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Kenneth W. Johnson

Abstract

Development of a methodology for the optimal placement of multivariate sensors as an aid in the design of geophysical field experiments is shown. The optimal placement methodology relies on spatial correlation estimates, interpolation error estimates as provided by a multivariate optimal interpolation scheme, and optimization techniques using nonlinear programming. Atmospheric fields and their associated statistics are simulated by analytic functions to demonstrate the capabilities of the methodology. These include the ability to design new networks, to add sensors optimally to existing networks, and to place restrictions on the region in which sensors can be located by introducing physical and economical constraints on the nonlinear programming problem. It is demonstrated that the mean and variance of the interpolation error for all fields is generally smaller for analyses whose input is derived from optimal sampling locations rather than from subjectively chosen locations.

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David B. Johnson and Kenneth V. Beard

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When raindrops collide, some of the kinetic energy involved in the collision will be available to initiate or sustain oscillations in the surviving drops. This paper presents results of a simple model of drop collisions that generates an estimate of the expected distribution of energies in an ensemble of colliding raindrops as a function of drop size and rain intensity. The results indicate that drop collisions can be an effective source of raindrop oscillations and that within any one rain shaft, it tends to produce a range of oscillation energies from intense to imperceptible. In every case, however, the fraction of drops oscillating and the severity of the oscillations increase with increasing drop size and rainfall intensity.

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Ying Lin, Peter S. Ray, and Kenneth W. Johnson

Abstract

A method is developed to initialize convective storm simulations with Doppler radar-derived fields. Input fields for initialization include velocity, rainwater derived from radar reflectivity, and pressure and temperature fields obtained through thermodynamic retrieval. A procedure has been developed to fill in missing wind data, followed by a variational adjustment to the filled wind field to minimize “shocks” that would otherwise cause the simulated fields to deteriorate rapidly.

A series of experiments using data from a simulated storm establishes the feasibility of the initialization method. Multiple-Doppler radar observations from the 20 May 1977 Del City tornadic storm are used for the initialization experiments. Simulation results are shown and compared to observations taken at a later time. The simulated storm shows good agreement with the subsequent observations, though the simulated storm appears to be evolving faster than observed. Possible reasons for the discrepancies are discussed.

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Richard H. Johnson, Paul E. Ciesielski, and Kenneth A. Hart

Abstract

Soundings taken from the tropical western Pacific warm pool region during TOGA COARE reveal the common occurrence of temperature and moisture perturbations near the 0°C level. The perturbations frequently are characterized by shallow layers of increased stability (or occasionally temperature inversions) and reversals or inflections in the vertical profile of specific humidity. Similar temperature and moisture inversions have been observed elsewhere in the Tropics and midlatitudes but have not received much attention. Isothermal layers are known to exist just below the melting level in stratiform rain regions; however, not all stable layers observed over the warm pool are confined to precipitation systems. The perturbation in the specific humidity profile accounts for the often-observed double-peak structure in the apparent moisture sink Q 2 in tie Tropics.

Stratification of the data based on relative humidity criteria indicates that the stable layers near the 0°C level generally fall into two main classifications: anomalously cool–moist conditions at and slightly below the 0°C level and anomalously warm–dry conditions at and just above. The former occur primarily within or in close proximity to precipitating systems, suggesting they are a result of the direct effects of melting. Soundings in the latter group typically occur outside convective areas. Mechanisms for formation of the warm–dry stable layers are unclear at this time, but advective, radiative, gravity wave, and melting effects may all play some role. In some cases they may simply be remnant melting layers from past convection.

There is evidence to indicate that the stable layers near the 0°C level affect tropical cloud populations. Convection impinging upon or penetrating the stable layers may detrain significantly near the 0°C level, thereby contributing to perturbations in the moisture profile there. Midlevel cloud layers that are commonly observed in the Tropics may be evidence of this detrainment.

Another primary finding is the frequent occurrence of a trade wind stable layer over the warm pool. Though not widely recognized, this finding is consistent with the prevalence of trade cumulus clouds in the western Pacific region. The trade inversions often coexist with, but are distinct from, the inversions near the 0°C level.

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Kenneth W. Johnson, Peter S. Ray, Brenda C. Johnson, and Robert P. Davies-Jones

Abstract

Observations of the 20 May 1977 tornadic storms are used to evaluate recent theories on the initiation of rotation at mid-and low levels and to verify recent thermodynamic retrieval results. Using the lengthy data record from a variety of sensors available for this day, it appears that the mechanism that initiates low-level rotation is different from that at midlevels. Attempts to identify the source of the low-level rotation as vertical tilting baroclinically generated horizontal vorticity were inconclusive.

The recent thermodynamic retrieval results of Hane and Ray and of Brandes for these storms are in good agreement with independent measurements where available. However, verification is hindered by the sparseness of these measurements. Noticeable differences in the region of the rear-flank downdraft suggest that there is room for improvement in the retrieval methods.

Investigation of the cyclic generation of rotation along gust fronts indicates that the source of low-level rotation is not derived from baroclinically generated horizontal vorticity as seems to be the case with the initial mesocyclone core. Instead, vertical vorticity amplification along the gust front leading to successive generation of mesocyclone cores and discrete mesocyclone propagation is the result of the concentration of low-level preexisting vertical vorticity through convergence.

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James R. Scoggins, Joe Arellano Jr., Bernard Esposito, G. Alan Johnson, and Kenneth C. Brundidge
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William W. Kellogg, David Atlas, David S. Johnson, Richard J. Reed, and Kenneth C. Spengler
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Brian A. Colle, Rosemary Auld, Kenneth Johnson, Christine O’Connell, Temis G. Taylor, and Joshua Rice

Abstract

It is challenging to communicate uncertainty for high-impact weather events to the public and decision makers. As a result, there is an increased emphasis and training within the National Weather Service (NWS) for “impact-based decision support.” A Collaborative Science, Technology, And Research (CSTAR) project led by Stony Brook University (SBU) in collaboration with the Alan Alda Center for Communicating Science, several NWS forecast offices, and NWS operational centers held two workshops at SBU on effective forecast communication of probabilistic information for high-impact weather. Trainers in two 1.5-day workshops helped 15-20 forecasters learn to distill their messages, engage audiences, and more effectively communicate risk and uncertainty to decision makers, media, and the general public. The novel aspect of the first workshop focused on using improvisational techniques to connect with audiences along with exercises to improve communication skills using short, clear, conversational statements. The same forecasters participated in the second workshop, which focused on matching messages to intended audiences and stakeholder interaction. Using a recent high-impact weather event, representatives in emergency management, TV media, departments of transportation, and emergency services provided feedback on the forecaster oral presentations (2-3 minute) and a visual slide. This article describes our innovative workshop approach, illustrates some of the techniques used, and highlights participant feedback.

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Kenneth W. Johnson, Jeff Bauer, Gregory A. Riccardi, Kelvin K. Droegemeier, and Ming Xue

Abstract

This paper describes the parallelization of a mesoscale-cloud-scale numerical weather prediction model and experiments conducted to assess its performance. The model used is the Advanced Regional Prediction System (ARPS), a limited-area nonhydrostatic model suitable for cloud-scale and mesoscale studies. Because models such as ARPS are usually memory and CPU bound, the motivation here is to decrease the computer time required for running the model and/or increase the size of the problem that can be run. A domain decomposition strategy using a network of workstations produced a significant decrease in elapsed time and increase in problem size relative to a single-workstation run. The performance of the resulting program is described by deprived formulas (collectively known as a performance model), which predict the execution time and speedup for different numbers of processors and problem sizes. The interprocessor communication speeds are shown to be the major obstacle to achieving full processor use. The effect of faster communication networks on parallel performance is predicted based on this performance model. Parallelization experiments using the ARPS code were run on a cluster of IBM RS6000 workstations connected via Ethernet. The message-passing paradigm implemented here made use of the library of routines from the Parallel Virtual Machine software package.

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Kenneth S. Johnson, Joshua N. Plant, Stephen C. Riser, and Denis Gilbert

Abstract

Aanderaa optode sensors for dissolved oxygen show remarkable stability when deployed on profiling floats, but these sensors suffer from poor calibration because of an apparent drift during storage (storage drift). It has been suggested that measurement of oxygen in air, during the period when a profiling float is on the surface, can be used to improve sensor calibration and to determine the magnitude of sensor drift while deployed in the ocean. The effect of air calibration on oxygen measurement quality with 47 profiling floats that were equipped with Aanderaa oxygen optode sensors is assessed. Recalibrated oxygen concentration measurements were compared to Winkler oxygen titrations that were made at the float deployment stations and to the World Ocean Atlas 2009 oxygen climatology. Recalibration of the sensor using air oxygen reduces the sensor error, defined as the difference from Winkler oxygen titrations in the mixed layer near the time of deployment, by about tenfold when compared to errors obtained with the factory calibration. The relative error of recalibrated sensors is <1% in surface waters. A total of 29 floats were deployed for time periods in excess of one year in ice-free waters. Linear changes in the percent of atmospheric oxygen reported by the sensor, relative to the oxygen partial pressure expected from the NCEP air pressure, range from −0.9% to +1.3% yr−1 with a mean of 0.2% ± 0.5% yr−1. Given that storage drift for optode sensors is only negative, it is concluded that there is no evidence for sensor drift after they are deployed and that other processes are responsible for the linear changes.

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