Search Results

You are looking at 1 - 10 of 31 items for

  • Author or Editor: C. A. Davis x
  • Refine by Access: All Content x
Clear All Modify Search
S. B. Trier
,
C. A. Davis
, and
W. C. Skamarock

Abstract

Idealized numerical simulations are used to quantify the effect of quasi-balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in Part I. Maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly, near the radius of maximum tangential vortex winds. The location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear, and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90° to the left of downshear for strong vortices in weak ambient vertical shear. Although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear, maximum upward displacements are associated with large vortices and occur in approximately average vertical shear for MCV environments.

The simulations suggest that in larger and stronger than average MCVs, the lifting that results from the MCV being embedded in a weakly baroclinic environment is, alone, sufficient to saturate initially moist and conditionally unstable layers immediately above the boundary layer. The horizontal location of the resulting thermodynamic instability is approximately coincident with the maximum lower-tropospheric upward displacements. Since in the absence of sustained deep convection the vortices develop substantial vertical tilt, the destabilized region in the lower troposphere lies nearly underneath the vortex center at its level of maximum strength, consistent with observations that redevelopment of organized, long-lived (e.g., t ≥ 6 h) deep convection is most often found near the midtropospheric MCV center. This location for convectively induced stretching of preexisting vertical vorticity is optimal for maintaining the vortex against the deleterious effect of differential advection by the ambient shear.

Full access
S. B. Trier
,
C. A. Davis
, and
D. A. Ahijevych

Abstract

The diurnal cycle of warm-season precipitation in the Rocky Mountains and adjacent Great Plains of the United States is examined using a numerical modeling framework designed to isolate the role of terrain-influenced diurnally varying flows within a quasi-stationary longwave pattern common to active periods of midsummer convection. Simulations are initialized using monthly averaged conditions and contain lateral boundary conditions that vary only with the diurnal cycle. Together these attributes mitigate effects of transient weather disturbances originating upstream of the model domain. After a spinup period, the final 7 days of the 10-day model integration are analyzed and compared with observations. Results indicate that many salient features of the monthly precipitation climatology are reproduced by the model. These include a stationary afternoon precipitation frequency maximum over the Rocky Mountains followed overnight by an eastward-progressing zone of maximum precipitation frequencies confined to a narrow latitudinal corridor in the Great Plains. The similarity to observations despite the monthly averaged initial and lateral boundary conditions suggests that although progressive weather disturbances (e.g., mobile cold fronts and midtropospheric short waves) that originate outside of the region may help enhance and focus precipitation in individual cases, they are not crucial to the general location and diurnal cycle of midsummer precipitation. The roles of persistent daily features such as the nocturnal low-level jet and the thermally induced mountain–plains vertical circulation on both convection and a mesoscale water budget of the central Great Plains (where the heaviest rain occurs) are discussed.

Full access
S. B. Trier
,
C. A. Davis
, and
J. D. Tuttle

Abstract

Observations from the modernized United States National Weather Service (NWS) data network are used to assess the frequency and general characteristics of midtropospheric cyclonic vortices (MCVs) generated by mesoscale convective systems (MCSs). Results from the 1998 convective season (15 May–15 September) over the central United States suggest that long-lived MCVs, which persist after the dissipation of the initiating MCS, are more common than previously documented. These MCVs occur in weaker ambient vertical shear (both in the lower troposphere and through a nominal vortex layer) than MCSs from which no detectable MCVs are spawned.

An important aspect of MCVs is that they may focus subsequent convective development within long-lived discontinuous heavy precipitation episodes. Subsequent deep convection is observed in the vicinity of MCVs in slightly greater than 1/2 of the MCV cases. This subsequent convection occurs in thermodynamic environments of moderate-to-large convective available potential energy and small convective inhibition, and is located in a region from the center of the MCV circulation outward to its downshear periphery. This location is consistent with lower-tropospheric ascent arising from the interaction of a quasi-balanced vortex with the ambient vertical shear.

Long-lived organized convection near the MCV center is likely crucial in either sustaining or reinvigorating vortices during the relatively rare MCV events that persist longer than a diurnal cycle. Examples from the 1998 convective season are used to illustrate differences in the relationship between the MCV circulation and the organization of subsequent convection among different MCV cases.

Full access
Christopher A. Davis
,
Sarah C. Jones
, and
Michael Riemer

Abstract

Simulations of six Atlantic hurricanes are diagnosed to understand the behavior of realistic vortices in varying environments during the process of extratropical transition (ET). The simulations were performed in real time using the Advanced Research Weather Research and Forecasting (WRF) model (ARW), using a moving, storm-centered nest of either 4- or 1.33-km grid spacing. The six simulations, ranging from 45 to 96 h in length, provide realistic evolution of asymmetric precipitation structures, implying control by the synoptic scale, primarily through the vertical wind shear.

The authors find that, as expected, the magnitude of the vortex tilt increases with increasing shear, but it is not until the shear approaches 20 m s−1 that the total vortex circulation decreases. Furthermore, the total vertical mass flux is proportional to the shear for shears less than about 20–25 m s−1, and therefore maximizes, not in the tropical phase, but rather during ET. This has important implications for predicting hurricane-induced perturbations of the midlatitude jet and its consequences on downstream predictability.

Hurricane vortices in the sample resist shear by either adjusting their vertical structure through precession (Helene 2006), forming an entirely new center (Irene 2005), or rapidly developing into a baroclinic cyclone in the presence of a favorable upper-tropospheric disturbance (Maria 2005). Vortex resiliency is found to have a substantial diabatic contribution whereby vertical tilt is reduced through reduction of the primary vortex asymmetry induced by the shear. If the shear and tilt are so large that upshear subsidence overwhelms the symmetric vertical circulation of the hurricane, latent heating and precipitation will occur to the left of the tilt vector and slow precession. Such was apparent during Wilma (2005).

Full access
William C. Ackermann
,
Stanley A. Changnon Jr.
, and
Ray Jay Davis

The Illinois State Water Survey, a state water resources research agency, initiated efforts in 1971 to develop and secure a law for Illinois that would permit and regulate weather modification activities. Such legislation was deemed a prime requirement, not only for the proper execution of scientific experiments on weather modification in Illinois but for the general benefit of citizens of Illinois through encouragement to properly conducted activities and protection from improperly conducted weather modification operations. (It was our intention to develop a “model law” that reflected the best aspects of weather modification legislation and experience in other states, and which would serve as a model for future legislation in other states.) The efforts began in October 1971 and were completed in September 1973 with the signing of the Illinois Weather Modification Control Bill and its accompanying appropriation bill. This paper describes the type of law desired, the activities performed to secure the law, and the primary aspects of the enacted Illinois law.

Full access
Daran L. Rife
,
Christopher A. Davis
, and
Jason C. Knievel

Abstract

The study describes a method of evaluating numerical weather prediction models by comparing the characteristics of temporal changes in simulated and observed 10-m (AGL) winds. The method is demonstrated on a 1-yr collection of 1-day simulations by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) over southern New Mexico. Temporal objects, or wind events, are defined at the observation locations and at each grid point in the model domain as vector wind changes over 2 h. Changes above the uppermost quartile of the distributions in the observations and simulations are empirically classified as significant; their attributes are analyzed and interpreted.

It is demonstrated that the model can discriminate between large and modest wind changes on a pointwise basis, suggesting that many forecast events have an observational counterpart. Spatial clusters of significant wind events are highly continuous in space and time. Such continuity suggests that displaying maps of surface wind changes with high temporal resolution can alert forecasters to the occurrence of important phenomena. Documented systematic errors in the amplitude, direction, and timing of wind events will allow forecasters to mentally adjust for biases in features forecast by the model.

Full access
K. D. Musgrave
,
C. A. Davis
, and
M. T. Montgomery

Abstract

This study examines the formation of Hurricane Gabrielle (2001), focusing on whether an initial disturbance and vertical wind shear were favorable for development. This examination is performed by running numerical experiments using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Gabrielle is chosen as an interesting case to study since it formed in the subtropics only a few days before making landfall in Florida. Three simulations are run: a control run and two sensitivity experiments. The control run is compared with observations to establish the closeness of the model output to Gabrielle’s observed formation. The two sensitivity experiments are designed to test the response of the developing tropical cyclone to alterations in the initial conditions. The first sensitivity experiment removes the initial (or precursor) disturbance, a midtropospheric vortex located over Florida. The second sensitivity experiment reduces the vertical wind shear over the area of formation. The control run produces a system comparable to Gabrielle. The convection in the control run is consistently located downshear of the center of circulation. In the first sensitivity experiment, with the removal of the initial disturbance, no organized system develops. This indicates the importance of the midtropospheric vortex in Gabrielle’s formation. The second sensitivity experiment, which reduces the vertical wind shear over the area of Gabrielle’s formation, produces a system that can be identified as Gabrielle. This system, however, is weaker than both the control run and the observations of Gabrielle. This study provides direct evidence of a favorable influence of modest vertical wind shear on the formation of a tropical cyclone in this case.

Full access
S. B. Trier
,
C. A. Davis
,
D. A. Ahijevych
,
M. L. Weisman
, and
G. H. Bryan

Abstract

A large-domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one-week period of successive zonally propagating heavy precipitation episodes occurring over the central United States. Similar to climatological studies of phase-coherent warm-season convection, the longest-lived precipitation episodes initiate during the late afternoon over the western Great Plains (105°–100°W), reach their greatest intensity at night over the central Great Plains (100°–95°W), and typically weaken around or slightly after sunrise over the Midwest (95°–85°W). The longest-lived episodes exhibit average zonal phase speeds of ∼20 m s−1, consistent with radar observations during the period.

Composite analysis of the life cycle of five long-lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east–west-oriented lower-tropospheric frontal zone. An elevated ∼2-km-deep layer of high-θe air helps sustain convection during its period of greatest organization overnight. Trajectory analysis for individual episodes reveals that the high-θe air originates both from within the frontal zone and to its south where, in this latter case, it is transported northward by the nocturnal low-level jet (LLJ).

The mature (nocturnal) stage composite evinces a thermally direct cross-frontal circulation, within which the trajectories ascend 0.5–2 km to produce the elevated conditionally unstable layer. This transverse vertical circulation is forced by deformation frontogenesis, which itself is supported by the intensification of the nocturnal LLJ. The frontal zone also provides an environment of strong vertical shear, which helps organize the zonally propagating component of convection. Overnight the convection exhibits squall-line characteristics, where its phase speed is typically consistent with that which arises from deep convectively induced buoyancy perturbations combined with the opposing environmental surface flow. In a large majority of cases convection weakens as it reaches the Midwest around sunrise, where environmental thermodynamic stability is greater, and environmental vertical shear, frontogenesis, and vertical motions are weaker than those located farther west overnight.

Full access
D. A. Ahijevych
,
C. A. Davis
,
R. E. Carbone
, and
J. D. Tuttle

Abstract

The western and central United States experience a pronounced diurnal cycle in rainfall during the warm season. Over the higher terrain west of 105°W, most precipitation occurs in the afternoon, whereas the central United States experiences more nocturnal events. This coherent phase transition between the Rocky Mountains and the U.S. Great Plains is well defined for all warm seasons between 1996 and 2003, provided that the rainfall observations are remapped relative to the elevated terrain in the western United States prior to north–south averaging. Due to the westward shift of the Continental Divide north of 42°N and its intersection with the warm season storm track for 2002, the diurnal coherence greatly improves after remapping the 2002 rainfall observations. This speaks to the long-range influence of orography on precipitation frequency and suggests that the primary east–west corridor of precipitation for an individual warm season intersects the cordillera over a fairly narrow latitude range.

Full access
S. B. Trier
,
F. Chen
,
K. W. Manning
,
M. A. LeMone
, and
C. A. Davis

Abstract

A coupled land surface–atmospheric model that permits grid-resolved deep convection is used to examine linkages between land surface conditions, the planetary boundary layer (PBL), and precipitation during a 12-day warm-season period over the central United States. The period of study (9–21 June 2002) coincided with an extensive dry soil moisture anomaly over the western United States and adjacent high plains and wetter-than-normal soil conditions over parts of the Midwest. A range of possible atmospheric responses to soil wetness is diagnosed from a set of simulations that use land surface models (LSMs) of varying sophistication and initial land surface conditions of varying resolution and specificity to the period of study.

Results suggest that the choice of LSM [Noah or the less sophisticated simple slab soil model (SLAB)] significantly influences the diurnal cycle of near-surface potential temperature and water vapor mixing ratio. The initial soil wetness also has a major impact on these thermodynamic variables, particularly during and immediately following the most intense phase of daytime surface heating. The soil wetness influences the daytime PBL evolution through both local and upstream surface evaporation and sensible heat fluxes, and through differences in the mesoscale vertical circulation that develops in response to horizontal gradients of the latter. Resulting differences in late afternoon PBL moist static energy and stability near the PBL top are associated with differences in subsequent late afternoon and evening precipitation in locations where the initial soil wetness differs among simulations. In contrast to the initial soil wetness, soil moisture evolution has negligible effects on the mean regional-scale thermodynamic conditions and precipitation during the 12-day period.

Full access