Search Results

You are looking at 1 - 10 of 18 items for :

  • Author or Editor: William M. Frank x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: All Content x
Clear All Modify Search
William M. Frank

Abstract

Rawinsonde and satellite data from GATE are used to construct composite analyses of the Intertropical Convergence Zone (ITCZ) of the summertime eastern Atlantic. The latitude of the ITCZ varies substantially with time, and the data are analyzed relative to the latitude of maximum convection. Sensible heat, moisture and kinetic energy budgets are performed.

Significant mean upward motion occurs over a band approximately 900 km wide. The ITCZ is centered in a broad surface low pressure trough with a weak upper-level warm core structure. It is found to be a direct circulation which generates an excess of kinetic energy, most of which is exported meridionally in the upper troposphere by transient eddy circulations.

Full access
William M. Frank

Abstract

Vertical fluxes of mass, static energy and angular momentum in tropical cyclones and their environments are analyzed using composite rawinsonde data. Cumulus-scale eddy fluxes are derived using a spectral cloud model incorporating the effects of downdrafts, overshoot cooling, multiple-level cloud detrainment and ice-phase transitions.

Deep convection exists in all regions in approximately direct proportion to the low-level mass convergence, and large amounts of shallow cloudiness are found everywhere. A bimodal cloud distribution is found only in the outer convective area. Direct warming of the troposphere through cloud-induced subsidence is very small in the upper troposphere inside a radius of 4° latitude.

Angular momentum appears to be conserved in deep and middle-level convection, while shallow clouds and dry turbulence transport momentum downward in the lower levels. These principles could be incorporated into cumulus parameterization schemes.

Full access
William M. Frank

Abstract

The life cycles of GATE squall lines and loosely organized cloud clusters are analyzed and documented using radar and composited rawinsonde data. Time variations of the temperature, moisture, wind and vertical motion fields are presented for both types of systems. The convective systems are usually triggered by the approach of a middle-level trough in the easterlies. Low-level convergence increases prior to intensification of convection, possibly by several hours. Once the convection begins, the systems largely cause the changes in their upper and lower level divergence profiles through cumulus-induced and radiational heating. Squall line and cluster systems are found to be essentially similar except for their propagation speeds and their vertical wind shears.

Full access
William M. Frank

Abstract

The net vertically integrated temperature of the GATE B army is analyzed at each rawinsonde observation time for all three phases. Temperatures are adjusted to remove persistent intership biases and errors induced by solar heating of the rawinsondes. Modulation of the net temperature by direct solar heating, secondary circulation processes and latent heat release are explored.

Latent heat release does not warm the troposphere on the observed time (3–6 h) and space scales indicating that condensation heating is dispersed very rapidly from cloud-cluster-scale systems to larger circulations. The net temperature undergoes a regular diurnal temperature oscillation modulated primarily by direct radiational heating and indirect circulation. These processes are of comparable magnitudes.

Full access
John L. McBride
and
William M. Frank

Abstract

Rawinsonde data from the Australian Monsoon Experiment are analyzed to determine the manner in which the atmospheric stratification of density and moisture respond to large amounts of convective latent heat release. The study focuses on time series of data from a ship located at the northern end of the Gulf of Carpentaria during active and break periods of the monsoon.

Variations in lapse rate or vertical stratification through the depth of the troposphere are found to occur mainly between active and break periods, rather than on a day-to-day basis. This is interpreted as being due to midtropospheric temperature being adjusted by dynamical processes over large scales rather than in situ response to localized convection. Between active and break periods large changes occurred in midtropospheric moisture. Variations in convective activity are well related to variations in lower and middle tropospheric moisture content. The break coincided with a drying due to large-scale horizontal advection.

Convective activity is weakly but inversely related to convective available potential energy variations. Day-to-day variations in CAPE are dominated by variations in equivalent potential temperature of the source level (boundary layer) air. The physical effect is one of changing the moist adiabat along which the air parcel rises. In temperature–log pressure space, moist adiabats diverge in the upper half of the troposphere. Since CAPE variations are dominated by changes in the moist adiabat of the rising parcel, the day-to-day CAPE changes occur almost totally in positive area variations above the 600-hPa level.

The above results are discussed in the context of other studies in the literature. It is proposed that stabilization of the atmosphere in response to deep convection occurs almost entirely through the modification of CAPE through decreasing θ e of the source air in the boundary layer. This occurs over relatively small spatial scales, whereas variations in lapse rate through the deep troposphere are hypothesized to occur over the relatively large scales associated with monsoon active and break events.

Full access
Young C. Kwon
and
William M. Frank

Abstract

The energy flows of a simulated moist hurricane-like vortex are analyzed to examine the processes that change the intensity and structure of tropical cyclones. The moist vortex used in this study is initially axisymmetric on an f plane and is placed on a uniform surface—an ocean with constant sea surface temperature of 29°C. Two simulations are performed using the following different environmental flows: one in a calm environment and the other in weak environmental vertical shear. The differences between the intensities and structures of the two simulated vortices are discussed in terms of energy flows.

While the structure and intensity of the vortex without shear are relatively steady, those of the vortex with shear experience dramatic changes. The sheared vortex shows delayed weakening, persistent wavenumber 1 asymmetry with maximum rainfall on the downshear left side, and top-down breakdown. In both vortices barotropic energy conversion is stronger than baroclinic energy conversion. However, baroclinic processes in the upper levels of the sheared vortex play an important role in weakening the vortex. The energy flow diagram and the cross section of energy conversion terms show the existence of multiple baroclinic eddy life cycles at the upper levels of the sheared vortex. The activity of the baroclinic eddies continues until ventilation of the upper-level warm-core structure is sufficient to weaken the sheared vortex.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access