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John W. Nielsen-Gammon

A 20-yr loop of the global tropopause, defined in terms of potential vorticity (PV), is constructed using the NCEP–NCAR reanalysis dataset. This method of visualizing observed upper-tropospheric dynamics is useful for studying a wide range of phenomena. Examples are given of the structure of jet streams and planetary-scale tropopause folds, the propagation of a high-amplitude Rossby wave packet partway around a hemisphere, several subtropical wave breaking events, the similarities between exceptional cases of rapid cyclogenesis, favorable regions for cross-equatorial propagation of Rossby waves, the annual cycle of the tropical tropopause, the structure of the Tibetan anticyclone and equatorial easterly jet associated with the Asian monsoon, the meridional structure of the upper branch of the Hadley cell, the interaction of a hurricane and midlatitude trough to form the “Perfect Storm,” and the upper-tropospheric PV changes associated with El Niño and La Niña.

Plumes of anticyclonic potential vorticity are frequently seen to be pulled from the subtropical reservoir and roll up into large anticyclones. These previously undescribed plumes may be particularly relevant to jet streak dynamics and stratosphere-troposphere exchange.

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Richard C. Igau and John W. Nielsen-Gammon

Abstract

The evolution of the southerly low-level jet (LLJ) during a return flow event is studied using output from the Penn State/NCAR Mesoscale Model (Version 4). Three geographically different southerly LLJs develop in the simulation: one over the southern Plains of the United States, a second southwest of Brownsville, Texas, and a third over the western Gulf of Mexico. The LLJ over the Plains is found to form first as an inertial oscillation and later as a response to lee troughing and an elevated mixed layer that develops over the region. Over Mexico, the temperature structure over the Altiplanicie Mexicana (Mexican High Plain) is responsible for a locally intense low-level pressure gradient east of the High Plain which remains nearly stationary over two diurnal cycles. The LLJ over the western Gulf of Mexico results largely from topographic blocking of the low-level southerly flow by the eastern end of the Neovolcanic Cordillera northwest of Veracruz, Mexico.

The evolution of the lower troposphere over the southern Plains resembles the Carlson and Ludlam conceptual model for a severe storm environment, but the structure of the return flow is complex. When midlevel westerlies are weak, mesoscale and boundary layer processes govern the development of LLJs. As the west and southwesterly winds increase with an approaching upper-level disturbance, synoptic influences overwhelm the mesoscale processes leading to a single, larger-scale LLJ.

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John W. Nielsen-Gammon and Randy J. Lefevre

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The intensification and evolution of midlatitude upper-tropospheric mobile troughs may be viewed in terms of the isentropic advection and deformation of the tropopause potential vorticity gradient. The potential vorticity viewpoint allows one to qualitatively assess observed events in the context of existing theories of mobile trough genesis, such as baroclinic instability or downstream development. In order to quantitatively determine the role of distinct dynamical process, the method of piecewise tendency diagnosis, or PTD, is developed. PTD is an extension of piecewise potential vorticity inversion applied to height tendencies, with the forcing terms in the quasigeostrophic height tendency equation partitioned into potential vorticity advection associated with distinct dynamical processes.

A particular case of mobile trough genesis, which occurred during 1–4 December 1980 over North America is diagnosed using PTD. Although about 20% of the intensification of the trough was due to superposition and amplification of the low-level cyclone during surface cyclogenesis, the diagnosis focuses on the height perturbation induced by the upper-level PV anomaly. The trough is found to have formed primarily through down-stream propagation of Rossby wave energy from disturbances over the, northwest Pacific. As the trough amplified, it interacted with an existing surface temperature gradient over the central United States and produced a front wave. As the frontal wave intensified, the favorable vertical tilt allowed mutual baroclinic amplification of the upper and lower systems. Eventually, the upper-level trough grew to sufficient amplitude that it began to lose energy downstream through wave propagation and the trough began to weaken even though a favorable tilt remained between the upper and lower systems. Horizontal deformation and small-scale vortex interaction were less important to the overall development of the mobile trough, but contributed significantly to intensification at various times in its lift cycle. The direct effects of the remaining dynamical processes excluding latent heating and friction, which were not diagnosed) were insignificant.

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Boksoon Myoung and John W. Nielsen-Gammon

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Identifying dynamical and physical mechanisms controlling variability of convective precipitation is critical for predicting intraseasonal and longer-term changes in warm-season precipitation and convectively driven large-scale circulations. On a monthly basis, the relationship of convective instability with precipitation is examined to investigate the modulation of convective instability on precipitation using the Global Historical Climatology Network (GHCN) and NCEP–NCAR reanalysis for 1948–2003. Three convective parameters—convective inhibition (CIN), precipitable water (PW), and convective available potential energy (CAPE)—are examined. A lifted index and a difference between low-tropospheric temperature and surface dewpoint are used as proxies of CAPE and CIN, respectively.

A simple correlation analysis between the convective parameters and the reanalysis precipitation revealed that the most significant convective parameter in the variability of monthly mean precipitation varies by regions and seasons. With respect to region, CIN is tightly coupled with precipitation over summer continents in the Northern Hemisphere and Australia, while PW or CAPE is tightly coupled with precipitation over tropical oceans. With respect to seasons, the identity of the most significant convective parameter tends to be consistent across seasons over the oceans, while it varies by season in Africa and South America. Results from GHCN precipitation data are broadly consistent with reanalysis data where GHCN data exist, except in some tropical areas where correlations are much stronger (and sometimes signed differently) with reanalysis precipitation than with GHCN precipitation.

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John W. Nielsen-Gammon and David M. Schultz

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David A. Gold and John W. Nielsen-Gammon

Abstract

A potential vorticity (PV) diagnostic framework is used to explore the sensitivity of the 3 May 1999 Oklahoma City tornado outbreak to the strength of a particular PV anomaly proximate to the geographical region experiencing the tornado outbreak. The results derived from the balanced PV diagnosis agree broadly with those obtained previously in a numerical simulation of the same event, while offering additional insight into the nature of the sensitivity. Similar to the findings of other cases, the balanced diagnosis demonstrates that intensifying (removing) the PV anomaly of interest increases (decreases) the balanced CAPE over the southwestern portion of the outbreak region, reduces (increases) the storm-relative helicity, and increases (reduces) ascent. The latter finding, coupled with the results of the modeling study, demonstrates that intensifying a PV anomaly proximate to an outbreak environment can increase the likelihood that more widespread and possibly less tornadic convection will ensue. The overall results of the balanced diagnosis complement those of other case studies, leading to the formulation of a conceptual model that broadly anticipates how the convective regime will respond to changes in intensity of upper-tropospheric weather features.

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John W. Nielsen-Gammon and David A. Gold

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Idealized numerical experiments are conducted to understand the effect of upper-tropospheric potential vorticity (PV) anomalies on an environment conducive to severe weather. Anomalies are specified as a single isolated vortex, a string of vortices analogous to a negatively tilted trough, and a pair of string vortices analogous to a position error in a negatively tilted trough. The anomalies are placed adjacent to the tropopause along a strong upper-level jet at a time just prior to a major tornado outbreak and inverted using the nonlinear balance equations.

In addition to the expected destabilization beneath and adjacent to a cyclonic PV anomaly, the spatial pattern of the inverted balanced streamfunction and height fields is distorted by the presence of the horizontal PV gradient along the upper-tropospheric jet stream. Streamfunction anomalies are elongated in the cross-jet direction, while height and temperature anomalies are elongated in the along-jet direction. The amplitude of the inverted fields, as well as the changes in CAPE associated with the inverted temperature perturbations, are linearly proportional to the amplitudes of the PV anomalies themselves, and the responses to complex PV perturbation structures are approximated by the sum of the responses to individual simple PV anomalies. This is true for the range of PV amplitudes tested, which was designed to mimic typical 6-h forecast or analysis errors and produced changes in CAPE beneath the trough of well over 100 J kg−1. Impacts on inverted fields are largest when the PV anomaly is on the anticyclonic shear side of the jet, where background PV is small, compared with the cyclonic shear side of the jet, where background PV is large.

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David A. Gold and John W. Nielsen-Gammon

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Observational and modeling studies have shown that shear and instability are powerful predictors of the likelihood of severe weather and tornadoes. To the extent that upper-tropospheric forecast errors can be described as potential vorticity (PV) anomalies on the forecasted PV field, knowing (and being able to quantify) the effects of such errors on shear and instability would allow forecasters to anticipate the effects of those errors on the likely mode of severe weather. To test the sensitivity of the severe convective environment to PV fluctuations, a PV inversion framework is adopted that utilizes nonlinear balance. The observed PV field is modified in a way that mimics realistic perturbations of trough intensity, location, or shape. Soundings, including moisture profiles, are reconstructed from the balanced geopotential height field assuming that air parcels conserve mixing ratio while their isentropic surfaces are displaced upward or downward by the addition of anomalous PV. Unperturbed balanced soundings agree reasonably well with full, unbalanced soundings, and differences are attributable to departures from nonlinear balance in areas of strong vorticity or acceleration. Balanced vertical wind profiles do not include the effects of friction, so the vertical shear of the balanced wind departs unacceptably from total shear within the lowest 1 km of the troposphere. The balanced wind perturbations are added to the total analyzed shear profile to estimate the effect of PV perturbations on shear and storm-relative helicity. By this process, the importance of typical or hypothesized upper-tropospheric forecast errors may be addressed in an idealized, case-study, or operational context.

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John W. Nielsen-Gammon and Daniel Keyser

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Effective stratification can be interpreted as the resistance to upward motion of saturated air parcels experiencing condensation. Previously published expressions for effective stratification conflict with each other, and the most widely distributed expression contains an O(1) error. A derivation of effective stratification is presented that exposes its physical interpretation and that reveals the origin of the flaw in the incorrect derivation.

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John W. Nielsen-Gammon and William L. Read

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Left-moving supercells, which rotate anticyclonically, are much less common than their right-moving counterparts but are nevertheless capable of producing severe weather. On 26 May 1992, a severe left-moving thunderstorm over east Texas developed within range of the WSR-88D (Weather Surveillance Radar-1988 Doppler) radar at League City, Texas. The evolution of the left-moving thunderstorm, including its split from its parent thunderstorm, is presented using standard WSR-88D products. The storm produced wind damage and large hail, whose presence in the thunderstorm caused a flare echo in the return signal. No automated WSR-88D algorithms exist to detect mesoanticyclones or flares, so the subjective interpretation of these radar signatures as indicators of severe weather can be critical for the proper issuance of warnings for such storms.

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