Search Results

You are looking at 1 - 10 of 35 items for

  • Author or Editor: Scott Stephens x
  • Refine by Access: All Content x
Clear All Modify Search
Scott W. Tinis
and
Stephen Pond

Abstract

The energy budget of a tidally active, shallow silled fjord is discussed. Constriction of the flow over the shallow sill causes a reduction in tidal amplitude and a phase lag across the sill. A generalized expression for the total power extracted from the barotropic tide by dissipation at the sill is derived as a function of the tidal amplitude difference and phase lag of the tidal elevation. Using tide gauge data from both sides of the sill at the entrance to Sechelt Inlet, British Columbia, this generalized expression yields estimates for the energy flux of the barotropic tide, which approach 100 MW during spring tides. From direct measurements of the currents, the estimated frictional dissipation is equal to the flux out of the barotropic tide (within experimental error). A small amount of the energy flux (∼5%) is estimated to go into the generation of a tidal jet, which dissipates within a few kilometers of the sill and contributes to the formation of a mid-water layer.

Full access
Graeme L. Stephens
and
John C. Scott

Abstract

The overall design of a radiometer and data acquisition system for use on an aircraft platform is described. The instrument is a fast response spectrally scanning radiometer providing measurements at 48 spectral intervals between 400 and 1200 nm. The radiometer has a narrow field of view in the nadir of approximately 7 mrad and a hemispheric field of view in the zenith. This paper describes the optical, mechanical and electronic design of the instrument and presents some early results from data gathered using the instrument on board the CSIRO F27 aircraft to illustrate its performance.

Full access
Scott J. Weaver
,
Stephen Baxter
, and
Arun Kumar

Abstract

Variability of springtime tornadic activity over the United States is assessed through the connectivity of preferred modes of North American low-level jet (NALLJ) variability to the local thermodynamic environment and remote SST variations. The link between regional tornado activity and NALLJ variability as diagnosed from a consistent reanalysis system (i.e., NCEP–NCAR) serves as dynamical corroboration in light of the inhomogeneous tornado database. The analysis reveals a multidecadal variation in the strength of the NALLJ–tornado connection, highlighted by tornado activity in the southern Great Plains region nearly doubling its correlation with NALLJ principal component 1 (PC 1) in recent decades. Locally, this is a result of a southward shift of NALLJ variability modes during the recent period. Motivated by these epochal shifts in NALLJ activity, a comparison of the early (1950–78) and late (1979–2010) tornado and NALLJ SST linkages indicates an Atlantic decadal SST variability influence during the early epoch, with Pacific decadal variability thereafter, highlighting the remote SST influence on the shifts in geographic placement and strength of NALLJ variability. The remote SST variability linkages further reveal that the observed global-scale SST trend pattern over the last 61 years may be contributing to a shift toward weaker tornadoes during spring in the northern Great Plains region. Tornado activity over the southeast region of the United States shows no such relationship to the SST trend pattern during spring, an immunity that is unexpected if spurious trends in the tornado database were influencing the SST linkage.

Full access
Scott J. Weaver
,
Stephen Baxter
, and
Kirstin Harnos

Abstract

Intensification of regional springtime precipitation variability over the United States and the role of North American low-level jets (NALLJs) are investigated for the 1950–2010 period. The analysis reveals that the primary modes of NALLJ fluctuations are related to the strengthening of AMJ precipitation variability over the northern Great Plains and southeastern United States during the last 60 years. Examination of the epochal change in NALLJ variations shows a stronger connectivity to SST variability during 1980–2010 than in the 1950–79 period. In the context of the first three NALLJ variability modes it appears that the role of decadal SST variations (NALLJ mode 1) and the recent emergence of tropical Pacific connectivity (NALLJ modes 1 and 2) via SST-induced atmospheric heating and large-scale circulation changes may act to strengthen and spatially shift the NALLJ variability modes southward and/or eastward, intensifying regional precipitation variability in the recent epoch. Although notable NALLJ variability also exists in the earlier epoch, the upper-level height field is significantly lacking in meridional gradients, leading to weak upper-level zonal wind anomalies over the United States and diminished NALLJ variability. Conversely, the intensified and spatially shifted upper-level height anomaly in the recent epoch produces enhanced meridional height gradients in all three modes, strengthening NALLJ variability—highlighting that seemingly subtle shifts in hemispheric-scale atmospheric circulation changes can have important impacts on regional climate variability and change.

Full access
Stephen Baxter
,
Scott Weaver
,
Jon Gottschalck
, and
Yan Xue

Abstract

Lagged pentad composites of surface air temperature and precipitation are analyzed for the winter season (December–February) to assess the influence of the Madden–Julian oscillation (MJO) on the climate of the contiguous United States. Composites are based on the Wheeler and Hendon MJO index as well as an index developed and maintained at NOAA’s Climate Prediction Center (CPC), which is based on extended empirical orthogonal function analysis of upper-level velocity potential. Significant positive temperature anomalies develop in the eastern United States 5–20 days following Wheeler and Hendon MJO index phase 3, which corresponds to enhanced convection centered over the eastern Indian Ocean. At the same lag, positive precipitation anomalies are observed from the southern Plains to the Great Lakes region. Negative temperature anomalies appear in the central and eastern United States 10–20 days following Wheeler and Hendon MJO phase 7. These impacts are supported by an analysis of the evolution of 200-hPa geopotential height and zonal wind anomalies. Composites based on the CPC velocity potential MJO index generally yield similar results; however, they capture more cases since the index contains both interannual and subseasonal variability. There are some cases where the CPC index differs from that of WH in both MJO phase identification and its North American impacts, especially near the West Coast. This analysis suggests that MJO-related velocity potential anomalies can be used without the Wheeler and Hendon MJO index to predict MJO impacts.

Full access
Ziad S. Haddad
,
Eastwood Im
,
Stephen L. Durden
, and
Scott Hensley

Abstract

This paper describes a computationally efficient nearly optimal Bayesian algorithm to estimate rain (and drop size distribution) profiles, given a radar reflectivity profile at a single attenuating wavelength. In addition to estimating the averages of all the mutually ambiguous combinations of rain parameters that can produce the data observed, the approach also calculates the rms uncertainty in its estimates (this uncertainty thus quantifies the “amount of ambiguity” in the “solution”). The paper also describes a more general approach that can make estimates based on a radar reflectivity profile together with an approximate measurement of the path-integrated attenuation, or a radar reflectivity profile and a set of passive microwave brightness temperatures. This more general “combined” algorithm is currently being adapted for the Tropical Rainfall Measuring Mission.

Full access
Scott A. Stephens
,
Robert G. Bell
,
Douglas Ramsay
, and
Nigel Goodhue

Abstract

A technique to produce high-water alerts from coinciding high astronomical tide and high mean sea level anomaly is demonstrated for the Pacific Islands region. Low-lying coastal margins are vulnerable to episodic inundation that often coincides with times of higher-than-normal high tides. Prior knowledge of the dates of the highest tides can assist with efforts to minimize the impacts of increased exposure to inundation. It is shown that the climate-driven mean sea level anomaly is an important component of total sea level elevation in the Pacific Islands region, which should be accounted for in medium-term (1–7 months) sea level forecasts. An empirical technique is applied to develop a mean sea level–adjusted high-water alert calendar that accounts for both sea level components and provides a practical tool to assist with coastal inundation hazard planning and management.

Full access
Robert W. Scott
,
Edward C. Krug
, and
Stephen L. Burch

Abstract

Long-term data collection of volumetric soil moisture under sod has been conducted in Illinois for more than 25 years. Numerous applied and modeling studies have been undertaken with these data, often relating results to regional conditions under a variety of surface covers. However, the actual level of representation of these data to nearby areas with different surface covers is unknown. In 2006/07, the Soil Moisture under Sod Experiment was conducted at Bondville, Illinois, to increase understanding of soil moisture variability across a very small area of seemingly uniform surface and near-surface conditions. Ten locations were chosen at random within a 5.9-ha sodded field for twice weekly neutron probe soil moisture observations over a period of more than 13 months. Measurements were taken at the surface and at 20-cm intervals down to 2 m, precisely matching the historic Illinois depth observations. A detailed surface terrain analysis was conducted to consider effects on soil moisture attributable to surface slope or ponding potential at each monitoring location across the very low relief surface. The near-surface water table level at the field location was monitored. At the end of observations, soil property heterogeneity (i.e., soil porosity, bulk density, and soil color) was determined by digging trenches and extracting soil cores immediately adjacent to each monitoring site at all observation levels within the predominantly loess soil.

Results indicate a strong temporal consistency in intrasite trends of volumetric soil moisture at all depths throughout the experiment. However, intersite spatial variability increased with depth, indicated by an average standard deviation of all temporal observations of 2.26% in the top 30 cm of soil and 5.19% in the 170–200-cm layer. Differences between the average field soil moisture at all primary randomly selected sites and the historic Bondville site were 2.39% and 6.51%, respectively. In addition, an apparent strong relationship was observed between soil moisture in deeper layers and surface terrain slope, and to a lesser extent with soil porosity and bulk density.

The question of representativeness of soil moisture under sod to adjacent surface covers was not answered with this work, but the large differences measured across this seemingly uniform field suggest that proper use of the historic Illinois dataset by future research related to adjacent areas may need greater attention. Most of Illinois is under an agricultural cover, not sod. Adequate data monitoring of surface terrain slope, soil profiles, and water table climatology under various major surface covers within a region may be necessary prior to the installation of new soil moisture monitoring networks and before useful assumptions concerning spatial representation can be made that attribute individual soil moisture datasets to adjacent areas. These results highlight the importance of a strict globally unified protocol for soil moisture network design and data collection in support of quality in situ global soil moisture assessment, a primary goal of the International Soil Moisture Working Group of the Global Energy and Water Cycle Experiment.

Full access
Arunchandra S. Chandra
,
Pavlos Kollias
,
Scott E. Giangrande
, and
Stephen A. Klein

Abstract

A long-term study of the turbulent structure of the convective boundary layer (CBL) at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility is presented. Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. A conditional sampling method is applied to the original Doppler velocity dataset to extract coherent vertical velocity structures and to examine plume dimension and contribution to the turbulent transport. Overall, the derived turbulent statistics are consistent with previous aircraft and lidar observations. The observations provide unique insight into the daytime evolution of the convective boundary layer and the role of increased cloudiness in the turbulent budget of the subcloud layer. Coherent structures (plumes–thermals) are found to be responsible for more than 80% of the total turbulent transport resolved by the cloud radar system. The extended dataset is suitable for evaluating boundary layer parameterizations and testing large-eddy simulations (LESs) for a variety of surface and cloud conditions.

Full access
C. M. R. Platt
,
A. C. Dilley
,
J. C. Scott
,
I. J. Barton
, and
G. L. Stephens

Abstract

The infrared properties and structures of some anvils emanating from local thunderstorms were studied by lidar and infrared radiometry at Darwin, tropical Northern Australia. The anvils were typically from 1 to 2 km deep, at altitudes from 7 to 16 km and at temperatures from −15 to −70°C. There was a rough dependence of infrared emittance on temperature, but there was also a dependence on the age of the anvil. The average altitude and calculated wide-band greybody flux emittance were 11 km and 0.65 respectively.

One dense cloud appeared “superblack” when observed from below, due to reflection of upwelling warm radiation from the surface. The magnitude of the effect agreed within experimental error with that predicted from computations on a model cloud of ice cylinders, but was about twice that computed for a model of ice spheres.

Calculated rates of heating in the very cold clouds were very high, reaching 4°C h−1 near cloud base. The survival of these clouds for several hours suggests that the absorbed radiant heat was converted largely into sensible heat in the atmosphere rather than causing evaporation of the crystals.

Full access