Charlotte, North Carolina, ∼330 km inland.

Accurate determination of surface wind distribution in land-falling hurricanes is dependent upon the spatial density and quality of surface wind measurements and techniques to adjust reconnaissance flight-level winds to the surface. Improvements should allow forecasters to prepare more-accurate warnings and advisories and allow more-thorough documentation of poststorm effects. Empirical adjustments to reconnaissance aircraft measurements may replace surface data voids if the vertical profile of the horizontal wind is known. Expanded use of the airborne stepped-frequency microwave radiometer for remote sensing of ocean surface winds could fill data voids without relying upon empirical methods or models. A larger network of offshore, coastal, and inland surface platforms at standard (10-m) elevations with improved sampling strategies is envisioned for better resolution of hurricane wind fields. A rapid-response automatic station network, deployed at prearranged coastal locations by local universities with meteorology and/or wind engineering programs, could further supplement the fixed platform network and avoid the logistical problems posed by sending outside teams into threatened areas.

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Richard J. Hutcheon, Richard H. Johnson, William P. Lowry, Charles H. Black, and Donald Hadley
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Peter G. Black, Russell L. Elsberry, Lynn K. Shay, Ray P. Partridge, and Jeffrey D. Hawkins

Abstract

Three drifting buoys were successfully air-dropped ahead of Hurricane Josephine. This deployment resulted in detailed simultaneous measurements of surface wind speed, surface pressure and subsurface ocean temperature during and subsequent to storm passage. This represents the first time that such a self-consistent data set of surface conditions within a tropical cyclone has been collected. Subsequent NOAA research overflights of the buoys, as part of a hurricane planetary boundary-layer experiment, showed that aircraft wind speeds, extrapolated to the 20 m level, agreed to within ±2 m s−1, pressures agreed to within ±1 mb, and sea-surface temperatures agreed to within ±0.8°C of the buoy values. Ratios of buoy peak 1 min wind (sustained wind) to one-half h mean wind > 1.3 were found to coincide with eyewall and principal rainband features.

Buoy trajectories and subsurface temperature measurements revealed the existence of a series of mesoscale eddies in the subtropical front. Buoy data revealed storm-generated, inertia-gravity-wave motions superposed upon mean current fields, which reached a maximum surface speed > 1.2 m s−1 immediately following storm passage. A maximum mixed-layer-temperature decrease of 1.8°C was observed to the right of the storm path. A temperature increase of 3.5°C at 100 m and subsequent decrease of 4.8°C following storm passage indicated a combination of turbulent mixing, upwelling and horizontal advection processes.

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