Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: A. Fong x
  • Refine by Access: All Content x
Clear All Modify Search
Derek A. Fong and W. Rockwell Geyer

Abstract

The alongshore transport of a surface-trapped river plume is studied using a three-dimensional model. Model simulations exhibit the previously observed rightward veering (in the Northern Hemisphere) of the freshwater and establishment of a downstream geostrophically balanced coastal current. In the absence of any ambient current, the plume does not reach a steady state. The downstream coastal current only carries a fraction of the discharged freshwater; the remaining fraction recirculates in a continually growing “bulge” of freshwater in the vicinity of the river mouth.

The river mouth conditions influence the amount of freshwater transported in the coastal current relative to the growing bulge. For high Rossby number [O(1)] discharge conditions, the bulge shape is circular and the coastal current transport is smaller than for the model runs of low Rossby number discharges. For all model runs conducted without an ambient current, the freshwater transport in the coastal current is less than the freshwater discharged at the river mouth.

The presence of an ambient current (in the same direction as the geostrophic coastal current) augments the transport in the plume such that its downstream freshwater transport matches the freshwater source, and the plume evolves to a steady-state width. The steady-state transport accounted for by the ambient current is independent of the strength of the ambient current. The amplitude of the ambient current only determines the time required to reach a steady-state plume width. A key result of this study is that an external forcing agent (e.g., wind or ambient current) is required in order for the entire freshwater volume discharged by a river to be transported downstream.

Full access
Derek A. Fong and Stephen G. Monismith

Abstract

The accuracy of an acoustic Doppler current profiler (ADCP) used with an internal bottom-tracking system is considered. The boat speed measured using bottom tracking is extremely accurate, comparable to the speeds measured by a high-resolution, real-time kinematic global positioning system (KGPS). The accuracy in the direction of boat motion reported by the bottom tracking is limited to the accuracy of the internal compass of the ADCP. Directional differences (after correcting for local magnetic declination) are about 3° between the ADCP bottom tracking and KGPS. An error of this magnitude is shown to result in a maximal measurement error in water velocity of less than 6%.

Nonetheless, an unexplained water velocity error is observed that is significantly larger than can be explained by a simple compass error. Repeated transects in opposing directions show a bias in measured water velocities in the direction of boat motion. The bias cannot be explained by an error in the compass or the bottom-tracked boat velocities. The difference in recorded velocity between two repeated transects with the boat moving in opposite directions exhibits an error of up to ±5 cm s−1 that has vertical variability.

Full access
Fong-Chiau Chang and Eric A. Smith

Abstract

A drought pattern and its time evolution over the U.S. Great Plains are investigated from time series of climate divisional monthly mean surface air temperature and total precipitation anomalies. The spatial pattern consists of correlated occurrences of high (low) surface air temperature and deficit (excess) rainfall. The center of maximum amplitude in rain fluctuation is around Kansas City; that of temperature is over South Dakota. Internal consistency between temperature and precipitation variability is the salient feature of the drought pattern. A drought index is used to quantify drought severity for the period 1895–1996. The 12 severest drought months (in order) during this period are June 1933, June 1988, July 1936, August 1983, July 1934, July 1901, June 1931, August 1947, July 1930, June 1936, July 1954, and August 1936. Hydrological conditions are examined using National Centers for Environmental Prediction (NCEP) reanalysis precipitable water (PW) and monthly surface observations from Kansas City, Missouri, and Bismarck, North Dakota, near the drought centers. This analysis explains why droughts exhibit negative surface relative humidity anomalies accompanied by larger than normal monthly mean daily temperature ranges and why maximum PWs are confined to a strip of about 10° longitude from New Mexico and Arizona into the Dakotas and Minnesota.

Dynamical conditions are examined using NCEP reanalysis sea level pressures and 500- and 200-mb geopotential heights. The analysis indicates a midtroposphere wave train with positive centers situated over the North Pacific, North America, and the North Atlantic, with negative centers in the southeastern Gulf of Alaska and Davis Strait. Above-normal sea level pressures over New Mexico, the North Atlantic, and the subtropical Pacific along with below-normal sea level pressures over the Gulf of Alaska eastward to Canada, Davis Strait, and Greenland are present during drought periods. The most prominent feature is the strong anticyclone over central North America.

On a regional scale, midtropospheric westerly winds are weakened (or become easterly) south of a thermal heat low centered in South Dakota during drought episodes because of the north–south temperature reversal perturbation. The associated westward displaced Bermuda high leads to enhanced low-level warm flow into the Dakotas, thus helping to maintain the reversal in the meridional temperature gradient and the concomitant thermal wind reversal. Enhanced moisture transport from the Gulf of California into the western plains (part of the Great Basin monsoon process) results from the large-scale perturbation pressure pattern. Middle-upper level convergence maintains the water vapor strip east of the Rocky Mountains, while the Mississippi valley undergoes moisture cutoff from both this process and the westward shift in the Bermuda high. The strip of maximum PW then undergoes enhanced solar and infrared absorption that feeds back on the thermal heat low. Surface air temperatures warm while sinking motion balances middle-upper level radiative cooling around the Kansas City area. This is the dynamical coupling that leads to reduced surface relative humidities. The centers of high surface air temperature and deficit rainfall are dynamically consistent with patterns in geopotential heights, vertical velocities, and water vapor amounts.

Full access
A. I. Carswell, A. Fong, S. R. Pal, and I. Pribluda

Abstract

This paper summarizes the results of a statistical analysis of lidar-determined cloud geometrical properties measured during the 1989 and 1991 campaigns of the Experimental Cloud Lidar Pilot Study. Useful lidar descriptors are introduced to specify the bottom-, top-, and midcloud altitudes. These are used to describe the behavior of cloud vertical location and vertical extent during several months of observations using a dual wavelength (1064 and 532 nm) Nd:YAG lidar at Toronto. Frequency distributions of cloud height and cloud thickness are presented and the relationship of the lidar descriptors to cloud properties are discussed. These data are compared with other information on cloud geometry available in the literature.

Full access
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.

Full access
Nicholas J. Nidzieko, Derek A. Fong, and James L. Hench

Abstract

A field experiment was conducted to directly compare the effects of different sampling modes on Reynolds stress estimates calculated from acoustic Doppler current profilers (ADCPs). Two 1.2-MHz ADCPs were deployed concurrently over a fortnightly cycle: one collected single-ping measurements using mode 1 and a second ADCP employed the fast-ping rate mode 12 with subping-averaged data recorded at the same sample rate as the first ADCP. While mode 12 clearly has a lower noise floor for the estimate of mean velocities, it has been an open question whether the averaging of subpings leads to a biased estimate of turbulence quantities, due to the temporal averaging inherent in this approach. Using the variance method, Reynolds stresses were estimated from the two ADCP datasets and compared with stresses computed directly from the velocity records obtained with a pair of fast sampling acoustic Doppler velocimeters (ADVs) collocated with the ADCPs. Mode-12 stresses were more accurate than mode 1 in comparison to ADV-derived stresses, and mode 12 exhibited much lower measurement uncertainty than mode 1. Mode 1 appears to overestimate stresses by 20% in this study. The lower noise floor associated with mode 12 suggests that the variance method may be used with mode 12 to resolve smaller stresses than would be possible with mode 1.

Full access
Gary A. Morris, Walter D. Komhyr, Jun Hirokawa, James Flynn, Barry Lefer, Nicholay Krotkov, and Fong Ngan

Abstract

This paper reports on the development of a new technique for inexpensive measurements of SO2 profiles using a modified dual-ozonesonde instrument payload. The presence of SO2 interferes with the standard electrochemical cell (ECC) ozonesonde measurement, resulting in −1 molecule of O3 reported for each molecule of SO2 present (provided [O3] > [SO2]). In laboratory tests, an SO2 filter made with CrO3 placed on the inlet side of the sonde removes nearly 100% of the SO2 present for concentrations up to 60 ppbv and remained effective after exposure to 2.8 × 1016 molecules of SO2 [equivalent to a column ∼150 DU (1 DU = 2.69 × 1020 molecules m−2)]. Flying two ECC instruments on the same payload with one filtered and the other unfiltered yields SO2 profiles, inferred by subtraction. Laboratory tests and field experience suggest an SO2 detection limit of ∼3 pbb with profiles valid from the surface to the ozonopause [i.e., ∼(8–10 km)]. Two example profiles demonstrate the success of this technique for both volcanic and industrial plumes.

Full access
Sarah N. Giddings, Stephen G. Monismith, Derek A. Fong, and Mark T. Stacey

Abstract

Residual (subtidal) circulation profiles in estuaries with a large tidal amplitude-to-depth ratio often are quite complex and do not resemble the traditional estuarine gravitational circulation profile. This paper describes how a depth-normalized σ-coordinate system allows for a more physical interpretation of residual circulation profiles than does a fixed vertical coordinate system in an estuary with a tidal amplitude comparable to the mean depth. Depth-normalized coordinates permit the approximation of Lagrangian residuals, performance of empirical orthogonal function (EOF) analysis, estimation of terms in the along-stream momentum equations throughout depth, and computation of a tidally averaged momentum balance. The residual mass transport velocity has an enhanced two-layer exchange flow relative to an Eulerian mean because of the Stokes wave transport velocity directed upstream at all depths. While the observed σ-coordinate profiles resemble gravitational circulation, and pressure and friction are the dominant terms in the tidally varying and tidally averaged momentum equations, the two-layer shear velocity from an EOF analysis does not correlate with the along-stream density gradient. To directly compare to theoretical profiles, an extension of a pressure–friction balance in σ coordinates is solved. While the barotropic riverine residual matches theory, the mean longitudinal density gradient and mean vertical mixing cannot explain the magnitude of the observed two-layer shear residual. In addition, residual shear circulation in this system is strongly driven by asymmetries during the tidal cycle, particularly straining and advection of the salinity field, creating intratidal variation in stratification, vertical mixing, and shear.

Full access
Ryan J. Moniz, Derek A. Fong, C. Brock Woodson, Susan K. Willis, Mark T. Stacey, and Stephen G. Monismith

Abstract

Autonomous underwater vehicle measurements are used to quantify lateral dispersion of a continuously released Rhodamine WT dye plume within the stratified interior of shelf waters in northern Monterey Bay, California. The along-shelf evolution of the plume’s cross-shelf (lateral) width provides evidence for scale-dependent dispersion following the 4/3 law, as previously observed in both surface and bottom layers. The lateral dispersion coefficient is observed to grow to 0.5 m2 s−1 at a distance of 700 m downstream of the dye source. The role of shear and associated intermittent turbulent mixing within the stratified interior is investigated as a driving mechanism for lateral dispersion. Using measurements of time-varying temperature and horizontal velocities, both an analytical shear-flow dispersion model and a particle-tracking model generate estimates of the lateral dispersion that agree with the field-measured 4/3 law of dispersion, without explicit appeal to any assumed turbulence structure.

Full access