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John Molinari

Abstract

A closure is proposed for the b parameter of Kuo (1974), using the framework developed by Krishnamurti et al. (1976). Emphasis is placed on the time-dependent behavior of the solutions. The proposed closure is found to be the only one of several tested to produce an approach to moist neutrality in both temperature and moisture under strong external forcing. The sensitivity of the grid-scale evolution to the partitioning of moisture defined by the b parameter suggests that such partitioning must be carefully dealt with in any method for computing the effects of cumulus convection, whether or not b is explicitly present.

By including entrainment in the cloud lapse rate, the observed large-scale behavior of the vertical profile of moist static energy under disturbed conditions is simulated. The approach is shown to be easily invertible when precipitation rate is specified, thus insuring internal consistency in a model when such a procedure is used as part of a dynamic initialization.

Because it is relatively simple and general, and reproduces observed large-scale θe variations under strong forcing, the approach may be particularly suitable for large-scale models. An economical way to extend the procedure to mesoscale models is proposed.

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John Molinari

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Rainfall rates determined from airborne radar and infrared satellite images are combined to construct a space- and time-dependent heating function for Hurricane Anita (1977). The heating is assimilated into a three-dimensional primitive equation prediction during a 12 h pre-forecast integration, after which the heating rate is computed internally by the model. The specified heating forces initial wind and mass fields toward their observed values, and produces improved 12 and 24 h forecasts of both track and intensity compared to a control integration, for which the heating is computed internally for the entire period.

Calculations indicate that model adjustment during the period of heating can be viewed as a slow response of the vorticity field to continuous forcing of the divergence by the heating. The location and pattern of the heating relative to the center appear to be of greater importance than the magnitude of the heating. This may be of significance because remotely-sensed rainfall estimates are more likely to be accurate in the positioning of heavy rainfall than in its intensity. The initialization procedure appears capable of producing useful improvement in short-term hurricane prediction, particularly prior to landfall, when data coverage is best and accuracy is of greatest concern.

A number of authors have noted the importance of upper-level inward eddy momentum fluxes for hurricane intensification. Calculations from the simulated storm indicate that such eddy fluxes are present in Hurricane Anita and are associated in part with an anticyclonic outflow eddy over an intense local rainfall area 300 km east of the center.

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John Molinari

Abstract

A formulation of Kuo's cumulus parameterization is described which satisfies arbitrary vertical profiles of apparent heat source (Q 1) and apparent moisture sink (Q 2). The approach requires little calculation, and for a given Q 1 and Q 2, contains only one parameter, the partitioning of available moisture between storage and precipitation. The proposed method is tested in the prediction of a mesoscale convective complex and its advantages and limitations are discussed.

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John Molinari and Michael Dudek

Abstract

No abstract available

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Patrick Duran and John Molinari

Abstract

High-vertical-resolution rawinsondes were used to document the existence of low–bulk Richardson number (R b) layers in tropical cyclones. The largest frequency of low R b existed in the inner 200 km at the 13.5-km level. This peak extended more than 1000 km from the storm center and sloped downward with radius. The presence of an extensive upper-tropospheric low-R b layer supports the assumption of Richardson number criticality in tropical cyclone outflow by Emanuel and Rotunno.

The low-R b layers were found to be more common in hurricanes than in tropical depressions and tropical storms. This sensitivity to intensity was attributed to a reduction of upper-tropospheric static stability as tropical cyclones intensify. The causes of this destabilization include upper-level cooling that is related to an elevation of the tropopause in hurricanes and greater longwave radiative warming in the well-developed hurricane cirrus canopy. Decreased mean static stability makes the production of low R b by gravity waves and other perturbations easier to attain.

The mean static stability and vertical wind shear do not exhibit diurnal variability. There is some indication, however, that low Richardson numbers are more common in the early morning than in the early evening, especially near the 200–300-km radius. The location and timing of this diurnal variability is consistent with previous studies that found a diurnal cycle of infrared brightness temperature and rainfall in tropical cyclones.

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Patrick Duran and John Molinari

Abstract

Dropsondes with horizontal spacing as small as 4 km were released from the stratosphere in rapidly intensifying Hurricane Patricia (2015) during the Office of Naval Research Tropical Cyclone Intensity experiment. These observations provide cross sections of unprecedented resolution through the inner core of a hurricane. On 21 October, Patricia exhibited a strong tropopause inversion layer (TIL) across its entire circulation, with a maximum magnitude of 5.1 K (100 m)−1. This inversion weakened between 21 and 22 October as potential temperature θ increased by up to 16 K just below the tropopause and decreased by up to 14 K in the lower stratosphere. Between 22 and 23 October, the TIL over the eye weakened further, allowing the tropopause to rise by 1 km. Meanwhile over Patricia’s secondary eyewall, the TIL restrengthened and bulged upward by about 700 m into what was previously the lower stratosphere. These observations support many aspects of recent modeling studies, including eyewall penetration into the stratosphere during rapid intensification (RI), the existence of a narrow inflow layer near the tropopause, and the role of subsidence from the stratosphere in developing an upper-level warm core. Three mechanisms of inner-core tropopause variability are hypothesized: destabilization of the TIL through turbulent mixing, weakening of the TIL over the eye through upper-tropospheric subsidence warming, and increasing tropopause height forced by overshooting updrafts in the eyewall. None of these processes are seen as the direct cause of RI, but rather part of the RI process that includes strong increases in boundary layer moist entropy.

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Michael Dickinson and John Molinari

Abstract

A large-amplitude mixed Rossby–gravity wave packet is identified in the western Pacific using 6–10-day bandpass-filtered winds. Individual disturbances of 2300–3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden–Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east.

West of 150°E, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby–gravity waves to off-equatorial tropical depression (TD)-type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby–gravity waves remained outside the circulation centers, and three tropical cyclones formed on the edges of three consecutive cyclonic gyres as they moved off the equator. Although the expected Rossby–Kelvin wave structure was present in the background winds within the active MJO, tropical cyclone genesis did not occur within the trailing Rossby gyres, but 2500 km to the west and north.

This case study provides evidence that equatorial modes, under the right conditions, can supply precursor disturbances for repeated formation of tropical cyclones. It is argued based on previous work in the literature that this sequence of events is not uncommon.

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John Molinari and Michael Dudek

Abstract

The ability of several explicit formulations of convective heating to predict the precipitation associated with a mesoscale convective complex was compared to that of a cumulus parameterization on a ½ deg latitude-longitude mesh. In the explicit approaches, prediction equations were present for both water vapor and cloud water, or vapor alone. The simplest explicit approach, for which any condensed water was assumed to fall immediately as rain, produced localized excessive rainfall. This explicit heating instability arose as a result of the requirements of saturation prior to rainfall, which delayed condensation and allowed excessive convective instability to build, and neglect of fluxes, which prevented the instability from being released in a realistic manner. These results, combined with those of previous investigators, indicate that the simplest form of explicit heating is prone to instability and unsuitable for mesoscale models.

Instability problems were significantly reduced by the inclusion of the inhibiting effects of rainwater evaporation and a cloud phase with hydrostatic water loading, Nevertheless, bemuse significant nor occurred in nature in the absence of area-averaged saturation, rainfall was unrealistically delayed when a 100 percent saturation criterion was used. Reducing the saturation criterion improved the phase error of the rainfall prediction, but sometimes reintroduced local instability.

Although only simple explicit formulations were used, inclusion of more sophisticated microphysical parameterizations from cloud models may be unrepresentative of processes in nature for meso-α scale models, for which the grid spacing exceeds 50 km. It is proposed for such models that implicit approaches offer the greatest potential for improvement. For meso-β scale models the optimum choice remains uncertain.

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John Molinari and David Vollaro

Abstract

It is frequently stated that 70%–80% of western North Pacific tropical cyclones form “within the monsoon trough,” but without an objective definition of the term. Several definitions are tested here. When the monsoon trough (MT) is defined as the contiguous region where long-term (1988–2010) mean July–November 850-hPa relative vorticity is positive, 73% of all July–November tropical cyclones form within the MT. This percentage varies interannually, however, from as low as 50% to nearly 100%. The percentage correlates with the Niño-3.4 index, with more storms forming within the MT during warm periods. When the MT is defined instead using long-term monthly mean ζ 850, more than 80% of tropical cyclones form within the MT in all months except July and August, when more than 30% of storms form poleward of the MT. It is hypothesized that the known peak in the frequency of upper-tropospheric midlatitude wave breaking in July and August is responsible. It is argued that any long-term mean provides a suitable definition of the MT. Defining it on less than seasonal time scales, however, creates a lack of conceptual separation between the MT and other tropical disturbances such as the MJO, equatorial waves, and easterly waves. The term monsoon trough should represent a climatological feature that provides an asymmetric background state within which other disturbances evolve.

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John Molinari and David Vollaro

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This paper describes a large cyclonic gyre that lasted several days in the northwest Pacific during July 1988. Cyclonic winds at 850 hPa extended beyond the 2000-km radius with a radius of maximum winds of 700–800 km. The gyre exhibited clear skies within and north of its center. Active convection extended 4000 km in longitude to its south.

The Madden–Julian oscillation (MJO) was in its active phase in the Indian Ocean prior to gyre formation. Consistent with earlier studies, diabatic heating in the MJO was associated with an anomalous upper-tropospheric westerly jet over the northeast Asian coast and a jet exit region over the northwest Pacific. Repeated equatorward wave-breaking events developed downwind of the jet exit region. One such event left behind a region of lower-tropospheric cyclonic vorticity and convection in the subtropics that played a key role in the gyre formation. A second wave-breaking event produced strong subsidence north of the mature gyre that contributed to its convective asymmetry.

Gyres from 1985 and 1989 were compared to the 1988 case. All three gyres developed during an active MJO in the Indian Ocean. Each gyre displayed the same strong convective asymmetry. Each developed in July or August during the climatological peak in breaking Rossby waves in the northwest Pacific. Finally, all of the gyres developed during La Niña at nearly the same location. This location and the convective structure of the gyres closely matched composite La Niña anomalies during boreal summer.

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