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J. A. Milbrandt and R. McTaggart-Cowan

Abstract

The computation of hydrometeor sedimentation in one-moment, two-moment, and three-moment bulk microphysics parameterizations is examined in the context of a 1D model, with no other microphysical processes active. The solution from an analytic bin model is used as a reference against which the bulk model simulations are compared. Errors in the computed (nonprognostic) moments from 0 to 7 from the bulk model runs are examined. In addition to the commonly used predicted variables (number concentration, mass, and reflectivity), bulk scheme configurations with alternative combinations of prognostic moments are considered.

While the extra degree of freedom in a two-moment scheme adds realism to the simulation of sedimentation over a one-moment scheme, the standard practice of imposing a constant relative dispersion in the particle size distribution results in considerable errors in some of the computed moments. The error can be shifted to different moments by selecting different prognostic moments. For three-moment schemes, the error is considerably reduced over a wide range of computed moments and there is much less sensitivity to the choice of prognostic variables.

Two alternative approaches are proposed for modifying the computation of sedimentation in two-moment schemes to reduce problems associated with excess size sorting. The first approach uses a diagnostic relative dispersion (shape) parameter, generalized for any pair of prognostic moments. The second involves progressively reducing the differential fall velocities between the moments and is therefore applicable for schemes that hold the shape parameter constant. Both approaches greatly reduce the errors in the computed moments, including those on which microphysical process rates depend, and are easily applied to existing two-moment schemes.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau
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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

The multitude of tropical–extratropical interactions that occur during an extratropical transition (ET) complicate the prediction and diagnosis of these extreme events. This study focuses on the analysis of a double ET and reintensification event that took place between 5 and 7 September 1998. Ex-Hurricanes Earl and Danielle reintensified rapidly over the western and eastern North Atlantic, respectively. A set of simulations designed to test the sensitivity of Earl's ET to features in the downstream state was run using a set of idealizations for a numerical model's initial and boundary conditions downstream of ex-Hurricane Earl. Dynamic tropopause analyses and the “PV thinking” paradigm applied under the Eady model highlight important developmental and structural differences between the tests.

In fact, two distinct solution modes are diagnosed both in the control and in the sensitivity tests. Earl's ET proceeds according to a “baroclinic mode” of redevelopment, whereas Danielle displays distinct “tropical mode” ET signatures throughout the period of investigation. The presence of a strong zonal jet immediately downstream of the transitioning cyclone is found to be sufficient to induce a baroclinic mode of redevelopment characterized by cyclonic potential vorticity rollup and strong near-surface frontogenesis. Under the influence of an upstream jet in isolation, the reintensification takes on distinctly tropical characteristics as the enhanced northward intrusion of warm, moist air ahead of the system creates a local environment favorable for a tropical mode of redevelopment. A description of the dynamics associated with these two distinct redevelopment modes may aid in the understanding and prediction of these events.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

The role that atmospheric water, in both its liquid and vapor phases, plays in cyclogenesis is difficult to determine because of the complex interactions between dynamic and thermodynamic forcings. From a potential vorticity (PV) perspective, it is possible to decompose the atmospheric state into a set of superposed PV anomalies. The modification of these anomalies allows for sensitivity testing using numerical models. Although this approach allows for the determination of cyclogenetic contributions from individual PV features, its application has not accounted for the dynamically consistent modification of the moisture field. This paper develops a PV-based variable that describes the effects of water vapor, cloud, and rainwater on balanced dynamics. A special-case analytic form of this “moist component” PV is developed and interpreted using an idealized model of the atmosphere. The application of the moist component methodology developed here provides the basis for future work, which includes sensitivity tests designed to separate the impacts of dynamics and thermodynamics on cyclogenesis.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

The importance of remnant tropical cyclone (TC) circulation and moisture structures is investigated for a simultaneous extratropical transition (ET) event involving ex-Hurricanes Danielle and Earl (September 1998). Although both storms undergo prolonged periods of reintensification following ET, the forcings involved in each of their redevelopment processes differ fundamentally. A review of the tropical and baroclinic ET modes in the North Atlantic stresses the importance of jet/front structures to the nature of the reintensification process. Ex-Hurricane Danielle begins to redevelop in the eastern half of the basin in the downstream, poleward sector of an intensifying polar jet. The system undergoes a tropical mode of reintensification, resulting in a troposphere-deep warm environment surrounding the storm, devoid of near-surface fronts and maintained by strong tropopause folds at its periphery. Ex-Hurricane Earl reintensifies near the eastern seaboard according to a baroclinic mode, under the influence of an upshear upper-level trough. A rapid cyclonic rollup of upper-level potential vorticity over the reintensifying low-level center results in a strong baroclinic system with well-defined frontal boundaries.

The two elements of the remnant TCs considered here are circulation and moisture. Potential vorticity-based modifications are made to the initial atmospheric state of the Mesoscale Compressible Community model in order to remove either one or both of these possible cyclogenetic forcings. The resulting set of sensitivity tests is analyzed in terms of system intensity and structure. It is found that the tropical-mode reintensification (ex-Hurricane Danielle) process requires the presence of the remnant's circulation and moisture for rapid redevelopment. However, the baroclinic-mode transition studied (ex-Hurricane Earl) is remarkably insensitive to the removal of the ex-tropical vorticity and moisture structures of the TC remnant.

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R. McTaggart-Cowan, J. R. Gyakum, and M. K. Yau

Abstract

This study uses the Mesoscale Compressible Community model to simulate the extratropical transition and reintensification of Hurricane Earl (1998) for the purposes of testing sensitivity to modification of the model's initial conditions. Though relatively strong “classical” cyclogenetic forcings were present in this case, operational forecasts seriously underpredicted the severity of the reintensification. Employing a piecewise potential vorticity (PV) inversion, the authors remove localized PV anomaly (PV′) maxima from the initial conditions and rebalance the fields for input to the model. Several PV′ structures in an upstream trough, and the PV′ associated with the hurricane, are removed individually and the model is rerun. Comparison of the resulting output with that of the control integration allows for a quantification of the impact of each PV anomaly on the regeneration of Earl. It is found that the existence of an upstream trough is of primary importance to the storm's reintensification, while the presence of the low-level circulation associated with the decaying hurricane plays only a secondary role.

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David R. Novak, Brian A. Colle, and Ron McTaggart-Cowan

Abstract

The role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast U.S. cyclones is investigated using piecewise potential vorticity (PV) inversion, modeling experiments, and potential temperature tendency budgets. Snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700-hPa low. This 700-hPa trough was associated with intense frontogenetical forcing for ascent. A variety of PV evolutions among the cases contributed to midlevel trough formation and associated frontogenesis. However, in each case the induced flow from diabatic PV anomalies accounted for a majority of the midlevel frontogenesis during the band’s life cycle, highlighting the important role that latent heat release plays in band evolution. Simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band. However, changes in the meso-α-scale flow associated with the development of diabatic PV anomalies east of the band contributed to frontolysis and band dissipation. Conditional stability was reduced near 500 hPa in each case several hours prior to band formation. This stability remained small until band formation, when the stratification generally increased in association with the release of conditional instability. Previous studies have suggested that the dry slot is important for the initial stability reduction at midlevels, but this was not evident for the three banding cases examined. Rather, differential horizontal temperature advection in moist southwest flow ahead of the upper trough was the dominant process that reduced the midlevel conditional stability.

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Ron McTaggart-Cowan, John R. Gyakum, and Richard W. Moore

Abstract

As subsaturated air ascends sloping isentropic surfaces, adiabatic expansion results in cooling and relative moistening. This process is an effective way to precondition the atmosphere for efficient moist processes while bringing parcels to saturation, and thereafter acts to maintain saturation during condensation. The goal of this study is to develop a diagnostic quantity that highlights circulations and regions in which the process of parcel moistening by isentropic ascent is active. Among the many features that rely on this process for the generation of an important fraction of their energy are oceanic cyclones, transitioning tropical cyclones, warm conveyor belts, diabatic Rossby vortices, and predecessor rain events. The baroclinic moisture flux (BMF) is defined as moisture transport by the component of vertical motion associated with isentropic upgliding. In warm conveyor belt and diabatic Rossby vortex case studies, the BMF appears to be successful in identifying the portion of the circulation in which this process is actively bringing parcels to saturation to promote the formation of clouds and precipitation. On a broader scale, the climatological maxima of the BMF highlight regions in which parcel moistening by isentropic ascent is anticipated to have a nonnegligible impact on the atmospheric state either through the action of the mean flow or via the repeated occurrence of isolated large-BMF events. The process-centric foundation of the BMF makes it useful as a filtering or exploratory variable, with the potential for extension into predictive applications.

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D. J. Kirshbaum, T. M. Merlis, J. R. Gyakum, and R. McTaggart-Cowan

Abstract

Idealized simulations are used to examine the sensitivity of moist baroclinic wave growth to environmental temperature and moisture content. With relative humidity held fixed, the surface temperature at 45°N, denoted T 0, is varied from 275 to 290 K. As T 0 increases, the atmospheric moisture content, moist instability, and moist available potential energy also increase. For the chosen initial configuration, moist waves develop larger eddy kinetic energy K e than corresponding dry waves, but enhanced diabatic heating at larger T 0 does not further increase K e. This finding is linked to a warm-frontal cyclonic potential vorticity (PV) anomaly that strengthens and shifts downstream at larger T 0 owing to increased diabatic heating along the frontal cloud band. This eastward shift feeds back negatively on the parent cyclone by increasing the downstream export of mechanical energy aloft and degrading the phasing between dry baroclinic vertical motion and buoyancy within the warm sector. The latter suppresses the conversion from eddy potential energy to K e [C(P e, K e)], offsetting a direct enhancement of C(P e, K e) by diabatic heating. Compared to their dry counterparts, isolated moist waves (initiated by a single finite-amplitude PV anomaly) display a similar sensitivity to T 0, while periodic wave trains (initiated by multiple such anomalies) exhibit a stronger negative relationship. The latter stems from anticyclonic diabatic PV anomalies aloft that originate along the warm front and recirculate through the system to interact with the upper-level trough. This interaction leads to a horizontal forward wave tilt that enhances the conversion of wave K e into zonal-mean kinetic energy.

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J. A. Milbrandt, M. K. Yau, J. Mailhot, S. Bélair, and R. McTaggart-Cowan

Abstract

This is the second in a series of papers examining the behavior of the Milbrandt–Yau multimoment bulk microphysics scheme for the simulation of the 13–14 December 2001 case of orographically enhanced precipitation observed during the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) experiment. The sensitivity to the number of predicted moments of the hydrometeor size spectra in the bulk scheme was investigated. The triple-moment control simulations presented in were rerun using double- and single-moment configurations of the multimoment scheme as well the single-moment Kong–Yau scheme. Comparisons of total precipitation and in-cloud hydrometeor mass contents were made between the simulations and observations, with the focus on a 2-h quasi-steady period of heavy stratiform precipitation. The double- and triple-moment simulations were similar; both had realistic precipitation fields, though generally overpredicted in quantity, and had overprediction of snow mass and an underprediction of cloud water aloft. Switching from the triple- to single-moment configuration resulted in a simulation with a precipitation pattern shifted upwind and with a larger positive bias, but with hydrometeor mass fields that corresponded more closely to the observations. Changing the particular single-moment scheme used had a greater impact than changing the number of moments predicted in the same scheme, with the Kong–Yau simulations greatly overpredicting the total precipitation in the lee side of the mountain crest and producing too much snow aloft. Further sensitivity tests indicated that the leeside overprediction in the Kong–Yau runs was most likely due to the combination of the absence of the latent heat effect term in the diffusional growth rate for snow combined with the assumption of instantaneous snow melting in the scheme.

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