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Michael J. Dickinson and David J. Knight

Abstract

A two-dimensional, hydrostatic, nearly adiabatic primitive equation model is used to study the evolution of a front passing across topography. Frontogenesis is forced by shearing deformation associated with the nonlinear evolution of an Eady wave. This study extends previous work by including an upper-level potential vorticity (PV) anomaly and a growing baroclinic wave in a baroclinically unstable basic state.

Results for the Eady wave simulations show that the mountain retards and blocks the approaching front at the surface while the upper-level PV anomaly associated with the front moves across the domain unaffected. Warm advection ahead of the lee trough forces convergence and cyclonic vorticity growth near the base of the lee slope. This vorticity growth is further encouraged by the approach of the upper-level PV anomaly. The upper-level PV anomaly then couples with this new surface vorticity center and propagates downstream. The original surface front remains trapped on the windward slope. Thus when the upstream blocking is strong, frontal propagation is discontinuous across the ridge. This evolution occurs for tall mountains and narrow mountains, as well as weak fronts. For low mountains, wide mountains, and strong fronts, only weak retardation is observed on the windward slope. The surface front remains coupled with the upper-level PV anomaly. The front moves continuously across the mountain.

The net result, regardless of mountain size and shape, is that the front reaches the base of the lee slope stronger, sooner, and with a decreased cross-front scale compared to the “no-mountain” case. Well downstream of the mountain, no position change of the surface front is observed.

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John Molinari, David Knight, Michael Dickinson, David Vollaro, and Steven Skubis

Abstract

A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean.

During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

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Roy Rasmussen, Michael Dixon, Steve Vasiloff, Frank Hage, Shelly Knight, J. Vivekanandan, and Mei Xu

Abstract

This paper describes and evaluates an algorithm for nowcasting snow water equivalent (SWE) at a point on the surface based on a real-time correlation of equivalent radar reflectivity (Z e) with snow gauge rate (S). It is shown from both theory and previous results that Z eS relationships vary significantly during a storm and from storm to storm, requiring a real-time correlation of Z e and S. A key element of the algorithm is taking into account snow drift and distance of the radar volume from the snow gauge. The algorithm was applied to a number of New York City snowstorms and was shown to have skill in nowcasting SWE out to at least 1 h when compared with persistence. The algorithm is currently being used in a real-time winter weather nowcasting system, called Weather Support to Deicing Decision Making (WSDDM), to improve decision making regarding the deicing of aircraft and runway clearing. The algorithm can also be used to provide a real-time ZS relationship for Weather Surveillance Radar-1988 Doppler (WSR-88D) if a well-shielded snow gauge is available to measure real-time SWE rate and appropriate range corrections are made.

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Thomas R. Karl, Wei-Chyung Wang, Michael E. Schlesinger, Richard W. Knight, and David Portman

Abstract

Important surface observations such as the daily maximum and minimum temperature, daily precipitation, and cloud ceilings often have localized characteristics that are difficult to reproduce with the current resolution and the physical parameterizations in state-of-the-art General Circulation climate Models (GCMs). Many of the difficulties can be partially attributed to mismatches in scale, local topography. regional geography and boundary conditions between models and surface-based observations. Here, we present a method, called climatological projection by model statistics (CPMS), to relate GCM grid-point flee-atmosphere statistics, the predictors, to these important local surface observations. The method can be viewed as a generalization of the model output statistics (MOS) and perfect prog (PP) procedures used in numerical weather prediction (NWP) models. It consists of the application of three statistical methods: 1) principle component analysis (FICA), 2) canonical correlation, and 3) inflated regression analysis. The PCA reduces the redundancy of the predictors The canonical correlation is used to develop simultaneous relationships between linear combinations of the predictors, the canonical variables, and the surface-based observations. Finally, inflated regression is used to relate the important canonical variables to each of the surface-based observed variables.

We demonstrate that even an early version of the Oregon State University two-level atmospheric GCM (with prescribed sea surface temperature) produces free-atmosphere statistics than can, when standardized using the model's internal means and variances (the MOS-like version of CPMS), closely approximate the observed local climate. When the model data are standardized by the observed free-atmosphere means and variances (the PP version of CPMS), however, the model does not reproduce the observed surface climate as well. Our results indicate that in the MOS-like version of CPMS the differences between the output of a ten-year GCM control run and the surface-based observations are often smaller than the differences between the observations of two ten-year periods. Such positive results suggest that GCMs may already contain important climatological information that can be used to infer the local climate.

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Alexander Ryzhkov, Sergey Y. Matrosov, Valery Melnikov, Dusan Zrnic, Pengfei Zhang, Qing Cao, Michael Knight, Clemens Simmer, and Silke Troemel

Abstract

A new methodology for estimating the depolarization ratio (DR) by dual-polarization radars with simultaneous transmission/reception of orthogonally polarized waves together with traditionally measured differential reflectivity Z DR, correlation coefficient ρ , and differential phase ΦDP in a single mode of operation is suggested. This depolarization ratio can serve as a proxy for circular depolarization ratio measured by radars with circular polarization. The suggested methodology implies the use of a high-power phase shifter to control the system differential phase on transmission and a special signal processing to eliminate the detrimental impact of differential phase on the estimate of DR. The feasibility of the suggested approach has been demonstrated by retrieving DR from the standard polarimetric variables and the raw in-phase I and quadrature Q components of radar signals and by implementing the scheme on a C-band radar with simultaneous transmission/reception of horizontally and vertically polarized waves. Possible practical implications of using DR include the detection of hail and the determination of its size above the melting layer, the discrimination between various habits of ice aloft, and the possible identification and quantification of riming, which is associated with the presence of supercooled cloud water. Some examples of these applications are presented.

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Simon F. B. Tett, Alexander Falk, Megan Rogers, Fiona Spuler, Calum Turner, Joshua Wainwright, Oscar Dimdore-Miles, Sam Knight, Nicolas Freychet, Michael J. Mineter, and Caroline E. R. Lehmann
Open access
David R. Perkins IV, Jennifer Vanos, Christopher Fuhrmann, Michael Allen, David Knight, Cameron C. Lee, Angela Lees, Andrew Leung, Rebekah Lucas, Hamed Mehdipoor, Sheila Tavares Nascimento, Scott Sheridan, and Jeremy Spencer
Open access
Sarah A. Tessendorf, Roelof T. Bruintjes, Courtney Weeks, James W. Wilson, Charles A. Knight, Rita D. Roberts, Justin R. Peter, Scott Collis, Peter R. Buseck, Evelyn Freney, Michael Dixon, Matthew Pocernich, Kyoko Ikeda, Duncan Axisa, Eric Nelson, Peter T. May, Harald Richter, Stuart Piketh, Roelof P. Burger, Louise Wilson, Steven T. Siems, Michael Manton, Roger C. Stone, Acacia Pepler, Don R. Collins, V. N. Bringi, M. Thurai, Lynne Turner, and David McRae

As a response to extreme water shortages in southeast Queensland, Australia, brought about by reduced rainfall and increasing population, the Queensland government decided to explore the potential for cloud seeding to enhance rainfall. The Queensland Cloud Seeding Research Program (QCSRP) was conducted in the southeast Queensland region near Brisbane during the 2008/09 wet seasons. In addition to conducting an initial exploratory, randomized (statistical) cloud seeding study, multiparameter radar measurements and in situ aircraft microphysical data were collected. This comprehensive set of observational platforms was designed to improve the physical understanding of the effects of both ambient aerosols and seeding material on precipitation formation in southeast Queensland clouds. This focus on gaining physical understanding, along with the unique combination of modern observational platforms utilized in the program, set it apart from previous cloud seeding research programs. The overarching goals of the QCSRP were to 1) determine the characteristics of local cloud systems (i.e., weather and climate), 2) document the properties of atmospheric aerosol and their microphysical effects on precipitation formation, and 3) assess the impact of cloud seeding on cloud microphysical and dynamical processes to enhance rainfall. During the course of the program, it became clear that there is great variability in the natural cloud systems in the southeast Queensland region, and understanding that variability would be necessary before any conclusions could be made regarding the impact of cloud seeding. This article presents research highlights and progress toward achieving the goals of the program, along with the challenges associated with conducting cloud seeding research experiments

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Melanie Wetzel, David Dempsey, Sandra Nilsson, Mohan Ramamurthy, Steve Koch, Jennie Moody, David Knight, Charles Murphy, David Fulker, Mary Marlino, Michael Morgan, Doug Yarger, Dan Vietor, and Greg Cox

An education-oriented workshop for college faculty in the atmospheric and related sciences was held in Boulder, Colorado, during June 1997 by three programs of the University Corporation for Atmospheric Research. The objective of this workshop was to provide faculty with hands-on training in the use of Web-based instructional methods for specific application to the teaching of satellite remote sensing in their subject areas. More than 150 faculty and associated scientists participated, and postworkshop evaluation showed it to have been a very successful integration of information and activities related to computer-based instruction, educational principles, and scientific lectures.

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Robert M. Rauber, Bjorn Stevens, Jennifer Davison, Sabine Goke, Olga L. Mayol-Bracero, David Rogers, Paquita Zuidema, Harry T. Ochs III, Charles Knight, Jorgen Jensen, Sarah Bereznicki, Simona Bordoni, Humberto Caro-Gautier, Marilé Colón-Robles, Maylissa Deliz, Shaunna Donaher, Virendra Ghate, Ela Grzeszczak, Colleen Henry, Anne Marie Hertel, Ieng Jo, Michael Kruk, Jason Lowenstein, Judith Malley, Brian Medeiros, Yarilis Méndez-Lopez, Subhashree Mishra, Flavia Morales-García, Louise A. Nuijens, Dennis O'Donnell, Diana L. Ortiz-Montalvo, Kristen Rasmussen, Erin Riepe, Sarah Scalia, Efthymios Serpetzoglou, Haiwei Shen, Michael Siedsma, Jennifer Small, Eric Snodgrass, Panu Trivej, and Jonathan Zawislak

The Rain in Cumulus over the Ocean (RICO) field campaign carried out a wide array of educational activities, including a major first in a field project—a complete mission, including research flights, planned and executed entirely by students. This article describes the educational opportunities provided to the 24 graduate and 9 undergraduate students who participated in RICO.

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