Search Results

You are looking at 1 - 10 of 24 items for

  • Author or Editor: Matthew Hitchman x
  • Refine by Access: All Content x
Clear All Modify Search
Lawrence Coy and Matthew Hitchman

Abstract

Atmospheric Kelvin waves, as revealed by temperatures obtained from the recent Limb Infrared Monitor of the Stratosphere (LIMS) experiment, commonly occur in packets. A simple two-dimensional gravity-wave model is used to study the upward propagation of these packets through different zonal mean wind profiles derived from the LIMS data. The observed prevalence of high frequency waves in the lower mesosphere and low frequency waves in the lower stratosphere can be explained by dispersion of energy associated with the range of frequencies comprising a packet. Dominant wave frequencies at upper and lower levels are more distinctly separated if the packet propagates through a layer of westerly winds. Due to dispersion and shear effects, a packet of short temporal length at low levels will have a considerably extended impact on a layer of westerly winds at higher levels. Observed and modeled westerly accelerations resulting from packet absorption occur in the same layer, and are similar in magnitude and duration. These results support the theory that Kelvin waves are responsible for the westerly phase of the semiannual oscillation.

Full access
Matthew H. Hitchman and Conway B. Leovy

Abstract

The distribution of day-night temperature differences in the middle atmosphere determined by the Nimbus 7 LIMS experiment is described. Day-night differences maximize at and are approximately symmetric about the equator. Successive centers of opposite sign increase in amplitude with altitude, the pattern having a vertical wavelength of ∼25 km. Profiles of rocket meridional wind at Kwajalein (8.7°N) and Ascension Island (8.0°S) taken near local noon and averaged over the LIMS data period, exhibit maxima which support the tidal interpretation of the equatorial temperature pattern. These characteristics are in general agreement with previous observational and theoretical results for the solar driven diurnal tide. Substantial time variations in amplitude and in location of the temperature maxima are observed. The diurnal tide near the equatorial stratopause appears to be influenced by the phase of the semiannual oscillation.

Full access
Shellie M. Rowe and Matthew H. Hitchman

Abstract

The stalling and rapid destruction of a potential vorticity (PV) anomaly in the upper troposphere–lower stratosphere (UTLS) by convectively detrained inertially unstable air is described. On 20 August 2018, 10–15 in. (~0.3–0.4 m) of rain fell on western Dane County, Wisconsin, primarily during 0100–0300 UTC 21 August (1900–2100 CDT 20 August), leading to extreme local flooding. Dynamical aspects are investigated using the University of Wisconsin Nonhydrostratic Modeling System (UWNMS). Results are compared with available radiosonde, radar, total rainfall estimates, satellite infrared, and high-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Using ECMWF analyses, the formation of the UTLS PV anomaly is traced to its origin a week earlier in a PV streamer over the west coast of North America. The rainfall maximum over southern Wisconsin was associated with this PV anomaly, whereby convection forming in the warm-upglide sector rotated cyclonically into the region. The quasi-stationarity of this rainfall feature was aided by a broad northeastward surge of inertially unstable convective outflow air into southeastern Wisconsin, which coincided with stalling of the eastward progression of the PV anomaly and its diversion into southern Wisconsin, extending heavy rainfall for several hours. Cessation of rainfall coincided with dilution of the PV maximum in less than an hour (2100–2200 CDT), associated with the arrival of negative PV in the upper troposphere. The region of negative PV was created when convection over Illinois transported air with low wind speed into northeastward shear. This feature is diagnosed using the convective momentum transport hypothesis.

Restricted access
Matthew H. Hitchman and Shellie M. Rowe

Abstract

The role of differential advection in creating tropopause folds and strong constituent gradients near midlatitude westerly jets is investigated using the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). Dynamical structures are compared with aircraft observations through a fold and subpolar jet (SPJ) during research flight 4 (RF04) of the Stratosphere–Troposphere Analyses of Regional Transport (START08) campaign. The observed distribution of water vapor and ozone during RF04 provides evidence of rapid transport in the SPJ, enhancing constituent gradients above relative to below the intrusion. The creation of a tropopause fold by quasi-isentropic differential advection on the upstream side of the trough is described. This fold was created by a southward jet streak in the SPJ, where upper-tropospheric air displaced the tropopause eastward in the 6–10 km layer, thereby overlying stratospheric air in the 3–6 km layer. The subsequent superposition of the subtropical and subpolar jets is also shown to result from quasi-isentropic differential advection. The occurrence of low values of ozone, water vapor, and potential vorticity on the equatorward side of the SPJ can be explained by convective transport of low-ozone air from the boundary layer, dehydration in the updraft, and detrainment of inertially unstable air in the outflow layer. An example of rapid juxtaposition with stratospheric air in the jet core is shown for RF01. The net effect of upstream convective events is suggested as a fundamental cause of the strong constituent gradients observed in midlatitude jets, with the aggregate divergence aloft causing upper-tropospheric air to flow over stratospheric air. Idealized diagrams illustrate the role of differential advection in creating tropopause folds and constituent gradient enhancement.

Restricted access
Gregory A. Postel and Matthew H. Hitchman

Abstract

Ten years (1986–95) of global analyses from the European Centre for Medium-Range Weather Forecasts are used to investigate the temporal and spatial distributions of Rossby wave breaking (RWB) at 350 K along the tropopause, herein defined by the ±1.5 potential vorticity (PV) unit (10−6 K m2 s−1 kg−1) contours. Though many studies acknowledge RWB as an important contributor to the complex of mixing processes in the atmosphere, there exists no prior climatological study of its distribution near the tropopause.

As in previous studies, RWB is identified in the global analyses by southward directed PV gradients. At 350 K, RWB along the tropopause occurs preferentially during summer over the midoceans, in relative proximity to the planetary-scale high pressure systems in the subtropics. Isentropic trajectories at 350 K show that outflow from the tops of these subtropical highs directly participates in RWB over the adjacent, downstream oceanic regions.

Two regions are highlighted in this study: the North Pacific during boreal summer and the South Atlantic during austral summer. Synoptic maps of breaking Rossby waves in these regions are provided to reveal the acute tropopause folding in the meridional plane, which characteristically accompanies RWB. The rich interaction between the tropical flow and the extratropical westerly current exhibited by these cases suggests that the subtropical highs serve as important agents in the coupling between the tropical troposphere and the extratropical stratosphere. As expected from theoretical considerations, the locations where RWB occurs most frequently, known as “surf zones,” are shown to coexist with regionally weak time-mean wind speeds and horizontal gradients of PV at 350 K.

Full access
Donal J. O'Sullivan and Matthew H. Hitchman

Abstract

A mechanistic model of the middle atmosphere is used to study the interaction between Rossby waves forced at the extratropical tropopause and inertial instability in the equatorial lower mesosphere. The impact of cross-equatorial shear strength and Rossby wave forcing amplitude is explored. Model results support the hypothesis, based on satellite temperature observations, that Rossby waves organize regions of equatorial inertial instability into coherent large-scale circulations. Horizontal convergence and divergence maxima are found stacked over the boundaries of regions of anomalous potential vorticity (PV). This supports observational diagnoses and theoretical expectations that parcel inertial accelerations arise in regions of anomalous PV, with divergence and convergence occurring at the boundaries. Although cross-equatorial shear determines the initial volume of inertially unstable air, Rossby waves arriving from the winter hemisphere deform PV contours such that zonally confined regions of anomalous PV extend well into the winter hemisphere and somewhat into the summer hemisphere. Significant parcel inertial accelerations are diagnosed to occur in anomalous PV tongues to 30° latitude in the winter hemisphere. Using a smaller cross-equatorial shear delays the penetration of anomalous PV into the winter hemisphere, while increasing the Rossby wave forcing amplitude causes a more rapid evolution of the low-latitude flow.

These results suggest that inertial instability is intimately involved in Rossby wave breaking in the subtropical winter mesosphere. The horizontal scales of inertially unstable regions coevolve with those of PV anomalies, and so affect the breaking process from inception. The vertical scale of inertial circulations is much smaller than that of Rossby waves; hence, Rossby wave PV anomalies are eroded by vertical mixing as well as horizontal mixing. Gravity waves radiate away from the inertially unstable regions, which effectively convert energy from rotational to divergent modes. The interplay between inertial and gravitational instabilities and their role in causing irreversible mixing in the winter subtropics is explored. The effects of inertial instability on the seasonal evolution of PV is discussed from the point of view of “PV thinking.”

Full access
Chang Hi Joung and Matthew H. Hitchman

Abstract

A composite of 16 strong East Asian polar outbreak occurrences, widely separated in time, reveals a clear sequence of events: beginning over the western North Atlantic six or seven days in advance of the key day (as defined by the cold frontal passage over Korea) troughs and ridges are seen to form, develop and decay successively downstream of one another across the Eurasian continent until the polar outbreak occurs. These troughs and ridges reach their maximum amplitude in much the same location and at the same time relative to the key day in the majority of the 16 cases. The center of the wave packet moves along a curved trajectory approximating the mean 300 mb flow at a nearly constant rate of 30° longitude per day. The perturbation moves as an essentially barotropic dispersive wave across most of Eurasia, but its evolution becomes highly baroclinic as it approaches the East Asian coast. The wavetrain nature of this perturbation breaks down as it propagates out over the Pacific Ocean. This breakdown coincides in 8 of the 16 cases with the formation of a large amplitude ridge of great meridional extent.

Full access
Matthew H. Hitchman and Amihan S. Huesmann

Abstract

The influence of the stratospheric quasi-biennial oscillation (QBO) on the polar night jets (PNJs), subtropical easterly jets (SEJs), and associated Rossby wave breaking (RWB) is investigated using global meteorological analyses spanning 10 recent QBO cycles. The seasonal dependence of the descent of the QBO is shown by using five layered shear indices. It is found that the influence of the QBO is distinctive for each combination of QBO phase, season, and hemisphere (NH or SH). The following QBO westerly (W) minus easterly (E) differences in the PNJs were found to be significant at the 97% level: When a QBO W (E) maximum is in the lower stratosphere (∼500 K or ∼50 hPa), the NH winter PNJ is stronger (weaker), in agreement with previous results (mode A). Mode A does not appear to operate in other seasons in the NH besides DJF or in the SH in any season. When a QBO W (E) maximum is in the middle stratosphere (∼700–800 K or ∼10–20 hPa), the PNJ in the SH spring is stronger (weaker), also in agreement with previous results (mode B). It is found that mode B also operates in the NH spring. A third distinctive mode is found during autumn in both hemispheres: a QBO W (E) maximum in the middle stratosphere coincides with a weaker (stronger) PNJ (mode C). The signs of wind anomalies are the same at low and high latitudes for modes A and B, but are opposite for mode C. This sensitive dependence on QBO phase and season is consistent with the nonlinear nature of the interaction between planetary waves and the shape of the seasonal wind structures.

During the solstices the meridional circulation associated with QBO connects primarily with the winter hemisphere, whereas during the equinoxes it is more symmetric about the equator. QBO W enhance the equatorial potential vorticity (PV) gradient maximum, but the time-mean maximum may be related to chronic instabilities in the subtropics. The equatorial PV gradient maximum and flanking RWB tend to be more pronounced in the Eastern Hemisphere in stratospheric analyses.

When QBO W are in the middle stratosphere, the flanking PV gradient minima (SEJs) are enhanced and RWB is more frequent and symmetric about the equator. When QBO W are in the upper stratosphere, a strong seasonal asymmetry is seen, with enhanced RWB in the summer SEJ, primarily during boreal winter. This is consistent with an upward increase of summer to winter flow and modulation by a strong “first” and weak “second” semiannual oscillation.

Full access
Restricted access
Matthew H. Hitchman and Shellie M. Rowe

Abstract

The structure and origin of mesoscale jets and associated potential vorticity (PV) dipoles in the upper troposphere and lower stratosphere (UTLS) in tropical cyclones (TCs) are investigated. UTLS PV dipole/jetlets, which occurred in Talas (2011), Edouard (2014), and Ita (2014), are simulated with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). PV dipoles are confined to the UTLS, where the jetlets oppose the ambient anticyclonic flow. They form ~100–250 km from the eye in convective asymmetries and are characterized by surges of air that accelerate in the updraft, overshoot, and extend radially outward. In these cases, the outflow jet merges with the subtropical westerly jet. Analysis of the structure of UTLS PV dipole/jetlets led to a new physical interpretation for their formation, based on the difference in momentum between the updraft and air in the UTLS: the convective momentum transport hypothesis. This view is complementary to the vorticity tilting hypothesis. A jetlet will form whenever an updraft carries horizontal winds to a level with different wind. Schematic diagrams show how to predict jetlet orientation based on horizontal speeds in the updraft and UTLS ambient air. In TCs, horizontal winds in the updraft are cyclonic, so a UTLS jetlet will be cyclonic and oppose the ambient flow. Each jetlet creates an anticyclonic, inertially unstable PV member, which lies radially outward. Estimates of terms in the PV conservation equation support the hypothesis that the dipoles arise from the curl of shear stress. Convective asymmetries associated with PV dipole/jetlets can significantly modify TC evolution by local thermodynamic acceleration.

Free access