Search Results

You are looking at 1 - 8 of 8 items for :

  • Author or Editor: Jerry M. Straka x
  • Journal of the Atmospheric Sciences x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Jerry M. Straka
and
John R. Anderson

Abstract

In the first part of this paper, the characteristics of microburst-producing storms are examined with a three-dimensional cloud model using soundings from the Cooperative Huntsville Meteorological Experiment (COHMEX). With a grid resolution of 500 m, it is shown that the general characteristics of observed vertical velocities, vertical draft sizes, water contents, radar reflectivities, and surface outflow strengths can be simulated. In addition, observed microburst precursors such as midlevel convergence and descending precipitation cores can also be simulated. Using a grid resolution of 250 m, the observed structure of a particularly well-documented storm on 20 July 1986 during COHMEX is simulated, including a hail shaft 1–2 km wide that descended to the ground.

In the second part of this paper, the influence of microphysical processes in the production of low-level downdrafts in simulated COHMEX storms is investigated. It is shown that low-level downdrafts are in some cases stronger and deeper in simulations made with the ice phase than in simulations made without the ice phase. These differences are due, in part, to the additional cooling associated with the melting of ice, and are consistent with findings of several other recent studies of low-level downdraft production in deep convective storms.

Full access
Erik N. Rasmussen
and
Jerry M. Straka

Abstract

The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

Full access
Katharine M. Kanak
,
Jerry M. Straka
, and
David M. Schultz

Abstract

Mammatus are hanging lobes on the underside of clouds. Although many different mechanisms have been proposed for their formation, none have been rigorously tested. In this study, three-dimensional numerical simulations of mammatus on a portion of a cumulonimbus cirruslike anvil are performed to explore some of the dynamic and microphysical factors that affect mammatus formation and evolution. Initial conditions for the simulations are derived from observed thermodynamic soundings. Five observed soundings are chosen—four were associated with visually observed mammatus and one was not. Initial microphysical conditions in the simulations are consistent with in situ observations of cumulonimbus anvil and mammatus. Mammatus form in the four model simulations initialized with the soundings for which mammatus were observed, whereas mammatus do not form in the model simulation initialized with the no-mammatus sounding. Characteristics of the modeled mammatus compare favorably to previously published mammatus observations.

Three hypothesized formation mechanisms for mammatus are tested: cloud-base detrainment instability, fallout of hydrometeors from cloud base, and sublimation of ice hydrometeors below cloud base. For the parameters considered, cloud-base detrainment instability is a necessary, but not sufficient, condition for mammatus formation. Mammatus can form without fallout, but not without sublimation. All the observed soundings for which mammatus were observed feature a dry-adiabatic subcloud layer of varying depth with low relative humidity, which supports the importance of sublimation to mammatus formation.

Full access
Paul M. Markowski
,
Jerry M. Straka
, and
Erik N. Rasmussen

Abstract

Idealized numerical simulations are conducted in which an axisymmetric, moist, rotating updraft free of rain is initiated, after which a downdraft is imposed by precipitation loading. The experiments are designed to emulate a supercell updraft that has rotation aloft initially, followed by the formation of a downdraft and descent of a rain curtain on the rear flank. In the idealized simulations, the rain curtain and downdraft are annular, rather than hook-shaped, as is typically observed. The downdraft transports angular momentum, which is initially a maximum aloft and zero at the surface, toward the ground. Once reaching the ground, the circulation-rich air is converged beneath the updraft and a tornado develops. The intensity and longevity of the tornado depend on the thermodynamic characteristics of the angular momentum-transporting downdraft, which are sensitive to the ambient low-level relative humidity and precipitation character of the rain curtain. For large low-level relative humidity and a rain curtain having a relatively small precipitation concentration, the imposed downdraft is warmer than when the low-level relative humidity is small and the precipitation concentration of the rain curtain is large. The simulated tornadoes are stronger and longer-lived when the imposed downdrafts are relatively warm compared to when the downdrafts are relatively cold, owing to a larger amount of convergence of circulation-rich downdraft air. The results may explain some recent observations of the tendency for supercells to be tornadic when their rear-flank downdrafts are associated with relatively small temperature deficits.

Full access
Leigh G. Orf
,
John R. Anderson
, and
Jerry M. Straka

Abstract

A parameter study of colliding microburst outflows is performed using a high-resolution three-dimensional model. The colliding microburst pairs me simulated in a domain of 18 km × 16 km × 4.25 km with 50-m resolution. Microburst pairs are examined in varying space and time separations, and the authors find that for certain geometries strong elevated wind fields are generated from the interactions between outflows. For a narrow range of space-time geometries, this elevated wind field is extremely divergent. An examination of the F-factor aircraft hazard parameter reveals that both the divergent wind fields and microburst downdraft cores are regions of danger to jet aircraft. Trajectory analysis reveals that the air composing the elevated jets can be traced back to the shallow outflow formed beneath each microburst core. An analysis of the parcel kinetic energy budget indicates that the pressure domes beneath and between the microbursts are the primary mechanisms for directing energy into the elevated jets.

Full access
Sonia G. Lasher-Trapp
,
Charles A. Knight
, and
Jerry M. Straka

Abstract

The growth of ultragiant aerosol (UGA) in a Lagrangian framework within a simulated three-dimensional cloud is analyzed and compared with radar and aircraft observations of a cumulus congestus collected during the Small Cumulus Microphysics Study (SCMS). UGA are ingested into the simulated cloud and grow by continuous collection; the resulting radar reflectivity factor and raindrop concentrations are evaluated at 1-min intervals. The calculations produce a substantial echo (>30 dBZ) within a short time (18 min), containing few raindrops (0.3 L−1). The calculated radar echo is very sensitive to the amount of UGA ingested into the modeled cloud and its liquid water content. The modeled radar echo and raindrop concentrations are consistent with the observations in that the differences fall within the modeling and measurement limitations and uncertainties.

Full access
Alexandre O. Fierro
,
Joanne Simpson
,
Margaret A. LeMone
,
Jerry M. Straka
, and
Bradley F. Smull

Abstract

An airflow trajectory analysis was carried out based on an idealized numerical simulation of the nocturnal 9 February 1993 equatorial oceanic squall line observed over the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) ship array. This simulation employed a nonhydrostatic numerical cloud model, which features a sophisticated 12-class bulk microphysics scheme. A second convective system that developed immediately south of the ship array a few hours later under similar environmental conditions was the subject of intensive airborne quad-Doppler radar observations, allowing observed airflow trajectories to be meaningfully compared to those from the model simulation. The results serve to refine the so-called hot tower hypothesis, which postulated the notion of undiluted ascent of boundary layer air to the high troposphere, which has for the first time been tested through coordinated comparisons with both model output and detailed observations.

For parcels originating ahead (north) of the system near or below cloud base in the boundary layer (BL), the model showed that a majority (>62%) of these trajectories were able to surmount the 10-km level in their lifetime, with about 5% exceeding 14-km altitude, which was near the modeled cloud top (15.5 km). These trajectories revealed that during ascent, most air parcels first experienced a quick decrease of equivalent potential temperature (θe ) below 5-km MSL as a result of entrainment of lower ambient θe air. Above the freezing level, ascending parcels experienced an increase in θe with height attributable to latent heat release from ice processes consistent with previous hypotheses. Analogous trajectories derived from the evolving observed airflow during the mature stage of the airborne radar–observed system identified far fewer (∼5%) near-BL parcels reaching heights above 10 km than shown by the corresponding simulation. This is attributed to both the idealized nature of the simulation and to the limitations inherent to the radar observations of near-surface convergence in the subcloud layer.

This study shows that latent heat released above the freezing level can compensate for buoyancy reduction by mixing at lower levels, thus enabling air originating in the boundary layer to contribute to the maintenance of both local buoyancy and the large-scale Hadley cell despite acknowledged dilution by mixing along updraft trajectories. A tropical “hot tower” should thus be redefined as any deep convective cloud with a base in the boundary layer and reaching near the upper-tropospheric outflow layer.

Full access
Alexandre O. Fierro
,
Edward J. Zipser
,
Margaret A. LeMone
,
Jerry M. Straka
, and
Joanne (Malkus) Simpson

Abstract

This paper addresses questions resulting from the authors’ earlier simulation of the 9 February 1993 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) squall line, which used updraft trajectories to illustrate how updrafts deposit significant moist static energy (in terms of equivalent potential temperature θe ) in the upper troposphere, despite dilution and a θe minimum in the midtroposphere. The major conclusion drawn from this earlier work was that the “hot towers” that Riehl and Malkus showed as necessary to maintain the Hadley circulation need not be undilute. It was not possible, however, to document how the energy (or θe ) increased above the midtroposphere. To address this relevant scientific question, a high-resolution (300 m) simulation was carried out using a standard 3-ICE microphysics scheme (Lin–Farley–Orville).

Detailed along-trajectory information also allows more accurate examination of the forces affecting each parcel’s vertical velocity W, their displacement, and the processes impacting θe , with focus on parcels reaching the upper troposphere. Below 1 km, pressure gradient acceleration forces parcels upward against negative buoyancy acceleration associated with the sum of (positive) virtual temperature excess and (negative) condensate loading. Above 1 km, the situation reverses, with the buoyancy (and thermal buoyancy) acceleration becoming positive and nearly balancing a negative pressure gradient acceleration, slightly larger in magnitude, leading to a W minimum at midlevels. The W maximum above 8 km and concomitant θ e increase between 6 and 8 km are both due to release of latent heat resulting from the enthalpy of freezing of raindrops and riming onto graupel from 5 to 6.5 km and water vapor deposition onto small ice crystals and graupel pellets above that, between 7 and 10 km.

Full access