Abstract

Observations have shown that thunderstorms sometimes undergo updraft splitting, where one updraft moves to the right of the mean tropospheric wind and the other to the left. Observations also show that the left-moving updraft tends to dissipate approximately 15 min after the splitting process. The right-moving cell, however, may exist for up to a few hours.

Idealized modeling studies suggest that this behavior is related to the clockwise turning of the environmental shear vectors with height. The interaction between the environmental shear and the storms updraft produces a high–low pressure couplet oriented downshear. This pressure pattern produces favorable vertical accelerations for the right mover. This same process inhibits upward motion for the left mover.

In this paper an idealized simulation is presented that suggests an additional process that contributed to the decay of the left-moving updraft. Analysis of low-level storm-relative winds for the left-moving cell indicated that the inflow was from the cool precipitating downdraft. This inflow was characterized by negatively buoyant air. Subsequently the updraft dissipated approximately 1500 s after the precipitating downdraft formed. In contrast, the inflow for the right-moving updraft was partly from the downdraft and the undisturbed environment. A second simulation was run in which no precipitation was allowed to form, thus no downdraft formed. In that simulation the left mover was long lived. These results suggest that the simulated left-moving cell dissipated by ingesting downdraft air.

Abstract

Previous studies have explained dryline movement to be a result of vertical turbulent mixing. Such mixing was shown to efficiently erode the western edge of the shallow moist layer above sloping terrain.

Two- and three-dimensional simulations have been used to demonstrate the impact of surface physiography on dryline evolution. Those simulations included changes in vegetation type, vegetation coverage, and soil moisture. In particular, dryline morphology has been shown to be dependent on the horizontal distribution of soil moisture. Modeling studies have also suggested that increases in the low-level horizontal water vapor gradient, associated with a dryline, are a result of frontogenetic forcing. The current study will extend past results by including more sensitivity experiments showing the dependence of dryline morphology on soil moisture.

In this paper, the Regional Atmospheric Modeling System was used to simulate the 26 April 1991 central plains dryline. Five simulations were conducted in which only soil moisture was varied. Results suggest that the movement of the dryline and the magnitude of the low-level water vapor gradient are sensitive to changes in soil moisture. Results from the constant soil moisture case show little movement of the 9.0 g kg⁻¹ water vapor mixing ratio isohume during the day. In that simulation the low-level horizontal gradient of water vapor displayed little change with time.

Simulated dryline evolution can be viewed as a two-step process. The first is the apparent eastward movement of drier air due to turbulent erosion of the shallow moist layer. The second step is the relatively rapid increase of the low-level horizontal gradient of water vapor. The increase of the gradient was found to be in response to vertically oriented thermally driven solenoids and frontogenetic forcing.

Abstract

A cloud-resolving model was used in conjunction with a radiative transfer (RT) modeling system to study 10.7-μm brightness temperatures computed for a simulated thunderstorm. A two-moment microphysical scheme was used that included seven hydrometeor types: pristine ice, snow, aggregates, graupel, hail, rain, and cloud water. Also, five different habits were modeled for pristine ice and snow. Hydrometeor optical properties were determined from an extended anomalous diffraction theory approach. Brightness temperatures were computed using a delta-Eddington two-stream model.

Results indicate that the enhanced “V,” a feature sometimes seen in satellite infrared observations, may be formed through an interaction between the overshooting dome and the upstream flanking region of high pressure. This idea is contrary to one in which the overshooting dome is viewed as an obstacle to the environmental flow. As expected, the radiative effects of pristine ice particles within the anvil largely determined the brightness temperature field. Although brightness temperatures were found to be insensitive to microphysical characteristics of moderate to thick portions of the anvil, a strong relationship did exist with column-integrated pristine ice mass for cloud optical depths below about 5. Precipitation-sized hydrometeors and surface precipitation rate, on the other hand, failed to exhibit any meaningful relationship with the cloud-top brightness temperature. The combined mesoscale model and RT modeling system used in this study may also have utility in satellite product development prior to launch of a satellite and in satellite data assimilation.

Abstract

This paper is the second part of a study on the dynamics of nonhydrostatic perturbations to dry, balanced, atmospheric vortices modeled after tropical cyclones. In , the stability and evolution of asymmetric perturbations were presented. This part is devoted to the stability and evolution of symmetric perturbations—particularly those that are induced by the wave–mean flow interactions of asymmetric perturbations with the symmetric basic-state vortex.

The linear model shows that the vortices considered in are stable to symmetric perturbations. Furthermore, the model can be used to derive the steady, symmetric response to stationary symmetric forcing, similar to the results from quasi-balanced dynamics as originally presented by Eliassen. The secondary circulations that develop act to oppose the effects of the forcing, but also to warm the core and intensify the vortex. The model is also used to simulate the response to impulsive symmetric forcings, that is, symmetric perturbations. Much like the asymmetries considered in , symmetric perturbations go through two kinds of adjustment: a fast adjustment that generates gravity waves, and then a slow adjustment leading to a final state that represents a net change in both the wind and mass fields of the symmetric vortex.

The nonhydrostatic, unsteady, symmetric response of the tropical-storm-like vortex to the evolving asymmetries from is presented. In contrast with results from previous studies with initially two-dimensional or balanced asymmetric vorticity perturbations, asymmetric temperature perturbations are found to have a negative effect on overall intensity. These changes are about two orders of magnitude smaller than those caused by symmetric perturbations of equal amplitude. The asymmetric/symmetric adjustment process for purely asymmetric temperature perturbations are also simulated with a fully nonlinear, compressible model. Excellent agreement is found between the linear, nonhydrostatic and the nonlinear, compressible models. The vortex intensification caused by a localized, impulsive thermal perturbation can be accurately estimated from the projection of this perturbation onto the purely symmetric motions.

Abstract

A two-way interactive, nested-grid simulation of a rotating supercell thunderstorm was performed. After 90 min the genesis of a descending incipient tornado vortex initially located aloft was simulated. The associated pressure-deficit tube subsequently built downward into the subcloud layer, where it continually fed upon a low-level source of vertical vorticity possibly introduced by the low-level downdraft. The pressure-deficit tube then drew in the low-level vorticity-rich air, allowing it to descend to the surface. A strong vortex thus formed in the subcloud field.

Abstract

Observations have shown that right moving thunderstorms are favored in environments characterized by clockwise-turning hodographs. There are, however, a few observational and numerical studies of long-lived, left moving storms within environments characterized by clockwise-turning hodographs. For example, a documented left mover that occurred on 26 May 1992, near Coldspring, Texas, with a mesoanticyclone and hail spike (also called a three-body scattering signature) produced severe weather. Although a few cases have been documented, left moving thunderstorms have received less study than right moving cells.

The long-lived, severe thunderstorm of 17 May 1996 is presented to improve documentation of left moving thunderstorms. The storm occurred over eastern Nebraska and will be referred to as the York County storm. This left mover resulted from storm splitting and moved to the west of a surface cold front. The relatively isolated storm subsequently split approximately 1 h later, yielding a new right moving thunderstorm. Doppler radial velocities suggested the existence of a mesoanticyclone within the York County storm. Hail, 1.75 in. in diameter, was produced by the storm around the time the updraft split.

There were many similarities between the York County storm and the 26 May 1992 Coldspring left moving severe thunderstorm. Both storms were relatively isolated, contained mesoanticyclones, and produced severe weather after the vertically integrated liquid water obtained a maximum value. Due to the dearth of material on left-moving storms, general statements concerning their evolution are lacking. This current study is a first step toward improving the sparse documentation of such thunderstorms. More work is needed in this area to help identify physical processes that lead to left moving thunderstorms, particularly those that become severe.

Abstract

Geostationary Operational Environmental Satellite-16 (GOES-16) was launched into geostationary orbit in late 2016 and began providing unprecedented spatial and temporal resolution imagery early in 2017. Its Advanced Baseline Imager has additional spectral bands including two in the “clear” window and “dirty window” portion of the infrared spectrum, and the difference of these two bands, sometimes called the split window difference, provides unique information about low-level water vapor. Under certain conditions, low-level convergence along a boundary can cause local water vapor pooling, and the signal of this pooling can sometimes be detected by GOES-16 prior to any cloud formation. This case study from 15 June 2017 illustrates how the technique might be used in an operational forecast setting. A boundary in western Kansas was detected using the split window difference more than 2 h before the first cloud formed.

Abstract

A new paradigm for the resiliency of tropical cyclone (TC) vortices in vertical shear flow is presented. To elucidate the basic dynamics, the authors follow previous work and consider initially barotropic vortices on an f plane. It is argued that the diabatically driven secondary circulation of the TC is not directly responsible for maintaining the vertical alignment of the vortex. Rather, an inviscid damping mechanism intrinsic to the dry adiabatic dynamics of the TC vortex suppresses departures from the upright state.

Recent work has demonstrated that tilted quasigeostrophic vortices consisting of a core of positive vorticity surrounded by a skirt of lesser positive vorticity align through projection of the tilt asymmetry onto vortex Rossby waves (VRWs) and their subsequent damping (VRW damping). This work is extended here to the finite Rossby number (Ro) regime characteristic of real TCs. It is shown that the VRW damping mechanism provides a direct means of reducing the tilt of intense cyclonic vortices (Ro > 1) in unidirectional vertical shear. Moreover, intense TC-like, but initially barotropic, vortices are shown to be much more resilient to vertical shearing than previously believed. For initially upright, observationally based TC-like vortices in vertical shear, the existence of a “downshear-left” tilt equilibrium is demonstrated when the VRW damping is nonnegligible.

On the basis of these findings, the axisymmetric component of the diabatically driven secondary circulation is argued to contribute indirectly to vortex resiliency against shear by increasing Ro and enhancing the radial gradient of azimuthal-mean potential vorticity. This, in addition to the reduction of static stability in moist ascent regions, increases the efficiency of the VRW damping mechanism.

Abstract

In a previous paper, the authors discussed the dynamics of an instability that occurs in inviscid, axisymmetric, two-dimensional vortices possessing a low-vorticity core surrounded by a high-vorticity annulus. Hurricanes, with their low-vorticity cores (the eye of the storm), are naturally occurring examples of such vortices. The instability is for asymmetric perturbations of azimuthal wavenumber-one about the vortex, and grows in amplitude as t ^1/2 for long times, despite the fact that there can be no exponentially growing wavenumber-one instabilities in inviscid, two-dimensional vortices. This instability is further studied in three fluid flow models: with high-resolution numerical simulations of two-dimensional flow, for linearized perturbations in an equivalent shallow-water vortex, and in a three-dimensional, baroclinic, hurricane-like vortex simulated with a high-resolution mesoscale numerical model.

The instability is found to be robust in all of these physical models. Interestingly, the algebraic instability becomes an exponential instability in the shallow-water vortex, though the structures of the algebraic and exponential modes are nearly identical. In the three-dimensional baroclinic vortex, the instability quickly leads to substantial inner-core vorticity redistribution and mixing. The instability is associated with a displacement of the vortex center (as defined by either minimum pressure or streamfunction) that rotates around the vortex core, and thus offers a physical mechanism for the persistent, small-amplitude trochoidal wobble often observed in hurricane tracks. The instability also indicates that inner-core vorticity mixing will always occur in such vortices, even when the more familiar higher-wavenumber barotropic instabilities are not supported.

Abstract

Cloud-top verification is inherently difficult because of large uncertainties in the estimates of observed cloud-top height. Misplacement of cloud top associated with transmittance through optically thin cirrus is one of the most common problems. Forward radiative models permit a direct comparison of predicted and observed radiance, but uncertainties in the vertical position of clouds remain. In this work, synthetic brightness temperatures are compared with forecast cloud-top heights so as to investigate potential errors and develop filters to remove optically thin ice clouds. Results from a statistical analysis reveal that up to 50% of the clouds with brightness temperatures as high as 280 K are actually optically thin cirrus. The filters successfully removed most of the thin ice clouds, allowing for the diagnosis of very specific errors. The results indicate a strong negative bias in midtropospheric cloud cover in the model, as well as a lack of land-based convective cumuliform clouds. The model also predicted an area of persistent stratus over the North Atlantic Ocean that was not apparent in the observations. In contrast, high cloud tops associated with deep convection were well simulated, as were mesoscale areas of enhanced trade cumulus coverage in the Sargasso Sea.