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Greg M. McFarquhar and Robert A. Black

Abstract

Mesoscale model simulations of tropical cyclones are sensitive to representations of microphysical processes, such as fall velocities of frozen hydrometeors. The majority of microphysical parameterizations are based on observations obtained in clouds not associated with tropical cyclones, and hence their suitability for use in simulations of tropical cyclones is not known. Here, representations of mass-weighted fall speed V m for snow and graupel are examined to show that parameters describing the exponential size distributions and fall speeds of individual hydrometeors [through use of relations such as V(D) = aD b] are identically important for determining V m. The a and b coefficients are determined by the composition and shape of snow and graupel particles; past modeling studies have not adequately considered the possible spread of a and b values. Step variations in these coefficients, associated with different fall velocity regimes, however, do not have a large impact on V m for observed size distributions in tropical cyclones and the values of a and b used here, provided that coefficients are chosen in accordance with the sizes where the majority of mass occurs. New parameterizations for V m are developed such that there are no inconsistencies between the diameters used to define the mass, number concentration, and fall speeds of individual hydrometeors. Effects due to previous inconsistencies in defined diameters on mass conversion rates between different hydrometeor classes (e.g., snow, graupel, cloud ice) are shown to be significant.

In situ microphysical data obtained in Hurricane Norbert (1984) and Hurricane Emily (1987) with two-dimensional cloud and precipitation probes are examined to determine typical size distributions of snow and graupel particles near the melting layer. Although well represented by exponential functions, there are substantial differences in how the intercept and slope of these distributions vary with mass content when compared to observations obtained in other locations; most notably, the intercepts of the size distributions associated with tropical cyclones increase with mass content, whereas some observations outside tropical cyclones show a decrease. Differences in the characteristics of the size distributions in updraft and downdraft regions, when compared to stratiform regions, exist, especially for graupel. A new representation for size distributions associated with tropical cyclones is derived and has significant impacts on the calculation of V m.

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Rebecca M. Westby and Robert X. Black

Abstract

During winter, anomalous temperature regimes (ATRs), which include cold-air outbreaks (CAOs) and warm waves (WWs), have important impacts in the southeastern United States. This study provides a synoptic–dynamic characterization of ATRs in the southeastern United States from 1949 to 2011 through composite time-evolution analyses. Events are categorized by the sign and amplitude of relevant low-frequency modes. During CAO (WW) onset, negative (positive) geopotential height anomalies are observed in the upper troposphere over the Southeast with oppositely signed anomalies in the lower troposphere over the central United States. In most cases, there is a surface east–west geopotential height anomaly dipole, with anomalous northerly (CAO) or southerly (WW) flow into the Southeast leading to cold or warm surface air temperature anomalies, respectively. Companion potential vorticity anomaly analyses reveal prominent features in the mid- to upper troposphere consistent with the coincident geopotential height anomaly patterns. Ultimately, synoptic-scale disturbances are found to serve as dynamic triggers for ATR events, while low-frequency modes provide a favorable environment for ATR onset. The results provide a qualitative indication of the role of low-frequency modes in ATR onset. In WW (CAO) events influenced by low-frequency modes, the North American geopotential height anomaly pattern arises in part as a downstream (regional) manifestation of the negative Pacific–North American pattern (North Atlantic Oscillation). Interestingly, the North Atlantic Oscillation contributes to both CAO onset and demise. Thus, these results indicate that low-frequency modes also affect event duration (CAOs). One general distinction found for ATRs is that CAOs involve substantial airmass transport while WW formation is more regional in nature.

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Randall M. Dole and Robert X. Black

Abstract

The present study extends our previous work on the life cycles of persistent anomalies by providing more comprehensive analyses of the synoptic and dynamical characteristics associated with the developments of the anomalies. We focus here on the developments of major cases of persistent negative height anomalies over the extratropical central North Pacific (PAC) region during wintertime. These ewes are generally manifested at the surface by an anomalously intense and eastward-displaced Aleutian low and, at upper levels, by an abnormally strong zonal jet that extends across most of the western and central Pacific at midlatitudes. The associated flow anomalies usually resemble particularly strong realizations of the Pacific-North American (PNA) teleconnection pattern.

The large-scale flow anomalies are typically preceded by a buildup of anomalously cold air over Asia and an intensification of the upper-level jet over southeastern Asia and the fox western Pacific. A few days prior to the larger-scale developments, a synoptic-scale disturbance intensifies over the northwest Pacific in a region of pronounced baroclinity on the cyclonic-shear side of the upper-level jet. As this disturbance propagates eastward into the mid-Pacific, it acquires a more zonally elongated, equivalent barotropic structure. During this period, the upper-level zonal wind anomalies initially over the western Pacific also extend eastward to the central Pacific.

The large-scale anomaly pattern that subsequently develops over the Pacific and North America resembles the most rapidly growing normal mode associated with barotropic instability of the climatological-mean wintertime flow. Diagnostic analyses confirm that, particularly at later stages in the developments, barotropic conversions from the time-mean flow contribute positively to the growth of the anomalies. These results support the idea that barotropic instability of the time-mean flow provides one mechanism for the developments.

Nevertheless, baroclinic processes also appear to play a significant role, particularly during the early stages of the developments. Temperature advection patterns associated with the growing disturbance tend to concentrate temperature gradients along the axis of the intensifying jet. Net eddy heat fluxes during the developments are both downgradient and upward, although most consistently so during the early stages of the developments. Net heat fluxes at later stages continue to have a substantial downgradient component although they also display a strong rotational (nondivergent) component, consistent with the more equivalent barotropic structure of the disturbance observed at later times.

The overall impression that emerges is of initial baroclinic development at long synoptic scales, followed by increasing barotropic contributions and decreasing baroclinic contributions to the growth of the anomalies after the disturbance reaches the jet exit region over the central Pacific. Additional baroclinic contributions to the developments may also occur at later stages. The observed characteristics are consistent with the hypothesis that the large-scale flow anomalies in these cases develop primarily as a result of an instability of the three-dimensional wintertime mean flow.

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Robert X. Black and Randall M. Dole

Abstract

Earlier studies of persistent large-scale flow anomalies have been extended, with the aim of identifying the primary mechanisms for persistent anomaly development. In this study the focus is on wintertime cases of persistent cyclonic flow anomalies over the North Pacific. These cases are typically manifested by an abnormally intense cyclonic circulation extending over the North Pacific basin, an unusually strong and eastward-extended East Asian jet, and a well-defined Pacific-North American teleconnection pattern. We have conducted extensive diagnostic analyses in order to determine the mechanisms responsible for development. In particular, these diagnostics examine the processes influencing the time evolution of eddy potential enstrophy and potential vorticity anomalies.

The cases are preceded by a buildup of anomalously high potential vorticity air at upper levels over eastern Asia. This high potential vorticity air is initially advected eastward in association with synoptic-scale cyclogenesis over the western North Pacific. As the disturbance propagates eastward into the central Pacific, it evolves toward a more zonally elongated and equivalent barotropic structure. Large-scale cyclogenesis ensues as the low becomes quasi-stationary near the Aleutians. In conjunction with large-scale development, the disturbance reacquires an upshear tilt with height.

Diagnostic analyses of wave activity fluxes indicate that the primary source region for the developments is over the extratropical North Pacific. Potential enstrophy analyses show that eddy enstrophy increases result mainly from downgradient potential vorticity fluxes by the large-scale eddy. The conversions are primarily baroclinic in nature, although barotropic processes also provide positive contributions. Anomalous nonconservative and nonlinear processes are relatively small and oppose the observed enstrophy changes.

Potential vorticity (PV) inversions are then performed to further clarify the dynamical mechanisms for large-scale development. A few days prior to large-scale development, anomalous upper-level northwesterly winds, associated with low-level thermal anomalies over the western North Pacific region, advect high PV air south-eastward from Asia into the western Pacific. As the PV maximum reaches the central Pacific, its associated circulation penetrates to the surface, resulting in a thermal advection pattern that produces a warm surface anomaly and associated surface cyclone downshear of the upper-level center. This is followed by strong baroclinic intensification. In several respects this behavior resembles a classical Petterssen Type B development, but occurs on a scale that is much larger than for typical synoptic-scale cyclogenesis.

The results indicate that the primary mechanism for the developments is a large-scale instability of (or initial value development upon) the three-dimensional time-mean flow, and suggest that nonmodal transient growth plays a significant role during development.

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Roger M. Wakimoto and Peter G. Black

A damage map documenting Hurricane Andrew's destructive landfall over southern Florida is presented. Vectors that represent the direction of winds causing damage to trees and structures are shown along with an F-scale rating in order to assess the strength of the near-surface winds. It is hypothesized that increased surface roughness once the hurricane made landfall may have contributed to a surface wind enhancement resulting in the strongest winds ever estimated (F3) for a landfall hurricane. This intense damage occurred primarily during the “second” period of strong winds associated with the east side of the eyewall. For the first time, a well-defined circulation inthe damage pattern by the second wind was documented. A superposition of radar data from Miami and Key West on top of the damage map provides the first detailed examination of the relationship between the eyewall and the surface flow field as estimated from the damage vectors.

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Robert X. Black and Randall M. Dole

Abstract

The relationship between the time-mean planetary-scale deformation field and the structure of midlatitude storm tracks is studied in wintertime simulations of the National Center for Atmospheric Research (NCAR) Community Climate Model and the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System model. Model biases are determined by contrasting model simulations (forced by observed SSTs) with parallel analyses of NCEP–NCAR reanalyses. Barotropic diagnostics are employed to identify potential dynamical linkages between regional biases in the midlatitude storm tracks and the horizontal deformation field. Initial observational analyses confirm that synoptic eddies are optimally configured to transfer kinetic energy to the mean flow in the jet exit regions, where strong stretching deformation exists. In these regions, the major axes of the synoptic eddies are aligned along the dilatation axes of the mean flow. Consequently, mean flow advection stretches synoptic eddies along their major axes, thereby increasing their anisotropy and weakening their kinetic energy.

A strong link is identified between model biases in the horizontal structure of the midlatitude storm tracks and the representation of upper-tropospheric barotropic deformation. In particular, model-simulated storm tracks extend too far downstream in regions where the zonal stretching deformation (associated with horizontal diffluence in jet exit regions) is either too weak in magnitude or displaced westward in comparison with observations. These biases are associated with anomalously weak or westward-displaced patterns of negative barotropic energy conversions, which normally act as a sink of synoptic eddy activity in the jet exit. The anomalous energy conversion patterns are primarily due to model biases in the winter-mean flow rather than the simulated horizontal eddy structures, which closely resemble observations.

The results indicate that the horizontal structure of midlatitude storm tracks in climate models is strongly controlled by the large-scale patterns of barotropic deformation in the upper troposphere. It is suggested that barotropic deformation analyses may provide a useful diagnostic measure for assessing climate simulation errors in atmospheric general circulation models.

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Matthew D. Eastin, William M. Gray, and Peter G. Black

Abstract

The buoyancy of hurricane convective vertical motions is studied using aircraft data from 175 radial legs collected in 14 intense hurricanes at four altitudes ranging from 1.5 to 5.5 km. The data of each leg are initially filtered to separate convective-scale features from background mesoscale structure. Convective vertical motion events, called cores, are identified using the criteria that the convective-scale vertical velocity must exceed 1.0 m s−1 for at least 0.5 km. A total of 620 updraft cores and 570 downdraft cores are included in the dataset. Total buoyancy is calculated from convective-scale virtual potential temperature, pressure, and liquid water content using the mesoscale structure as the reference state.

Core properties are summarized for the eyewall and rainband regions at each altitude. Characteristics of core average convective vertical velocity, maximum convective vertical velocity, and diameter are consistent with previous studies of hurricane convection. Most cores are superimposed upon relatively weak mesoscale ascent. The mean eyewall (rainband) updraft core exhibits small, but statistically significant, positive total buoyancy below 4 km (between 2 and 5 km) and a modest increase in vertical velocity with altitude. The mean downdraft core not superimposed upon stronger mesoscale ascent also exhibits positive total buoyancy and a slight decrease in downward vertical velocity with decreasing altitude. Buoyant updraft cores cover less than 5% of the total area in each region but accomplish ∼40% of the total upward transport.

A one-dimensional updraft model is used to elucidate the relative roles played by buoyancy, vertical perturbation pressure gradient forces, water loading, and entrainment in the vertical acceleration of ordinary updraft cores. Small positive total buoyancy values are found to be more than adequate to explain the vertical accelerations observed in updraft core strength, which implies that ordinary vertical perturbation pressure gradient forces are directed downward, opposing the positive buoyancy forces. Entrainment and water loading are also found to limit updraft magnitudes.

The observations support some aspects of both the hot tower hypothesis and symmetric moist neutral ascent, but neither concept appears dominant. Buoyant convective updrafts, however, are integral components of the hurricane’s transverse circulation.

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Vincent M. Brown, Barry D. Keim, and Alan W. Black

Abstract

This research introduces a climatology of hourly precipitation characteristics, investigates trends in precipitation hours (PH) and hourly accumulation, and uses four different time series to determine if precipitation intensity is changing across the southeastern United States from 1960 to 2017. Results indicate hourly intensity significantly increased at 44% (22/50) of the stations, accompanied by an increase in average hourly accumulation at 40% of the sites analyzed (20/50). The average duration of precipitation events decreased at 82% (41/50) of the stations. However, the frequency of 90th percentile hourly events and events above station-specific average hourly totals did not show a broad increase similar to hourly intensity. It seems hourly events are becoming heavier on average, while the duration of the average precipitation event is decreasing. Geographically, heavy hourly events are more frequent along the Gulf Coast and decrease inland. PH significantly decreased across South Carolina, Georgia, and northern Florida, mainly due to significant decreases in winter (DJF) and spring (MAM). Decreases in PH during spring were contained to Georgia and South Carolina and were accompanied by a decrease in accumulation. Decreases in PH during winter were more widespread and did not exhibit a broad decrease in accumulation, suggesting winter precipitation across that portion of the region is becoming more intense.

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Bradley M. Hegyi, Yi Deng, Robert X. Black, and Renjun Zhou

Abstract

Perpetual winter simulations using the NCAR Whole Atmosphere Community Climate Model (WACCM) are conducted to document the differences of the initial transient response of the boreal winter Northern Hemisphere stratospheric polar vortex to central (CPW) and eastern Pacific warming (EPW) events. Idealized patches of positive sea surface temperature (SST) anomalies are superimposed onto a climatological SST field to mimic canonical CPW and EPW forcings. A 20-member ensemble was created by varying initial atmospheric conditions for both CPW and EPW cases. In the ensemble average, the vortex weakens under both CPW and EPW forcing, indicated by a negative zonal mean zonal wind tendency. This tendency is mainly tied to changes in the eddy-driven mean meridional circulation (MMC). A negative anomaly in the eddy momentum flux convergence also plays a secondary role in the weakening. The vortex response, however, differs dramatically among individual ensemble members. A few ensemble members exhibit initial vortex strengthening although weaker in magnitude and shorter in duration than the initial weakening in the ensemble average. The initial state and the subsequent internal variation of the extratropical atmosphere is at least as important as the type of SST forcing in determining the transient response of the stratospheric polar vortex. Interactions between the internal variability of the vortex and SST-driven wave anomalies ultimately determine the nature of the initial transient response of the vortex to EPW and CPW forcing. This sensitivity to the initial atmospheric state has implications for understanding medium-range forecasts of the extratropical atmospheric response to emerging tropical SST anomalies, particularly over high-latitude regions.

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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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