Adam L. Houston

The tempest unmanned aircraft system for in situ observations of tornadic supercells: Design and VORTEX2 flight results

This paper reports results from field deployments of the Tempest Unmanned Aircraft System, the first of its kind of unmanned aircraft system designed to perform in situ sampling of supercell thunderstorms, including those that produce tornadoes. A description of the critical system components, consisting of the unmanned aircraft, ground support vehicles, communications network, and custom software, is given. The unique concept of operations and regulatory issues for this type of highly nomadic and dynamic system are summarized, including airspace regulatory decisions from the Federal Aviation Administration to accommodate unmanned aircraft system operations for the study of supercell thunderstorms. A review of the system performance and concept of operations effectiveness during flights conducted for the spring 2010 campaign of the VORTEX2 project is provided. These flights resulted in the first-ever sampling of the rear flank gust front and airmass associated with the rear flank downdraft of a supercell thunderstorm by an unmanned aircraft system. A summary of the lessons learned, future work, and next steps is provided.

The Collaborative Colorado–Nebraska Unmanned Aircraft System Experiment

The Collaborative Colorado–Nebraska Unmanned Aircraft System Experiment (CoCoNUE) was executed on 1 March and 30 September 2009. The principal objective of this project was to examine the feasibility of using a small unmanned aircraft operating semi-autonomously with an onboard autopilot to observe atmospheric phenomena within the terrestrial boundary layer covered by the United States National Airspace System. The application of an unmanned aircraft system (UAS; the aircraft along with the communications and logistics infrastructure required for operation) is beset by a number of engineering and regulatory challenges. This article discusses the strategies implemented to meet these challenges. Airmass boundaries served as the target of the flights conducted. These atmospheric phenomena have the fortuitous combination of an across-boundary scale that yields a coherent signal in the in situ meteorological data that can be collected by a UAS and an along-boundary scale that can be easily tracked via the exis...

The Sensitivity of Convective Initiation to the Lapse Rate of the Active Cloud-Bearing Layer

Numerical experiments are conducted using an idealized cloud-resolving model to explore the sensitivity of deep convective initiation (DCI) to the lapse rate of the active cloud-bearing layer [ACBL; the atmospheric layer above the level of free convection (LFC)]. Clouds are initiated using a new technique that involves a preexisting airmass boundary initialized such that the (unrealistic) adjustment of the model state variables to the imposed boundary is disassociated from the simulation of convection. Reference state environments used in the experiment suite have identical mixed layer values of convective inhibition, CAPE, and LFC as well as identical profiles of relative humidity and wind. Of the six simulations conducted for the experiment set, only the three environments with the largest ACBL lapse rates support DCI. The simulated deep convection is initiated from elevated sources (parcels in the convective clouds originate near 1300 m) despite the presence of a surface-based boundary. Thermal instability release is found to be more likely in the experiments with larger ACBL lapse rates because the forced ascent at the preexisting boundary is stronger (despite nearly identical boundary depths) and because the parcels’ LFCs are lower, irrespective of parcel dilution. In one experiment without deep convection, DCI failure occurs even though thermal instability is released. Results from this experiment along with the results from a heuristic Lagrangian model reveal the existence of two convective regimes dependent on the environmental lapse rate: a supercritical state capable of supporting DCI and a subcritical state that is unlikely to support DCI. Under supercritical conditions the rate of increase in buoyancy due to parcel ascent exceeds the reduction in buoyancy due to dilution. Under subcritical conditions, the rate of increase in buoyancy due to parcel ascent is outpaced by the rate of reduction in buoyancy from dilution. Overall, results demonstrate that the lapse rate of the ACBL is useful in diagnosing and/or predicting DCI.

Empirical Examination of the Factors Regulating Thunderstorm Initiation

AbstractInitiation is the part of the convective life cycle that is currently least understood and least well forecast. The inability to properly forecast the timing and/or location of deep convection initiation degrades forecast skill, especially during the warm season. To gain insight into what atmospheric parameters distinguish areas where storms initiate from areas where they do not initiate, over 55 000 thunderstorm initiation points over the central United States from 2005 to 2007 are found and a number of thermodynamic and kinematic parameters are computed from 20-km Rapid Update Cycle (RUC)-2 data. In addition to the initiation points, data are also collected at nearby locations where thunderstorms did not initiate (null points) for comparison. Thunderstorm identification and tracking are done using several tools within the Warning Decision Support Services–Integrated Information (WDSS-II) package and a thunderstorm tracking algorithm called Thunderstorm Observation by Radar (ThOR). The parameters...

The Dependence of Storm Longevity on the Pattern of Deep Convection Initiation in a Low-Shear Environment

The sensitivity of storm longevity to the pattern of deep convection initiation (e.g., multiple, quasi-linearly arranged initial deep convective cells versus an isolated deep convective cell) is examined using idealized cloud-resolving simulations conducted with a low-shear initial environment. When multiple deep convective cells are initialized in close proximity to one another using either a line of thermals or a shallow airmass boundary, long-lived storms are produced. However, when isolated deep convection is initiated, the resultant storm steadily decays following initiation. These results illustrate that a quasi-linear mechanism, such as a preexisting airmass boundary, that initiates multiple deep convective cells in close proximity can lead to longer-lived storms than a mechanism that initiates isolated deep convection. The essential difference between the experiments conducted is that an isolated initial storm produces a shallower cold pool than when a quasi-linear initiation is used. It is argued that the deep cold pools promote deep forced ascent, systematic convective cell redevelopment, and thus long-lived storms, even in environments with small values of vertical shear. The difference in cold pool depth between the simulations is attributed to differences in the horizontal flux of cold air to the gust front. With a single initial storm, the few convective cells that subsequently form provide only a limited source of cold air, leading to a cold pool that is shallow and incapable of fostering continued updraft redevelopment.

Evolution of a Long-Track Violent Tornado within a Simulated Supercell

AbstractTornadoes are among nature’s most destructive forces. The most violent, long-lived tornadoes form within supercell thunderstorms. Tornadoes ranked EF4 and EF5 on the Enhanced Fujita scale that exhibit long paths are the least common but most damaging and deadly type of tornado. In this article we describe an ultra-high-resolution (30-m gridpoint spacing) simulation of a supercell that produces a long-track tornado that exhibits instantaneous near-surface storm-relative winds reaching as high as 143 m s−1. The computational framework that enables this work is described, including the Blue Waters supercomputer, the CM1 cloud model, a data management framework built around the HDF5 scientific data format, and the VisIt and Vapor visualization tools. We find that tornadogenesis occurs in concert with processes not clearly seen in previous supercell simulations, including the consolidation of numerous vortices and vorticity patches along the storm’s forward-flank downdraft boundary and the intensificat...

The Impact of Airmass Boundaries on the Propagation of Deep Convection: A Modeling-Based Study in a High-CAPE, Low-Shear Environment

A suite of experiments conducted using a cloud-resolving model is examined to assess the role that preexistingairmassboundariescanplayinregulatingstormpropagation. The27May1997centralTexastornadic event is used to guide these experiments. The environment of this event was characterized by multiple preexisting airmass boundaries, large CAPE, and weak vertical shear. Only the experiments with preexisting airmass boundaries produce back-building storm propagation (storm motion in opposition to the mean wind). When both the cold front and dryline are present, storm maintenance occursthroughthe quasi-continuousmaintenance ofa setoflong-livedupdraftsandnotthrough discrete updraft redevelopment. Since the cold front is not required for back building, it is clear that back building in this environment does not require quasi-continuous updraft maintenance. The back-building storm simulated with both the cold front and dryline is found to be anchored to the boundary zipper (the intersection of the cold front and dryline). However,multiplepreexisting airmass boundaries are not required for back building since experiments with only a dryline also support back building. A conceptual model of back building and boundary zippering is developed that highlights the important role that preexisting boundaries can play in back-building propagation.

Sampling Severe Local Storms and Related Phenomena: Using Unmanned Aircraft Systems

Understanding and predicting the dynamic behavior of our planets environment over multiple spatial and temporal scales remains an outstanding scientific challenge [1], [2]. More than 50 years of investment and advancements in remote weather-sensing systems (satellite-based as well as ground-based radar) have resulted in remarkable capabilities; however, these systems cannot deliver observations to meet current requirements for timeliness, positional precision, and the acquisition of data that can only be obtained in situ. Highly mobile observations systems are needed to deliver in situ data that are critical for the verification and validation of current models and simulations. This is the challenge in engineering the tools of scientific discovery, one of the 14 Engineering Grand Challenges of the 21st Century posed by the National Academy of Engineering [2]. This article addresses specific challenges in designing and deploying unmanned aircraft systems (UASs) for sampling severe local storms.

An Energy-Aware Airborne Dynamic Data-Driven Application System for Persistent Sampling and Surveillance

This paper describes an energy-aware, airborne, dynamic data-driven application systems for persistent sensing in complex atmospheric conditions. The work combines i.) new onboard and remote real-time, wind sensing capabilities; ii.) online models for planning based on Gaussian processes for onboard data and dynamic atmospheric models that assimilate Doppler radar data; and iii.) a hierarchical guidance and control framework with algorithms that can adapt to environmental, sensing, and computational resources. The novel aspects of this work include real-time synthesis of multiple Doppler radar data into wind field measurements; creation of atmospheric models for online planning that can be run inside guidance loops; guidance algorithms based on stochastic dynamic programming and ordered upwind methods that can adapt planning horizons, cost function approximations, and mesh representations of the environment; and throttling algorithms that manage the adaptation of the models and guidance algorithms in response to computational resources.

Distributed Atmospheric Sensing using Small UAS and Doppler Radar

A distributed sensing system has been developed to probe an atmospheric airmass boundary with simultaneous dual-Doppler sensing and in-situ sampling using an Unmanned Vehicle System (UAS). In support of this effort, a suite of software was developed to allow for real time visualization of radar and UA information. Through this interface, controllers were able to effectively control a UA to an area of interest based upon meteorological information. An existing ad-hoc network was augmented to allow for the effective dissemination of telemetry, sensor data, and control throughout the multi-user network. Furthermore, a UA was developed that could carry the various sensors and conduct the required mission. These efforts were verified by flight operations conducted at the Pawnee National Grasslands under CoA 2008-WSA-51.