Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Edward I. Tollerud x
  • All content x
Clear All Modify Search
Edward I. Tollerud and Steven K. Esbensen

Abstract

A large-amplitude asymmetric vorticity couplet has been observed in the upper troposphere near a lame cloud cluster in GATE (GARP Atlantic Tropical Experiment) on 5 September 1974. The couplet consists of a cyclonic center to the north of the cluster and an anticyclonic center to the south. The couplet is a result of the deceleration of the upper-tropospheric easterly winds in a region near the center of the cluster. These features also appear on a composite of large slow-moving clusters occurring during Phase 3 of GATE.

Vorticity budget analysis shows that the couplets are produced by subcluster-scale process and, to a lesser degree, by cluster-scale twisting. On the basis of this finding and on the basis of the three-dimensional structure of horizontal momentum fields, it is suggested that a part of the deceleration producing the couplet is a result of momentum redistribution by subcluster-scale circulations (such as cumulonimbi or mesoscale cloud lines).

The composite wind fields of the large slow-moving cloud clusters during Phase 3 of GATE are found to be similar to horizontal composites made within easterly wave phase categories near the wave trough.

Full access
Edward I. Tollerud and Steven K. Esbensen

Abstract

The wind fields associated with cloud clusters observed during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. A compositing procedure is devised to isolate the cluster circulations. Satellite-observed cloud cover estimates by Cox and Griffith form the basis for the identification and classification of clusters and for the determination of their life cycles. The compositing criteria focus on the upper-tropospheric portions of anvil clouds that are a prominent feature of cloud clusters. The compositing procedure is applied to a set of objectively analyzed upper-air winds for Phase 3 of GATE prepared by K. V. Ooyama and J.-H. Chu.

The results show that slow-moving cloud clusters tend to form in regions of relatively small vertical wind shear and that the shear at the cluster center decreases during the cluster life cycle. Squall clusters, on the other hand, have significantly larger lower-tropospheric shear.

Changes in the total horizontal wind field in the middle and lower troposphere during cluster evolution appear to be primarily due to the advance of easterly waves relative to the slower-moving clusters. However, the horizontal divergence field is centered on the cluster during its lifetime. The maximum value of the upper-tropospheric divergence at the cluster center lags the maximum boundary layer convergence by 3–6 hours. During the mature and dissipating stages, a layer of convergence develops in the middle troposphere.

The vertical motion diagnosed in the growing, mature and dissipating stages of the clusters is found to be qualitatively similar to the life cycle hypothesized for the smaller-scale individual mesoscale precipitation features that are found within the clusters. Strong upward motion develops in the upper troposphere from the growing to the mature stage; in the lower troposphere, the upward velocities decrease dramatically from the mature to the dissipating stage.

The cluster-scale vertical motion in the mature stage of the slow-moving clusters is compared with large-scale values in the trough of a composite easterly wave, with Phase-3 averaged vertical velocities, and with vertical motion estimates in squall lines. In general, vertical motion increases rapidly with decreasing space and time scales. The squall and nonsquall vertical motion profiles are qualitatively similar.

Full access
Chungu Lu, Huiling Yuan, Edward I. Tollerud, and Ning Wang
Full access
Steven K. Esbensen, Lloyd J. Shapiro, and Edward I. Tollerud

Abstract

A physical and mathematical framework for the mutually consistent parameterization of the effects of cumulus convection on the large-scale momentum and vorticity fields is proposed. The key to achieving consistency is the understanding that the vorticity dynamics of the clouds below the spatial resolution of a large-scale dynamical model may be neglected in the vorticity budget when the clouds are considered to be independent buoyant elements sharing a common large-scale environment This simplified approach is used to obtain a consistent pair of large-scale momentum and vorticity equations based on Ooyama's theory of cumulus parameterization. The results focus attention on the need to obtain a better understanding of the detrainment process and the pressure interactions between the clouds and their environment.

Full access
Steven K. Esbensen, Jough-Tai Wang, and Edward I. Tollerud

Abstract

The heat and moisture budgets associated with five large nonsquall cloud clusters observed during Phase 3 of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. The input data for the budget computations are objectively analyzed fields of wind, temperature and relative humidity that were based on conventional upper-air soundings. Estimates of the radiative heating rate were obtained from Cox and Griffith. A compositing technique is used to summarize the budget results for the growing, mature and dissipating stages of the clusters.

The budgets in the growing stage are characterized by a very large low-level, apparent moisture sink separated in height from the region where the apparent heating is realized. In the mature stage, the apparent heating maximum shifts upward, accompanied by the development of a corresponding secondary maximum of apparent drying. A composite of radiative heating estimates from Cox and Griffith shows that the horizontal radiative heating gradients reach their maximum strength during the mature stage. In the dissipating stage, the apparent heat source is approximately balanced by the apparent moisture sink above the freezing level; below the freezing level, the implied vertical convective flux of sensible and latent heat is approximately constant with height.

The time-dependent behavior of the budgets gives support to the hypothesis of Leary and Houze that the widespread upper-level cloud decks associated with cloud clusters play an active and important role in determining large-scale beat and moisture budgets in the tropics.

Full access
Jennifer Luppens Mahoney, John M. Brown, and Edward I. Tollerud

Abstract

Case studies of heavy snowstorms at Denver and Colorado Springs, Colorado, indicate that they occur under different meteorological conditions. The authors examine the hypothesis that there are in fact fundamental differences between the synoptic evolution of events in these two storm types by compositing a total of 28 cases, 17 (11) of which are defined as heavy snowstorms (at least 20 cm of snowfall) at Denver (Colorado Springs). These composited fields were constructed using data at three times in the history of each case. Results show distinct differences in the composited synoptic evolution of the two groups. At low levels the Denver composite shows low static stabilities, warm advection, and high values of potential temperature in the lee of the Rockies. The Colorado Springs composite, on the other hand, shows cold, stable air and cold advection in the lee. At upper levels an eastward-progressing short-wave trough is found at different longitudes in the two composites.

The implied interaction between lower and upper levels of the two composites is also very different. For the Denver composite, the trajectory of the upper-level trough brings it close to the area of low static stability and high surface potential temperature at low levels. This implies strong interaction between the upper-level system and the warm unstable air at low levels and dramatic cyclogenesis east of the Rocky Mountains, typically in southeast Colorado. In contrast, the upper short-wave trough in the Colorado Springs composite is farther north, and a layer of cool stable air is found on the High Plains of Colorado. Not surprisingly, surface cyclogenesis is notably weaker in this composite. These conclusions, substantiated by inspection of the individual cases, have obvious implications for predicting the location of heavy snow along the Front Range of Colorado.

Full access
Steven K. Esbensen, Edward I. Tollerud, and Jan-Hwa Chu

Abstract

Objectively analyzed upper-air winds over the intertropical convergence zone of the eastern tropical Atlantic are separated into components representing the mean flow, westward moving synoptic-scale waves and cloud-cluster-scale motions during the third observational period of the GARP Atlantic Tropical Experiment. Vorticity budget analysis is performed for the three components of the flow.

At the scale of cloud clusters, the vorticity budgets show variability that is the same order of magnitude as the vorticity field changes caused by the passage of the easterly waves. The variability patterns appear to be closely related to the phase of the wave and to cloud-cluster activity.

It is concluded that the nonlinear contributions of cloud-cluster-scale motions to the average wave structure are small in the lower troposphere, but are as large as any of the vorticity budget terms involving wave divergence or vertical motion in the middle and upper troposphere.

The implications of the results for numerical modeling of easterly waves are discussed.

Full access
Edward I. Tollerud, Brian Etherton, Zoltan Toth, Isidora Jankov, Tara L. Jensen, Huiling Yuan, Linda S. Wharton, Paula T. McCaslin, Eugene Mirvis, Bill Kuo, Barbara G. Brown, Louisa Nance, Steven E. Koch, and F. Anthony Eckel
Full access
Edward I. Tollerud, Fernando Caracena, Steven E. Koch, Brian D. Jamison, R. Michael Hardesty, Brandi J. McCarty, Christoph Kiemle, Randall S. Collander, Diana L. Bartels, Steven Albers, Brent Shaw, Daniel L. Birkenheuer, and W. Alan Brewer

Abstract

Previous studies of the low-level jet (LLJ) over the central Great Plains of the United States have been unable to determine the role that mesoscale and smaller circulations play in the transport of moisture. To address this issue, two aircraft missions during the International H2O Project (IHOP_2002) were designed to observe closely a well-developed LLJ over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (most important, radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at 55-km intervals and a pair of onboard lidar instruments—High Resolution Doppler Lidar (HRDL) for wind and differential absorption lidar (DIAL) for moisture—observed the moisture transport in the LLJ at greater resolution. Using these observations, the authors describe the multiscalar structure of the LLJ and then focus attention on the bulk properties and effects of scales of motion by computing moisture fluxes through cross sections that bracket the LLJ. From these computations, the Reynolds averages within the cross sections can be computed. This allow an estimate to be made of the bulk effect of integrated estimates of the contribution of small-scale (mesoscale to convective scale) circulations to the overall transport. The performance of the Weather Research and Forecasting (WRF) Model in forecasting the intensity and evolution of the LLJ for this case is briefly examined.

Full access
Steven E. Koch, Brian D. Jamison, Chungu Lu, Tracy L. Smith, Edward I. Tollerud, Cecilia Girz, Ning Wang, Todd P. Lane, Melvyn A. Shapiro, David D. Parrish, and Owen R. Cooper

Abstract

High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model and a nested 1-km simulation with the nonhydrostatic Clark–Hall (CH) cloud-scale model.

Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia–gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appearance associated with these mesoscale gravity waves (horizontal wavelengths of ∼120–216 km). Smaller-scale gravity wave packets (horizontal wavelengths of 1–20 km) within the mesoscale wave field perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number conducive to the generation of turbulence. The wavelet analysis revealed that brief episodes of high turbulent energy were closely associated with gravity wave occurrences. Structure function analysis provided evidence that turbulence was most strongly forced at a horizontal scale of 700 m.

Fluctuations in ozone measured by the aircraft correlated highly with potential temperature fluctuations and the occurrence of turbulent patches at altitudes just above the jet core, but not at higher flight levels, even though the ozone fluctuations were much larger aloft. These results suggest the existence of remnant “fossil turbulence” from earlier events at higher levels, and that ozone cannot be used as a substitute for more direct measures of turbulence. The findings here do suggest that automated turbulence forecasting algorithms should include some reliable measure of gravity wave activity.

Full access