Stanley B. Trier

Inferences of Predictability Associated with Warm Season Precipitation Episodes

Herein preliminary findings are reported from a radar-based climatology of warm season precipitation ‘‘episodes.’’ Episodes are defined as time‐space clusters of heavy precipitation that often result from sequences of organized convection such as squall lines, mesoscale convective systems, and mesoscale convective complexes. Episodes exhibit coherent rainfall patterns, characteristic of propagating events, under a broad range of atmospheric conditions. Such rainfall patterns are most frequent under ‘‘weakly forced’’ conditions in midsummer. The longevity of episodes, up to 60 h, suggests an intrinsic predictability of warm season rainfall that significantly exceeds the lifetime of individual convective systems. Episodes are initiated primarily in response to diurnal and semidiurnal forcings. Diurnal forcing is dominant near the Rocky and Appalachian Mountains, whereas semidiurnal forcing is dominant between these cordilleras. A most common longitude of origin is at or near the east slope of the Continental Divide (1058W). These observations are consistent with a condition of continual thermal forcing, widespread hydrodynamic instability, and the existence of other processes that routinely excite, maintain, and regenerate organized convection. The propagation speed of major episodes is often in excess of rates that are easily attributable either to the phase speeds of large-scale forcing or to advection from low- to midlevel ‘‘steering’’ winds. It is speculated that wavelike mechanisms, in the free troposphere and/or the planetary boundary layer, may contribute to the rates of motion observed. Once understood, the representation of such mechanisms in forecast models offers the opportunity for improved predictions of warm season rainfall.

The Role of Environmental Shear and Thermodynamic Conditions in Determining the Structure and Evolution of Mesoscale Convective Systems during TOGA COARE

Abstract A collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5–6 km and primarily from radiosonde data at higher altitudes. The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low-level shear at a speed close to the wind maximum; when there is appreciable shear at middle levels (800–400 mb), convective bands form parallel to the shear. With...

Description and Evaluation of the Characteristics of the NCAR High-Resolution Land Data Assimilation System

Abstract This paper describes important characteristics of an uncoupled high-resolution land data assimilation system (HRLDAS) and presents a systematic evaluation of 18-month-long HRLDAS numerical experiments, conducted in two nested domains (with 12- and 4-km grid spacing) for the period from 1 January 2001 to 30 June 2002, in the context of the International H2O Project (IHOP_2002). HRLDAS was developed at the National Center for Atmospheric Research (NCAR) to initialize land-state variables of the coupled Weather Research and Forecasting (WRF)–land surface model (LSM) for high-resolution applications. Both uncoupled HRDLAS and coupled WRF are executed on the same grid, sharing the same LSM, land use, soil texture, terrain height, time-varying vegetation fields, and LSM parameters to ensure the same soil moisture climatological description between the two modeling systems so that HRLDAS soil state variables can be used to initialize WRF–LSM without conversion and interpolation. If HRLDAS is initialized...

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

A study of convection initiation in a mesoscale model using high-resolution land surface initial conditions

Abstract A coupled convection-resolving mesoscale atmosphere–land surface model (LSM) is used to investigate land surface–planetary boundary layer (PBL) interactions responsible for the initiation of deep, moist convection over the southern Great Plains of the United States on 19 June 1998. A high-resolution land data assimilation system provides initial conditions to the LSM, facilitating examination of soil moisture effects on forecasts of deep convection. During the late morning and early afternoon, the southwestern portion of a simulated southwest–northeast (SW–NE)-oriented surface water vapor gradient zone evolves into an intense dryline, unlike the northeastern portion, which remains relatively weak. Despite these regional differences, midafternoon convection initiation occurs within a ∼100-km-wide region of enhanced PBL depth along much of the SW–NE extent of the water vapor gradient zone. The afternoon PBL depth maximum results from a midmorning-to-early afternoon surface sensible heat flux maximu...

Mechanisms Supporting Long-Lived Episodes of Propagating Nocturnal Convection within a 7-Day WRF Model Simulation

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 convect...

Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Aircraft Observations of Precipitation, Circulation, and Surface Energy Fluxes

Abstract This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW–SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a “rear-inflow” that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with...

Structure and evolution of the 22 February 1993 TOGA COARE squall line : Numerical simulations

Abstract In this study a numerical cloud model is used to simulate the three-dimensional evolution of an oceanic tropical squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and investigate the impact of small-scale physical processes including surface fluxes and ice microphysics on its structure and evolution. The observed squall line was oriented perpendicular to a moderately strong low-level jet. Salient features that are replicated by the model include an upshear-tilted leading convective region with multiple updraft maxima during its linear stage and the development of a 30-km scale midlevel vortex and associated transition of the line to a pronounced bow-shaped structure. In this modeling approach, only surface flukes and stresses that differ from those of the undisturbed environment are included. This precludes an unrealistically large modification to the idealized quasi-steady base state and thus allows us to more easily isolate effects of ...

Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Organization Mechanisms Inferred from Numerical Simulation

Abstract Mechanisms responsible for meso- and convective-scale organization within a large tropical squall line that occurred on 22 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are investigated using a three-dimensional numerical cloud model. The squall line occurred in an environment typical of fast-moving tropical squall lines, characterized by moderate convective available potential energy and moderate-to-strong vertical shear beneath a low-level jet with weak reverse vertical shear above. A well-simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi-linear to a three-dimensional bow-shaped structure over a 2-h period. This transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40–60-km long bow-shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge, within enha...

Recent Advances in the Understanding of Near-Cloud Turbulence

Anyone who has flown in a commercial aircraft is familiar with turbulence. Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using examples, this article demonstrates how the...