Search Results

You are looking at 1 - 10 of 29 items for

  • Author or Editor: Robert D. Sharman x
  • Refine by Access: All Content x
Clear All Modify Search
Stanley B. Trier and Robert D. Sharman

Abstract

Geostationary Operational Environmental Satellite-14 (GOES-14) 1-km visible satellite data with 1-min frequency revealed horizontally propagating internal gravity waves emanating from tropopause-penetrating deep convection on 3–4 June 2015 during the Plains Elevated Convection at Night (PECAN) field experiment. These waves had horizontal wavelengths of ~6–8 km and approximate ground-relative phase speeds of 35 m s−1. PECAN radiosonde data are used to document the environment supporting the horizontally propagating gravity waves within the 200-km-long downstream thunderstorm anvil. Comparisons among soundings within the anvil core, at the downstream anvil edge, and outside of the anvil, together with supporting high-resolution numerical simulations, establish the importance of the storm-induced upper-tropospheric/lower-stratospheric (UTLS) outflow in providing conditions allowing vertical trapping of internal gravity waves over large horizontal distances within the mesoscale anvil. Turbulence was reported by commercial aviation in proximity to the gravity waves near the downstream anvil edge. The simulations suggest that the strongest turbulence was consistent with a mesoscale destabilization of the outer portion of the downstream anvil at elevations immediately below the outflow jet, where differential temperature advection owing to the strong associated vertical shear reduces static stability. The simulated gravity waves are trapped at this elevation and extend for several kilometers below. Local minima of moist gradient Richardson number occur immediately above the simulated warm gravity wave temperature perturbations at anvil base, suggesting a possible role these waves could play in establishing precise locations for the onset of turbulence.

Full access
Todd P. Lane and Robert D. Sharman

Abstract

Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.

Full access
Stanley B. Trier and Robert D. Sharman

Abstract

Widespread moderate turbulence was recorded on three specially equipped commercial airline flights over northern Kansas near the northern edge of the extensive cirrus anvil of a nocturnal mesoscale convective system (MCS) on 17 June 2005. A noteworthy aspect of the turbulence was its location several hundred kilometers from the active deep convection (i.e., large reflectivity) regions of the MCS. Herein, the MCS life cycle and the turbulence environment in its upper-level outflow are studied using Rapid Update Cycle (RUC) analyses and cloud-permitting simulations with the Weather Research and Forecast Model (WRF). It is demonstrated that strong vertical shear beneath the MCS outflow jet is critical to providing an environment that could support dynamic (e.g., shearing type) instabilities conducive to turbulence. Comparison of a control simulation to one in which the temperature tendency due to latent heating was eliminated indicates that strong vertical shear and corresponding reductions in the local Richardson number (Ri) to ∼0.25 at the northern edge of the anvil were almost entirely a consequence of the MCS-induced westerly outflow jet. The large vertical shear is found to decrease Ri both directly, and by contributing to reductions in static stability near the northern anvil edge through differential advection of (equivalent) potential temperature gradients, which are in turn influenced by adiabatic cooling associated with the mesoscale updraft located upstream within the anvil. On the south side of the MCS, the vertical shear associated with easterly outflow was significantly offset by environmental westerly shear, which resulted in larger Ri and less widespread model turbulent kinetic energy (TKE) than at the northern anvil edge.

Full access
Stanley B. Trier and Robert D. Sharman

Abstract

Mechanisms supporting a cold-season aviation turbulence outbreak over the northwest Atlantic Ocean and adjacent coastal regions of North America are investigated using high-resolution numerical simulations. Two distinct episodes of moderate-or-greater turbulence in the upper troposphere are observed, and the simulations suggest the turbulence is linked to eastward-translating mesoscale perturbations of negative potential vorticity (PV) emanating from upstream organized deep convection along the anticyclonic shear side of an upper-level jet. Within the exit region of the jet where the turbulence episodes occur, thermodynamic and kinematic fields in the vicinity of the PV perturbations exhibit structural characteristics of mesoscale inertia–gravity waves. These wavelike perturbations are shown to facilitate turbulence by influencing the vertical shear and static stability, which promotes mesoscale regions of banded cirrus clouds, near or within which the observed turbulence occurs.

The simulations also suggest that the turbulence arises from fundamentally different mechanisms in the two episodes. In the first and most severe turbulence episode, mesoscale wave-related vertical shear enhancements lead to Kelvin–Helmholtz instability (KHI) near aircraft cruising altitudes (~8.9–11.2 km MSL). Simulated KHI is most prevalent near relatively isolated areas of shallow, moist convection, where smaller-scale internal gravity waves originating in the middle troposphere in response to the shallow convection may play a role in excitation of the KHI located above. The second turbulence episode is consistent with simulated thermal-shear instability related to wave-induced mesoscale reductions in upper-tropospheric static stability. However, unlike for the earlier episode of enhanced turbulence, cloud-radiative feedbacks are necessary for the instability and mesoscale regions of banded cirrus to develop.

Full access
Teddie L. Keller, Richard Rotunno, Matthias Steiner, and Robert D. Sharman

Abstract

Previous studies have observed upstream-propagating modes in two-dimensional numerical simulations of idealized flow over topography with moist, nearly neutral conditions in the troposphere, topped by a stable stratosphere. The generation and propagation mechanisms for these modes were attributed to localized and dramatic changes in stability induced by the desaturation of the flow impinging on the mountain. In the present paper it is shown that these modes are transient upstream-propagating gravity waves, which are a fundamental feature of both moist and dry flow over topography of a two-layer troposphere–stratosphere atmospheric profile impulsively started from rest. The mode selection and propagation speeds of these transient waves are highly dependent on the tropospheric stability, as well as the wind speed and tropopause depth. In the moist case these modes appear to propagate according to an effective static stability that is intermediate to the normal dry stability and the lower moist stability. Comparisons with the linear, time-dependent, hydrostatic analytic solution show that these modes are similar to the transients observed in flow of a constant wind and stability layer over topography with a rigid upper boundary.

Full access
Domingo Muñoz-Esparza, Robert D. Sharman, and Julie K. Lundquist

Abstract

A better understanding and prediction of turbulence dissipation rate ε in the atmospheric boundary layer (ABL) is important for many applications. Herein, sonic anemometer data from the Experimental Planetary boundary layer Instrumentation Assessment (XPIA) field campaign (March–May 2015) are used to derive energy dissipation rate (EDR; =) within the first 300 m above the ground employing second-order structure functions. Turbulence dissipation rate is found to be strongly driven by the diurnal evolution of the ABL, presenting a distinct statistical behavior between daytime and nighttime conditions that follows log–Weibull and lognormal distributions, respectively. In addition, the vertical structure of EDR is characterized by a decrease with height above the surface, with the largest gradients occurring within the surface layer (z < 50 m). Convection-permitting mesoscale simulations were carried out with all of the 1.5-order turbulent kinetic energy (TKE) closure planetary boundary layer (PBL) schemes available in the Weather Research and Forecasting (WRF) Model. Overall, the three PBL schemes capture the observed diurnal evolution of EDR as well as the statistical behavior and vertical structure. However, the Mellor–Yamada-type schemes underestimate the large EDR levels during the bulk of daytime conditions, with the quasi-normal scale elimination (QNSE) scheme providing the best agreement with observations. During stably stratified nighttime conditions, Mellor–Yamada–Janjić (MYJ) and QNSE tend to exhibit an artificial “clipping” to their background TKE levels. A reduction in the model constant in the dissipation term for the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme did not have a noticeable impact on EDR estimates. In contrast, application of a postprocessing statistical remapping technique reduced the systematic negative bias in the MYNN results by 75%.

Open access
Domingo Muñoz-Esparza, Robert D. Sharman, and Stanley B. Trier

Abstract

Mesoscale numerical weather prediction (NWP) models are routinely exercised at kilometer-scale horizontal grid spacings (Δx). Such fine grids will usually allow at least partial resolution of small-scale gravity waves and turbulence in the upper troposphere and lower stratosphere (UTLS). However, planetary boundary layer (PBL) parameterization schemes used with these NWP model simulations typically apply explicit subgrid-scale vertical diffusion throughout the entire vertical extent of the domain, an effect that cannot be ignored. By way of an example case of observed widespread turbulence over the U.S. Great Plains, we demonstrate that the PBL scheme’s mixing in NWP model simulations of Δx = 1 km can have significant effects on the onset and characteristics of the modeled UTLS gravity waves. Qualitatively, PBL scheme diffusion is found to affect not only background conditions responsible for UTLS wave activity, but also to control the local vertical mixing that triggers or hinders the onset and propagation of these waves. Comparisons are made to a reference large-eddy simulation with Δx = 250 m to statistically quantify these effects. A significant and systematic overestimation of resolved vertical velocities, wave-scale fluxes, and kinetic energy is uncovered in the 1-km simulations, both in clear-air and in-cloud conditions. These findings are especially relevant for upper-level gravity wave and turbulence simulations using high-resolution kilometer-scale NWP models.

Open access
Domingo Muñoz-Esparza, Robert D. Sharman, and Wiebke Deierling

Abstract

We explore the use of machine learning (ML) techniques, namely, regression trees (RT), for the purpose of aviation turbulence forecasting at upper levels [20–45 kft (~6–14 km) in altitude]. In particular, we develop a series of RT-based algorithms that include random forests (RF) and gradient-boosted regression trees (GBRT) methods. Numerical weather prediction model prognostic variables and derived turbulence diagnostics based on 6-h forecasts from the 3-km High-Resolution Rapid Refresh model are used as features to train these data-driven models. Training and evaluation are based on turbulence estimates of eddy dissipation rate (EDR) obtained from automated in situ aircraft reports. Our baseline RF model, consisting of 100 trees with 30 layers of maximum depth, significantly reduces forecast errors for EDR < 0.1 m2/3 s−1 (which corresponds roughly to null and light turbulence) when compared with a simple regression model, increasing the probability of detection and in turn reducing the number of false alarms. Model complexity reduction via GBRT and feature-relevance analyses is performed, indicating that considerable execution speedups can be achieved while maintaining the model’s predictive skill. Overall, the ML models exhibit enhanced performance in discriminating the EDR forecast among the light, moderate, and severe turbulence categories. In addition, these artificial intelligence techniques significantly simplify the generation of new NWP and grid-spacing specific turbulence forecast products.

Open access
Stanley B. Trier, Robert D. Sharman, and Todd P. Lane

Abstract

The 9–10 March 2006 aviation turbulence outbreak over the central United States is examined using observations and numerical simulations. Though the turbulence occurs within a deep synoptic cyclone with widespread precipitation, comparison of reports from commercial aircraft with radar and satellite data reveals the majority of the turbulence to be in clear air. This clear-air turbulence (CAT) is located above a strong upper-level jet, where vertical shear ranged between 20 and 30 m s−1 km−1. Comparison of a moist simulation with a dry simulation reveals that simulated vertical shear and subgrid turbulence kinetic energy is significantly enhanced by the anticyclonic upper-level flow perturbation associated with the organized convection in regions of observed CAT.

A higher-resolution simulation is used to examine turbulence mechanisms in two primary clusters of reported moderate and severe turbulence. In the northern cluster where vertical shear is strongest, the simulated turbulence arises from Kelvin–Helmholtz (KH) instability. The turbulence farther south occurs several kilometers above shallow, but vigorous, moist convection. There, the simulated turbulence is influenced by vertically propagating gravity waves initiated when the convection impinges on a lowered tropopause. In some locations these gravity waves amplify and break leading directly to turbulence, while in others they aid turbulence development by helping excite KH instability within the layers of strongest vertical shear above them. Although both clusters of turbulence occur either above or laterally displaced from cloud, a shared characteristic is their owed existence to moist convection within the wintertime cyclone, which distinguishes them from traditional CAT.

Full access
Todd P. Lane, James D. Doyle, Robert D. Sharman, Melvyn A. Shapiro, and Campbell D. Watson

Abstract

Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.

Full access