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Paul E. Roundy
and
William M. Frank

Abstract

Propagating anomalies of moisture and moist deep convection in the Tropics are organized into a variety of large-scale modes. These include (but are not limited to) the so-called intraseasonal oscillations, convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane, and tropical-depression-type disturbances. Along with the annual and diurnal cycles, these modes act and interact to control much of the variance of tropical convection. Analyses of 10 yr of outgoing longwave radiation (OLR) and precipitable water (PW) data are carried out to develop comparative climatologies of these wavelike modes. The analysis relaxes the commonly used cross-equatorial symmetry constraints, which allows study of the portions of the wavelike processes that are asymmetric across the equator.

Mean background states are found for OLR and for PW as functions of day of the year. Examination of anomalies together with the background reveals much about how the waves are affected by their environments. Zonal wavenumber–frequency spectral analyses are performed on these anomalies. Following the spectral analyses, the OLR and the PW data are then filtered for specific regions of the wavenumber–frequency domain. Results show how variance generated by propagating modes is distributed in time and space, approximately illustrating the relative contributions of the wave modes to regional OLR and PW variability.

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Young C. Kwon
and
William M. Frank

Abstract

A series of numerical simulations of dry, axisymmetric hurricane-like vortices is performed to examine the growth of barotropic and baroclinic eddies and their potential impacts on hurricane core structure and intensity. The numerical experiments are performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with a 6-km horizontal grid. To examine internal effects on the stability of vortices, all external forcings are eliminated. Axisymmetric vortices that resemble observed hurricane structures are constructed on an f plane, and the experiments are performed without moist and boundary layer processes.

Three vortices are designed for this study. A balanced control vortex is built based on the results of a full-physics simulation of Hurricane Floyd (1999). Then, two other axisymmetric vortices, EXP-1 and EXP-2, are constructed by modifying the wind and mass fields of the control vortex. The EXP-1 vortex is designed to satisfy the necessary condition of baroclinic instability, while the EXP-2 vortex satisfies the necessary condition of barotropic instability. These modified vortices are thought to lie within the natural range of structural variability of hurricanes.

The EXP-1 and EXP-2 vortices are found to be unstable with respect to small imposed perturbations, while the control vortex is stable. Small perturbations added to the EXP-1 and EXP-2 vortices grow exponentially at the expense of available potential energy and kinetic energy of the primary vortex, respectively. The most unstable normal modes of both vortices are obtained via a numerical method. The most unstable mode of the EXP-1 (baroclinically unstable) vortex vertically tilts against shear, and the maximum growth occurs near a height of 14 km and a radius of 20 km. On the other hand, the most unstable normal mode of the EXP-2 (barotropically unstable) vortex has horizontal tilting against the mean angular velocity shear, and the maximum perturbations are located at a lower altitude (around 4 km) and at larger radius (around 100 km). Despite these differences, the normal modes of both vortices have a wavenumber-1 structure.

The energy budget analysis shows that the growing baroclinic and barotropic perturbations have opposite effects on the vortex intensity in terms of kinetic energy. Baroclinic eddies strengthen, whereas barotropic eddies weaken, the primary vortex. It is hypothesized that fluctuations in hurricane core structure and intensity can occur due to eddy processes triggered by alternating periods of barotropic and baroclinic eddy growth in the core. Once formed, these eddies may interact with the intense diabatic energy sources in real hurricanes. A similar study of eddy behaviors in a more realistic hurricane, which includes moist and boundary layer processes and uses a finer grid mesh, will be the topic of Part II.

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Paul E. Roundy
and
William M. Frank

Abstract

Intraseasonal oscillations (ISOs) control much of the large-scale variability of convection in the Tropics on time scales of about 15–100 days. These disturbances are often thought to be dominated by eastward-propagating modes, especially during austral summer, but disturbances that propagate westward are also important.

This work demonstrates by means of a multiple linear regression model and a brief case study that eastward- and westward-moving intraseasonal modes often cooperatively interact with one another to produce many of the characteristics of the observed Southern Hemisphere summer ISO. These interactions appear to be facilitated by topography and/or by the convective anomalies that are cooperatively induced by the eastward- and the westward-moving components of the oscillations. These interactions do not occur during every period of intraseasonal convective activity, but they do commonly occur during periods of high-amplitude convective anomalies. This analysis shows that eastward- and westward-moving intraseasonal modes should not be generally assumed to be linearly independent entities.

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Paul E. Roundy
and
William M. Frank

Abstract

Multiple linear regression models with nonlinear power terms may be applied to find relationships between interacting wave modes that may be characterized by different frequencies. Such regression techniques have been explored in other disciplines, but they have not been used in the analysis of atmospheric circulations. In this study, such a model is developed to predict anomalies of westward-moving intraseasonal precipitable water by utilizing the first through fourth powers of a time series of outgoing longwave radiation that is filtered for eastward propagation and for the temporal and spatial scales of the tropical intraseasonal oscillations. An independent and simpler compositing method is applied to show that the results of this multiple linear regression model provide a better description of the actual relationships between eastward- and westward-moving intraseasonal modes than a regression model that includes only the linear predictor.

A statistical significance test is applied to the coefficients of the multiple linear regression model, and they are found to be significant over broad regions of the Tropics. Correlations between the predictors are shown to not significantly influence results for this case.

Results show that this regression model reveals physical relationships between eastward- and westward-moving intraseasonal modes. The physical interpretation of these regression relationships is given in a companion paper.

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William M. Frank
and
Charles Cohen

Abstract

A new cumulus parametefization is developed for ffse in mesoscale model simulations of precipitating convective systems. It is designed to estimate convective properties using a cloud model that interacts with the mesoscale model in a physically consistent manner. The cloud model is initiated by and interacts with the grid-scalecirculation without being constrained in instantaneous equilibrium with the grid-scale flow. The parameterization is designed for use in conjunction with explicit moist processes. Simulations of tropical convective lines performed using the scheme are presented in a companion paper (Part II).

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Charles Cohen
and
William M. Frank

Abstract

The new cumulns parameterization of Part I of this paper is used in a setes of simulations of tropical mesoscale convective lines. The structures and life cycles of the simulated systems are quite similar to those observed in many studies of tropical convection, supporting the use of the parametedzation in mesoscale model.

The modeled cloud lines formed extensive upper-level nimbostratus clouds, primarily from air detrained below cloud top by the parameterized convection. These clouds produced copious rainfall and tended to shift the net diabafic heating predicted by the model to higher levels. When a cloud model lacking lateral detrainment was used, the simulated cloud lines grew rapidly, had short lifetimes, did not develop nimbostratus ions and concentrated the diabatic heating in the upper levels. Radiative processes, though crudely simulated, were shownto have potentially large effects on the intensity of the conyective system Decreasing the intercept, No, in the drop size distribution in the explicit moisture scheme had relatively small effects on the structure of the simulated systems, but it inctsed the rainfall and caused a small upward shir in the vertical heating probe.

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William M. Frank
and
Charles Cohen

Abstract

A simple cloud model is developed which is designed for both diagnostic studies and mesoscale cumulus parameterization experiments. The cloud model is combined with an observed population of tropical convective updrafts and used to examine the vertical distributions of convective beating and moistening produced by tropical cloud ensembles.

Although the cloud model ensembles are dominated by deep cumulonimbi, their vertical beating and moistening profiles differ significantly from those of individual clouds. These profiles and the total rainfall are sensitive to assumptions that affect the vertical mass flux distributions of the clouds. The ensemble heating and moistening profiles are in general agreement with large-scale budget analyses except for a tendency for the former to concentrate more of the heating above 600 mb.

Modeled convective properties are found to be highly sensitive to assumptions concerning the convective environment immediately surrounding the updrafts and downdrafts. This has important implications for cumulus parameterization experiments, particularly in coarse-grid models.

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William M. Frank
and
John L. McBride

Abstract

Tropical cloud clusters that occurred during the Australian Monsoon Experiment (AMEX) are composited and compared to a composite of the GARP Atlantic Tropical Experiment (GATE) systems. The analysis focuses on the evolution of the life cycles and upon the vertical heating profiles.

The AMEX and GATE systems were of comparable duration and magnitude, although the former produced more rainfall. However, AMEX convective systems produced maximum heating in the middle troposphere and showed only small variations in the heating with height. In contrast GATE systems began with heating concentrated in the lower troposphere and exhibited a marked upward shift in heating with time. GATE systems always had greater fractions of their total heating at lower levels than did AMEX systems, presumably due to differences in the large flow.

The vertical stratification of the atmosphere in both regions resembles that of a reversible moist adiabat at lower levels and of a pseudoadiabat above the freezing level. This agrees with results of recent studies of the tropical regions. During the life of the convective system, the atmosphere adjusts slightly toward these adiabats. This suggests that the abundant deep convection in the AMEX and GATE regions maintains the stratification near an equilibrium profile.

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Shuyi S. Chen
and
William M. Frank

Abstract

The purpose of this study is to understand the genesis of extratropical convective mesovortices and the large-scale environmental features that influence the vortex formation. A hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems (MCSs) due to the reduction of static stability that reduces the effective local Rossby radius in such regions. A conceptual model of the mesoscale convective cyclogenesis is introduced, which describes the three stages of the mesovortex formation.

A modified version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre-MCS large-scale environments. The model simultaneously incorporates parameterized convection and a grid-resolvable convective scheme containing the effects of hydrostatic water loading, condensation (evaporation), freezing (melting), and sublimation.

A control simulation is performed with a specified pre-MCS environment that is characterized by a midtro-pospheric short wave, a low-level jet ahead of the short-wave trough, a large area of conditionally unstable air, a deep layer of moisture, and small vertical wind shear. A mesovortex forms within a stratiform region behind a leading convective line. The evolution and structure of the mesovortex are similar to observations of the mesovortices associated with MCSs over land at midlatitudes. The results show that the mesovortex is produced by localized warming in a region of locally reduced Rossby radius, which induces convergence and, hence, creates rotational momentum via geostrophic adjustment.

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John Kaplan
and
William M. Frank

Abstract

Aircraft, rawinsonde, satellite, ship, and buoy data collected over a 40-h period were composited to analyze the inflow-layer structure of Hurricane Frederic (1979) within a radius of 10° latitude of the storm center. To improve the quality of the composite analyses, the low-level cloud-motion winds (CMWs) employed in this study were assigned a level of best fit (LBF). An LBF was assigned to each CMW by determining the level at which the closest agreement existed between CMW and ground-truth wind data (e.g., rawinsonde, aircraft, ship, and buoy). The CMWs were then adjusted vertically to uniform analysis levels, combined with ground-truth wind data, and objectively analyzed. These objectively analyzed wind fields were used to obtain kinematically derived fields of vorticity, divergence, and vertical velocity. An angular-momentum budget was also computed to obtain estimates of surface drag coefficients.

The low-level CMWs in this study were found to have LBFs ranging from 300 to 4000 m. It was shown that judicious use of this knowledge leads to substantial improvements in the estimates of the radial flow, but relatively insignificant improvement in the estimates of the rotational component of the wind. These results suggest that the common practice of assigning all low-level CMWs in a tropical cyclone environment to a constant level of 900–950 mb (approximately 500–1000 m) is probably appropriate for computations that depend primarily upon the rotational wind component. These findings, however, also indicate that failure to account for variations in LBFs of low-level CMWs could result in substantial errors in calculations that are sensitive to the radial wind.

The kinematic analyses showed that the asymmetric wind structure observed previously in studies of Frederic's inner core extends out to at least 10° latitude radius. Frederic was characterized by strong northeast-southwest radial flow through the storm and a pronounced northwest-southeast asymmetry of the tangential wind field at each analysis level. Analysis of Frederic's surface-560-m angular-momentum budget showed that the mean value of the surface drag coefficient beyond 2° radius was approximately 1.8 × 10−3.

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