Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Fernando Caracena x
  • Refine by Access: All Content x
Clear All Modify Search
Fernando Caracena
and
Michael W. Maier

Abstract

A microburst embedded in heavy rain in a humid environment struck very near the Field Observing Site (FOS) of the Florida Area Cumulus Experiment (FACE), producing a diverging pattern of wind damage in sugar cane.

While the dry, virga-type microburst is now beginning to be understood as a result of the SAWS project, the wet, or heavy-rain-embedded, microburst still remains a mystery. The fortuitous occurrence of a wet microburst in a humid environment, with a well-marked wind damage pattern and a well-instrumented site (including upper-air soundings), furnishes a means of gleaning some understanding of the larger-scale processes that are conducive to strong downdrafts in wet environments. In this case several features were present: 1) an elevated dry layer (above 500 mb), 2) overlying a nearly moist adiabatic lower tropospheric layer (below 500 mb), 3) a short-wave trough approaching the area from the north-northeast along the western side of a synoptic-scale trough with 4) increased shear in the lower troposphere, and 5) strong boundary-layer forcing, first by a lake breeze front off Lake Okeechobee, then by convective gust fronts. The site of the microburst itself was in the portion of the storm where a new cell was initiated by a strong gust front in an area where rain was still failing from an older, dissipating cell. The strong boundary-layer forcing may have generated an impulsive updraft surge in a very wet environment with lingering precipitation, which was followed by an impulsive collapse in a water-loaded downdraft. In this case, however, the negative buoyancy due to water loading was an order of magnitude less than that due to evaporation.

Full access
Charles A. Doswell III
and
Fernando Caracena

Abstract

Several aspects of the problem of estimating derivatives from an irregular, discrete sample of vector observations are considered. It is shown that one must properly account for transformations from one vector representation to another. if one is to preserve the original properties of a vector point function during such a transformation (e.g., from u and v wind components to speed and direction). A simple technique for calculating the linear kinematic properties of a vector point function (translation, cud, divergence, and deformation) is derived for any noncolinear triad of points. This technique is equivalent to a calculation done using line integrals, but is much more efficient.

It is shown that estimating derivatives by mapping the vector components onto a grid and taking finite differences is not equivalent to estimating the derivatives and mapping those estimates onto a grid, whenever the original observations are taken on a discrete, irregular network. This problem is particularly important whenever the data network is sparse relative to the wavelength of the phenomena. It is shown that conventional mapping/differencing fail to use all the information in the data, as well. Some suggesstions for minimizing the errors in derivative estimation for general (nonlinear) vector point functions are discussed.

Full access
T. Theodore Fujita
and
Fernando Caracena

Two aircraft accidents in 1975, one at John F. Kennedy International Airport in New York City on 24 June and the other at Stapleton International Airport in Denver on 7 August, were examined in detail. A third accident on 23 June 1976 at Philadelphia International Airport is being investigated. Amazingly, there was a spearhead echo just to the north of each accident site. The echoes formed from 5 to 50 min in advance of the accident and moved faster than other echoes in the vicinity. These echoes were photographed by National Weather Service radars, 130–205 km away. At closer ranges, however, one or more circular echoes were depicted by airborne and ground radars. These cells were only 3–5 km in diameter, but they were accompanied by downdrafts of extreme intensity, called downbursts. All accidents occurred as aircraft, either descending or climbing, lost altitude while experiencing strong wind shear inside downburst cells.

Full access
John A. Augustine
and
Fernando Caracena

Abstract

Composite analyses are examined to identify signals in the late afternoon surface and lower-tropospheric environments that indicate the expected location and degree of nocturnal mesoscale convective system (MCS) development over the central United States. The authors concentrate on two features: 1) the forcing for the low-level jet (LLJ), and 2) the frontogenetic character of lower-tropospheric fronts, or other types of airmass boundaries, with which MCSs are associated. Results show that very large, long-lived, nocturnal MCSs are likely to mature downwind of a late afternoon surface geostrophic wind maximum if that region is frontogenetic at 850 mb. The significance of the afternoon surface geostrophic wind maximum is that it identifies the region where the core of the elevated nocturnal LLJ will develop atop the surface-based nocturnal inversion. Where the forecast LLJ will encounter the frontogenetic boundary defines an area of potentially enhanced nocturnal low-level ascent through convergence and warm advection, which would predispose that region to significant mesoscale convective development and heavy rain. Composites and case studies show that smaller, less significant MCSs also mature north of maxima in the late afternoon surface geostrophic wind but that those regions appear to lack a strong frontogenetic signal at 850 mb. Case studies illustrate how well these indicators applied to four different situations during the summer of 1992. Finally, a tentative design for an operational product that incorporates key features of these findings for forecasting the location of heavy rain is proposed.

Full access
Stanley L. Barnes
,
Fernando Caracena
, and
Adrian Marroquin

Fine-mesh models, such as the eta model, are producing increasingly detailed predictions about mesoscale atmospheric motions. Mesoscale systems typically produce stronger vertical motions than do synoptic-scale storms, making it more difficult for forecasters to assess the strength of the latter's dynamics when the signals are overwhelmed by mesoscale processes. This paper describes a method for extracting synoptic-scale information from mesoscale model data. Predicted height fields from the 29-km eta model are investigated to determine the filtering and smoothing requirements necessary to resolve synoptic-scale patterns of vertical motions using quasigeostrophic (QG) diagnostics. The selected late-fall case includes a jet stream that enters the continent over the Pacific Northwest, resulting in orographically induced troughs in the lee of the Cascade Range and Rocky Mountains. Gravity waves are found to emanate from this region in arcs that reach Hudson Bay to the northeast and extend to the Caribbean in the southeast. Individual gravity wave crests (~240 km apart) are of sufficient amplitude (5 to 10 m at 500 mb) to dominate the expected synoptic-scale vertical motions by two orders of magnitude. A numerical filter based on a two-dimensional diffraction function is designed, tested, and found to eliminate the influence of the gravity waves effectively. The filtered model data are then able to reveal synoptic-scale vertical motion patterns in all areas except the vicinity of the lee troughs, which still dominate QG forcing near the jet axis.

Full access
Robert A. Maddox
,
Lee R. Hoxit
,
Charles F. Chappell
, and
Fernando Caracena

Abstract

Analyses and descriptions of the meteorological conditions that produced devastating flash floods in the Big Thompson Canyon on 31 July 1976 and in the Black Hills on 9 June 1972 are presented. The storms developed when strong low-level easterly winds pushed moist, conditionally unstable air masses upslope into elevated, mountainous terrain. Orographic uplift released the convective instability and light winds aloft allowed the storm complexes to remain nearly stationary. Meteorological conditions that produced these flash floods were found to have been very similar. A set of meteorological features is defined for the purpose of identifying the potential for this type flash flood along the eastern slopes of the Rocky Mountains.

Full access
Fernando Caracena
,
Robert A. Maddox
,
L. Ray Hoxit
, and
Charles F. Chappell

Abstract

Mesoscale analyses and descriptions of meteorological conditions that produced the devastating dash flood in the Big Thompson Canyon on 31 July 1976 are presented. The storm developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable air mass upslope into the Front Range of the Rocky Mountains. The main thrust of the moisture flux focused initially into the Big Thompson area. Orographic uplift released the convective instability, and light south-southeasterly winds at steering levels allowed the storm complex to remain nearly stationary over the foothills. Heavy rains fell within the storm along a narrow corridor only 5 km wide oriented north-northeast by south-southwest. Minimal entrainment of relatively moist air at middle and upper levels, very low cloud bases and a slightly tilted, updraft structure contributed to a high precipitation efficiency. A deep warm layer of convective cloud fostered precipitation growth through warm cloud processes. The greatest concentrations of precipitation size particles remained at low elevations and as a result of poor vertical beam resolution returned anomalously weak radar echoes to a WSR-57 located 110 n mi away.

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