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Gordon Mertz and Mohammed I. El-sabh

Abstract

The Gaspé Current is a buoyant jet driven primarily by the freshwater discharge from the St. Lawrence estuary. Previously, many incidents of unstable wave development in the3 Gaspé Current have been reported under summer, high runoff, conditions. Here we present satellite infrared images and discuss the occurrence and evolution of an autumn episode of current instability; in the fall, runoff is lower. This event is notable in that the pattern observed differs considerably from that of summer events, apparently due to the offshore shift that of the core of the jet in early autumn. A stability analysis suggests that the Gaspé Current instabilities may grow slightly faster in autumn conditions. The possible role of wind forcing in triggering wave growth is discussed.

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S. R. Pal, A. I. Carswell, I. Gordon, and A. Fong

Abstract

This paper presents the Statistical properties of lidar-derived values of cloud extinction coefficients σ and optical depths τ. The data were collected at Toronto during two measurement phases (phase 1: September–October 1989; phase 2: June–July 1991) or the Experimental Cloud Lidar Pilot Study. Although the small dataset limits general application of the statistical trends observed, the measurements demonstrate the valuable potential of lidar data for improving cloud parameterization in general circulation models. The measurements show the frequent occurrence of optically thin clouds (σ¯≤0.2 km−1 and τ≤0.2), demonstrating the ability of lidars to detect these dilute clouds and the importance of including them in radiative transfer models.

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In-Young Yeo, Steven I. Gordon, and Jean-Michel Guldmann

Abstract

The goal of this study is to develop and apply a methodology for delineating optimal land-use patterns that minimize peak runoff flow at watershed outlets by coupling a hydrological model and a land-use model. Under the assumption supported in prior research that nonpoint source (NPS) pollution is positively correlated with surface runoff volume, the model then yields land-use patterns that minimize nonpoint source pollution. A hydrological simulation model is developed with a modified and spatially explicit Soil Conservation Service (SCS) curve number method to analyze the geographical impacts of land uses. An optimization algorithm is integrated with the simulation model to evaluate different land-use patterns and their response to rainfall runoff events, and to search for optimal land-use patterns. This approach, applied to the southwestern basin of Lake Erie, Old Woman Creek Watershed (Ohio), yields optimal land-use patterns that reduce the peak runoff rate by 15%–20% under 1-, 2-, 5-, and 10-yr storms, compared to the current land-use pattern. The model results provide site-specific land-use guidelines and identify critical areas for conservation.

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Benjamin I. Cook, Gordon B. Bonan, and Samuel Levis

Abstract

The effects of increased soil moisture on wet season (October–March) precipitation in southern Africa are investigated using the Community Climate System Model version 3 (CCSM3). In the CTRL case, soil moisture is allowed to interact dynamically with the atmosphere. In the MOIST case, soil moisture is set so that evapotranspiration is not limited by the supply of water. The MOIST scenario actually results in decreased precipitation over the region of perturbed soil moisture, compared to CTRL. The increased soil moisture alters the surface energy balance, resulting in a shift from sensible to latent heating. This manifests in two ways relevant for precipitation processes. First, the shift from sensible to latent heating cools the surface, causing a higher surface pressure, a reduced boundary layer height, and an increased vertical gradient in equivalent potential temperature. These changes are indicative of an increase in atmospheric stability, inhibiting vertical movement of air parcels and decreasing the ability of precipitation to form. Second, the surface changes induce anomalous surface divergence and increased subsidence. This causes a reduction in cloud cover and specific humidity above 700 hPa and results in a net decrease of column-integrated precipitable water, despite the increased surface water flux, indicating a reduction in moisture convergence. Based on this and a previous study, soil moisture may act as a negative feedback to precipitation in southern Africa, helping to buffer the system against any external forcing of precipitation (e.g., ENSO).

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Irina I. Rypina, Lawrence J. Pratt, Julie Pullen, Julia Levin, and Arnold L. Gordon

Abstract

Techniques from dynamical systems theory have been applied to study horizontal stirring of fluid in the Philippine Archipelago. The authors’ analysis is based on velocity fields produced by two high-resolution (3 and 6 km) numerical models. Particular attention is paid to identifying robust surface flow patterns and associating them with dominant Lagrangian coherent structures (LCSs). A recurrent wind-driven dipole in the lee of the coastline is considered in detail. The associated LCSs form a template for stirring, exchange, and biological transport in and around the dipole. Chaotic advection is argued to provide a relevant framework for interpreting mesoscale horizontal stirring processes in an archipelago as a whole. Implications for the formation of filaments, the production of tracer variance, and the scale at which stirring leads to mixing are discussed in connection with an observed temperature record.

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Jan-Erik Tesdal, Ryan P. Abernathey, Joaquim I. Goes, Arnold L. Gordon, and Thomas W. N. Haine

Abstract

Examination of a range of salinity products collectively suggests widespread freshening of the North Atlantic from the mid-2000s to the present. Monthly salinity fields reveal negative trends that differ in magnitude and significance between western and eastern regions of the North Atlantic. These differences can be attributed to the large negative interannual excursions in salinity in the western subpolar gyre and the Labrador Sea, which are not apparent in the central or eastern subpolar gyre. This study demonstrates that temporal trends in salinity in the northwest (including the Labrador Sea) are subject to mechanisms that are distinct from those responsible for the salinity trends in the central and eastern North Atlantic. In the western subpolar gyre a negative correlation between near-surface salinity and the circulation strength of the subpolar gyre suggests that negative salinity anomalies are connected to an intensification of the subpolar gyre, which is causing increased flux of freshwater from the East Greenland Current and subsequent transport into the Labrador Sea during the melting season. Analyses of sea surface wind fields suggest that the strength of the subpolar gyre is linked to the North Atlantic Oscillation– and Arctic Oscillation–driven changes in wind stress curl in the eastern subpolar gyre. If this trend of decreasing salinity continues, it has the potential to enhance water column stratification, reduce vertical fluxes of nutrients, and cause a decline in biological production and carbon export in the North Atlantic Ocean.

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I. Gultepe, G. Pearson, J. A. Milbrandt, B. Hansen, S. Platnick, P. Taylor, M. Gordon, J. P. Oakley, and S. G. Cober

The main purpose of this work is to describe a major field project on fog and summarize the preliminary results. Three field phases of the Fog Remote Sensing and Modeling (FRAM) project were conducted over the following two regions of Canada: 1) the Center for Atmospheric Research Experiments (CARE), in Toronto, Ontario (FRAM-C), during the winter of 2005/06, and 2) Lunenburg, Nova Scotia (FRAM-L), during June 2006 and June 2007. Fog conditions observed during FRAM-C were continental in nature, while those conditions observed during FRAM-L were of marine origin. The main objectives of the project were to attain 1) a better description of fog environments, 2) the development of microphysical parameterizations for model applications, 3) the development of remote sensing methods for fog nowcasting/forecasting, 4) an understanding of issues related to instrument capabilities and improvement of the analysis, and 5) an integration of model data with observations to predict and detect fog areas and particle phase. During the project phases, various measurements at the surface, including droplet and aerosol spectra, ice crystal number concentration, visibility, 3D turbulent wind components, radiative fluxes, precipitation, liquid water content profiles, and cloud ceiling, were collected together with satellite measurements. These observations will be studied to better forecast/nowcast fog events in association with results obtained from numerical forecast models. It is suggested that improved scientific understanding of fog will lead to better forecasting/nowcasting skills, benefiting the aviation, land transportation, and shipping communities.

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Pedro L. Fernández-Cabán, A. Addison Alford, Martin J. Bell, Michael I. Biggerstaff, Gordon D. Carrie, Brian Hirth, Karen Kosiba, Brian M. Phillips, John L. Schroeder, Sean M. Waugh, Eric Williford, Joshua Wurman, and Forrest J. Masters

Abstract

While Hurricane Harvey will best be remembered for record rainfall that led to widespread flooding in southeastern Texas and western Louisiana, the storm also produced some of the most extreme wind speeds ever to be captured by an adaptive mesonet at landfall. This paper describes the unique tools and the strategy used by the Digital Hurricane Consortium (DHC), an ad hoc group of atmospheric scientists and wind engineers, to intercept and collect high-resolution measurements of Harvey’s inner core and eyewall as it passed over Aransas Bay into mainland Texas. The DHC successfully deployed more than 25 observational assets, leading to an unprecedented view of the boundary layer and winds aloft in the eyewall of a major hurricane at landfall. Analysis of anemometric measurements and mobile radar data during heavy convection shows the kinematic structure of the hurricane at landfall and the suspected influence of circulations aloft on surface winds and extreme surface gusts. Evidence of mesoscale vortices in the interior of the eyewall is also presented. Finally, the paper reports on an atmospheric sounding in the inner eyewall that produced an exceptionally large and potentially record value of precipitable water content for observed soundings in the continental United States.

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Lennart Bengtsson, Phil Arkin, Paul Berrisford, Philippe Bougeault, Chris K. Folland, Chris Gordon, Keith Haines, Kevin I. Hodges, Phil Jones, Per Kallberg, Nick Rayner, Adrian J. Simmons, Detlef Stammer, Peter W. Thorne, Sakari Uppala, and Russell S. Vose
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Adrian M. Tompkins, María Inés Ortiz De Zárate, Ramiro I. Saurral, Carolina Vera, Celeste Saulo, William J. Merryfield, Michael Sigmond, Woo-Sung Lee, Johanna Baehr, Alain Braun, Amy Butler, Michel Déqué, Francisco J. Doblas-Reyes, Margaret Gordon, Adam A. Scaife, Yukiko Imada, Masayoshi Ishii, Tomoaki Ose, Ben Kirtman, Arun Kumar, Wolfgang A. Müller, Anna Pirani, Tim Stockdale, Michel Rixen, and Tamaki Yasuda
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