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John E. Simpson

Abstract

Simple mathematical theory suggests that the direction of the sea breeze must veer (move in an anticyclonic sense) during the day, until it is blowing parallel to the coast. Many observations of sea-breeze hodographs have shown that this rotation often does not occur and may sometimes be in the opposite (cyclonic) direction. Papers by Neumann and coworkers showed that either anticyclonic or cyclonic rotation could result from suitable combinations of Coriolis force and pressure gradient. Some observed examples of such cyclonic rotation are explained in the shift of direction from a “local sea breeze” to a “continental sea breeze” of a different direction; the strong pressure gradients associated with mountains can also produce cyclonic rotation. A study of the measured pressure gradient and wind velocities from southern England are shown to agree with the analysis of Neumann.

It is concluded that topographical influences can often be responsible for cyclonic, rather than the expected anticyclonic, rotation.

Full access
Brian D. Scannell
,
Tom P. Rippeth
,
John H. Simpson
,
Jeff A. Polton
, and
Joanne E. Hopkins

Abstract

The combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estimates of ε will be biased by the vertical gradient in wave orbital velocities. To remove this bias, a modified structure function methodology is developed that exploits the differing length scale dependencies of the contributions to the structure function resulting from turbulent and wave orbital motions. The success of the modified method is demonstrated through a comparison of ε estimates based on data from instruments at three depths over a 3-month period under a wide range of conditions, with appropriate scalings for wind stress and convective forcing.

Open access
Tom P. Rippeth
,
John H. Simpson
,
Eirwen Williams
, and
Mark E. Inall

Abstract

Simultaneous measurements of the rates of turbulent kinetic energy (TKE) dissipation (ε) and production (P) have been made over a period of 24 h at a tidally energetic site in the northern Irish Sea in water of 25-m depth. Some ε profiles from ∼5 m below the surface to 15 cm above the seabed were obtained using a fast light yo-yo (FLY) microstructure profiler, while P profiles were determined from a bottom-mounted high-frequency acoustic Doppler current profiler (ADCP) using the variance method. In homogeneous flow of the kind observed, the turbulence regime should approximate to local equilibrium so that, with no buoyancy forces involved, ε and P are expected to covary with mean values that are equal. The results show a close tracking of ε and P for most of the observational period. For the second tidal cycle, when there was no significant surface wave activity, a mean ratio of ε/P ≃ 0.63 ± 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the ε measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum ε is interpreted using a simple model in terms of the decay rate of TKE. Consideration of inherent instrument noise has enabled an estimate of the lowest P threshold measurable using the variance technique. For the chosen averaging parameters a value of P min ∼ 7 × 10−5 W m−3 is estimated. Two other significant differences between the two sets of measurements are attributed to errors in the stress estimate. The first is a bias in the estimate of stress resulting from a combination of instrument tilt (1°–3.5°) and surface wave activity. The second are anomalously high stress estimates, covering nearly one-half of the water column at times, which are thought to be due to instrument noise associated with the large wave orbital velocities.

Full access
Anne M. Thompson
,
Wei-Kuo Tao
,
Kenneth E. Pickering
,
John R. Scala
, and
Joanne Simpson

Theoretical studies, aircraft, and space-borne measurements show that deep convection can be an effective conduit for introducing reactive surface pollutants into the free troposphere. The chemical consequences of convective systems are complex. For example, sensitivity studies show potential for both enhancement and diminution of ozone formation. Field observations of cloud and mesoscale phenomena have been investigated with the Goddard Cumulus Ensemble and Tropospheric Chemistry models. Case studies from the tropical ABLE 2, STEP, and TRACE-A experiments show that free tropospheric ozone formation should increase when deep convection and urban or biomass burning pollution coincide, and decrease slightly in regions relatively free of ozone precursors (often marine). Confirmation of post-convective ozone enhancement in the free troposphere over Brazil, the Atlantic, and southern Africa was a major accomplishment of the September–October 1992 TRACE-A (Transport and Atmospheric Chemistry near the Equator—Atlantic) aircraft mission. A flight dedicated to cloud outflow showed that deep convection led to a factor of 3–4 increase in upper tropospheric ozone formation downwind. Analysis of ozonesondes during TRACE-A was consistent with 20%–30% of seasonally enhanced ozone over the South Atlantic being supplied by a combination of biomass burning emissions, lightning, and deep convection over South America. With the Tropics the critical region for troposphere-to-stratosphere transfer of pollutants, these results have implications for the total ozone budget. Cloud-scale analyses will guide the development of more realistic regional and global chemical-transport models to assess the full impact of deep convection on atmospheric chemical composition.

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A. Gannet Hallar
,
Steven S. Brown
,
Erik Crosman
,
Kelley C. Barsanti
,
Christopher D. Cappa
,
Ian Faloona
,
Jerome Fast
,
Heather A. Holmes
,
John Horel
,
John Lin
,
Ann Middlebrook
,
Logan Mitchell
,
Jennifer Murphy
,
Caroline C. Womack
,
Viney Aneja
,
Munkhbayar Baasandorj
,
Roya Bahreini
,
Robert Banta
,
Casey Bray
,
Alan Brewer
,
Dana Caulton
,
Joost de Gouw
,
Stephan F.J. De Wekker
,
Delphine K. Farmer
,
Cassandra J. Gaston
,
Sebastian Hoch
,
Francesca Hopkins
,
Nakul N. Karle
,
James T. Kelly
,
Kerry Kelly
,
Neil Lareau
,
Keding Lu
,
Roy L. Mauldin III
,
Derek V. Mallia
,
Randal Martin
,
Daniel L. Mendoza
,
Holly J. Oldroyd
,
Yelena Pichugina
,
Kerri A. Pratt
,
Pablo E. Saide
,
Philip J. Silva
,
William Simpson
,
Britton B. Stephens
,
Jochen Stutz
, and
Amy Sullivan

Abstract

Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical–meteorological interactions that drive high-pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in western U.S. basins. Approximately 120 people participated, representing 50 institutions and five countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological–chemical linkages outlined here, nor to validate complex processes within coupled atmosphere–chemistry models.

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