Vertical Velocity Estimates Research Articles

The Effects of Nonuniform Beam Filling on Vertical Rainfall Velocity Measurements with a Spaceborne Doppler Radar

Abstract Information on the global distribution of vertical velocity of precipitating particles is needed in estimating latent heat fluxes, and therefore in the general study of energy transportation phenomena in the atmosphere. Such information is not currently available, but it can potentially be obtained by a spaceborne Doppler precipitation radar. In this paper, the expected performance for this type of Doppler radar for measuring vertical rainfall velocity is investigated. Although the high relative speed of the instrument with respect to the rainfall droplets contributes significantly to the spreading of the Doppler spectrum, accurate estimates of the average vertical velocity can be obtained when the rainfall intensity does not vary significantly within the resolution volume of the instrument. Such a result can be inferred through theoretical calculations and is confirmed by analyzing the Doppler spectra simulated using data gathered by the NASA/Jet Propulsion Laboratory (JPL) airborne rain radar i...

Deep Convection in the Labrador Sea as Observed by Lagrangian Floats

During the winters of 1997 and 1998, a total of 24 Lagrangian floats were deployed in the Labrador Sea. These floats were designed to match the buoyancy and compressibility of seawater. They measured temperature and three-dimensional position (pressure for vertical position and RAFOS acoustic tracking for latitude and longitude) as they followed water motions three-dimensionally. This data provides direct observation of mixed layer depth and excellent estimates of vertical velocity. Floats were repeatedly carried across the convecting layer by vertical velocities averaging several centimeters per second with vertical excursions of up to one kilometer. In the horizontal, several scales of eddy motion were resolved, as was a possible float predilection toward remaining in water preconditioned for convection. Heat flux estimates from this data reveal entrainment and surface heat fluxes similar in magnitude. The mixed layer acts as a vertical conveyor belt of temperature, transporting heat from depth to the surface without requiring a net change in mixed layer temperature, since incorporation of salt from below allows an increase in density without a net change in temperature. Comparison with NCEP reanalysis meteorological heat flux and wind magnitude data shows that the vertical velocity variance can be modeled with 80% skill as a linear function of lagged buoyancy flux (with the atmosphere leading the ocean by ∼1/2 day) without using the wind estimates. Mixed layer motions are clearly driven by the surface buoyancy flux, Bo. A nonrotating scaling of vertical velocity variance, (BoH)1/3, provides a marginally better fit than a rotating scaling, (Bo/f)1/2. Horizontal effects appear to play only a weak role during strong convection but result in rapid restratification when convective forcing weakens.

Evaluation of Vertical Winds near and inside a Cloud Deck Using Coherent Doppler Lidar

Abstract This paper summarizes high-resolution measurements of the vertical wind fields near and inside a cloud deck from a ground-based 2-μm solid state coherent Doppler lidar. Results include physical characterization of the deck and wind field statistics near and inside the cloud. The data suggest the possibility of wave conditions, which is supported by the corresponding National Oceanic and Atmospheric Administration profiler data. The temporal spectra of the vertical velocity estimates inside the cloud have a −5/3 power-law dependence, and estimates for the spatial structure function and energy dissipation rate are computed. Spectra are investigated for rain conditions where two spectral peaks are observed: one for aerosol target and one for rain.

Equatorial upwelling in the western Pacific warm pool

Vertical velocity on the equator in the western Pacific warm pool is investigated using data from the Coupled Ocean‐Atmosphere Response Experiment enhanced monitoring array (EMA) centered at 0°, 156°E. The data consist of hourly subsurface horizontal velocity time series from August 1991 until April 1994. Vertical velocity is calculated using horizontal velocity components and the application of the continuity equation. During the first year, from March 1992 until February 1993, data are available from five moorings of the EMA and thus provide nine different combinations of moorings from which to calculate vertical velocity. Four moorings were available during the remaining time period. Random errors are found to be <10−5 m s−1, while systematic errors (finite difference error, systematic instrument error, and error due to surface extrapolation) may be larger. It is suggested that errors, including finite difference errors, are not larger than the vertical velocity estimate. The estimates of vertical velocity are valid on spatial scales the size of the array (∼400 km) and timescales longer than a few days. They reveal a seasonal cycle manifested during a moderate El Niño. Results indicate upwelling, on average from 70 m down to 250 m over the 2 year time period, being slightly stronger in 1992 coincident with the stronger El Niño year. The divergence of horizontal velocity components, resulting in positive vertical velocity, is due to geostrophic divergence on the equator produced from a westward directed zonal pressure gradient force. Meridional divergence and zonal wind stress are uncorrelated, suggesting that Ekman convergence due to local westerly winds is only of partial influence. Consequently, downwelling is not found near the surface, where contributions from local winds and geostrophic divergence are in opposition. This estimate of vertical velocity indicates that water is upwelled in the warm pool from much deeper than but with comparable magnitude to the central and eastern Pacific.

The use of simulated drifters to estimate vorticity

A primitive equation ocean model is used to generate trajectories of simulated clusters of drifters in the California Current (CC) region. These trajectories allow us to evaluate a least squares (LS) method of estimating vorticity and vertical velocity along a cluster's path. Two clusters provide examples of successful and less successful estimates of vorticity and vertical velocity. Our analysis quantifies the dependence of estimate quality on several parameters that can be used as error predictors in the LS estimate of vorticity: cluster separation, number of drifters in a cluster, and cluster shape. A combination of cluster separation and ellipticity shows the most promise as an indicator of quality for the vorticity estimate.

Biologically induced circulation at fronts

Consider a frontal region that has high phytoplankton biomass on one side and low biomass on the other. Irradiance penetrates deeply through the water column on the low‐biomass side but is attenuated nearer the surface on the biomass‐rich side because of absorption by phytoplankton. Thus the near‐surface water is heated more on the biomass‐rich side than on the clearer side, resulting in lower‐density surface water on the biomass‐rich side. At greater depths the situation is reversed, with lower‐density water occurring on the biomass‐poor side. We model this situation and examine the resulting perturbations to the frontal circulation. Our aim is to provide an order‐of‐magnitude estimate of the feedbacks from the biological component of the ecosystem to the current field. The model consists of the steady state momentum equations, including Coriolis, pressure gradient, and viscous effects. We compute induced vertical velocities of up to 0.2 mm s−1, commensurate with field measurements and previous modeling estimates of vertical velocities at fronts. The horizontal along‐frontal velocities are of order 2 cm s−1 or less and so will not represent a major contribution to the overall flow field; however, such values are certainly not insignificant.

Correction Of Eddy-Covariance Measurements Incorporating Both Advective Effects And Density Fluxes

Equations are presented to correct eddy-covariance measurements for both fluctuations in density and non-zero mean advection, induced by convergence or divergence of flow, and spatial source/sink inhomogeneity, under steady-state and transient conditions. This correction collapses to the Webb-Pearman-Leuning expression if the mean vertical velocity is zero, and formally adds the Webb-Pearman-Leuning expression to the corrections suggested by Lee for conditions of non-zero vertical velocity and source/sink and mean scalar horizontal homogeneity. The equation requires measurement of the mean vertical gradients of the scalar concentration of interest (air temperature, humidity, CO2) as well as an accurate estimation of the mean vertical velocity, in addition to the vertical eddy covariance of the scalar. Simple methods for the approximation of sensor tilt and complex terrain flow angle are presented, to allow estimation of non-zero mean vertical velocities. The equations are applied to data from a maize crop and a forest to give examples of when the correction is significant. In addition, a term for the thermodynamic expansion energy associated with water vapour flux is derived, which implies that the sonic temperature derived sensible heat flux will accurately include this contribution.

Role of the climatological and current variability on shelf-slope exchanges of particulate matter: Evidence from the Rhône continental margin (NW Mediterranean)

Climatological, current and particulate flux data were gathered in the Grand-Rhône canyon on the Gulf of Lions continental margin for one year (Jan. 1988–Jan 1989). Time series were analyzed to determine the influence of physical exchange processes on particulate matter at the shelf-edge, with a special emphasis on the Northern Current variability. The synoptic variability of the Northern Current was linked to meanders of 2–5 day period. Its meso-scale activity presented a seasonal signal with maximum values in early spring. Peaks of particulate fluxes in the upper traps were little affected by large river and atmospheric inputs, but rather by enhanced shelf-slope exchanges at the shelf edge due to intense cross-slope fluctuations of the Northern Current. These fluctuations caused cross-isobath flows near the bottom, which appeared to be a potential mechanism in transporting particles off the shelf. At 900 m depth, high-flux events measured by sediment traps were primarily linked to periods of higher cross-slope current oscillations. This correlation suggests that vertical motions caused by these oscillations contribute to the suspended particulate matter transport through the process of bringing higher suspended material concentrations from above to greater depths. Vertical velocity estimates were derived through temperature fluctuations combined with vertical temperature gradient and from the kinematic boundary condition. A simple diffusion model indicates that the vertical turbulent mixing of suspended particulate matter, induced by the cross-slope current oscillations, yields downward fluxes of particles comparable to those collected by sediment traps.

A study of the omega equation for diagnosing vertical motions at ocean fronts

Estimation of vertical velocity is a key issue for understanding ocean physics and transport of biogeochemical tracers. We examine the accuracy of estimating vertical velocity in fronts with the omega equation. The diagnostic performance of the omega equation is evaluated by using vertical velocities obtained from simulation of frontal instabilities in a primitive equation model as a reference. We use two traditional quasigeostrophic methods in which the flow is either a geostrophic flow computed from density or a nondivergent flow derived from vorticity and also test two new formulations: a quasigeostrophic method using the total flow field and the semigeostrophic omega equation. Results show that all four formulations correctly diagnose the vertical velocity pattern. However, estimates provided by the traditional quasi-geostrophic formulations have systematic bias. In contrast, the two new techniques, which are practically equivalent, produce unbiased vertical velocity diagnostic at fronts. These results point out the importance of including higher order dynamics than quasigeostrophy to take into account the ageostrophic advection in the front. Since adequate filtering of ADCP data is not yet available to obtain a suitable total flow, the semigeostrophic omega equation is proposed as the most valuable tool to compute vertical velocities from high resolution density measurements.

Atmospheric sulfur cycling in the tropical Pacific marine boundary layer (12°S, 135°W): A comparison of field data and model results: 2. Sulfur dioxide

The atmospheric chemistry of sulfur dioxide over the tropical South Pacific Ocean is investigated by using results from field measurements and numerical models. Simultaneous real time measurements of sulfur dioxide and its biogenic precursor dimethylsulfide were made at 12°S, 135°W for a 6‐day period from March 3 through March 9, 1992. The mean SO2 and DMS mole fractions were 71 ± 56 pmol mol−1 (1σ) and 453 ± 93 pmol mol−1 (1σ) respectively. These concentrations are compared to those predicted by a time‐dependent photochemical box model of the marine boundary layer. Model estimates of the yield of SO2 from DMS oxidation range from 27% to 54%. Even with low yields, DMS is the dominant source of SO2 in this region. Estimates of vertical entrainment velocities based on the tropospheric ozone budget suggest that vertical entrainment is a minor source of SO2. The relative rates of various loss mechanisms for SO2 are dry deposition to the sea surface (58%), in‐cloud oxidation (9%), OH oxidation (5%), and uptake by sea‐salt aerosols (28%).

Deep Ocean Wave Measurements Using a Vertically Oriented Sonar

A vertically oriented 200-kHz sonar is used to make estimates of (one-dimensional) surface wave spectra in the deep ocean. Two independent approaches are used to determine wave spectra from this system: a direct measurement of range to the surface and the vertical velocity estimate of the surface. Estimated wave spectra are compared with observations from a Datawell Waverider buoy positioned directly above the acoustic instrument. Comparisons are provided for relatively calm conditions and also during a period of mixed swell and wind waves forced by a 13 m s−1 wind. Wave spectra from the three estimates appear qualitatively similar and agree to within 10% in total wave energy density.

Poststatistic steering wind estimation in the troposphere and lower stratosphere

Poststatistic steering (PSS) techniques are applied for estimating the wind vector. Since spatially separated receivers can be used to synthesize a radar beam in an arbitrary direction, Doppler beam swinging (DBS) techniques can be used in conjunction with PSS to obtain estimates of the wind vector. Data obtained with the middle and upper atmosphere (MU) radar in Japan are used to implement the technique in the troposphere and lower stratosphere at a VHF wavelength. To obtain an accurate estimate of the beam‐pointing direction, the synthesized two‐way antenna pattern is calculated. Also, the effects of aspect sensitivity on this technique are explored. The mode of operation of the MU radar was alternated between a typical DBS mode and a spatial interferometric (SI) mode, which was used for the PSS technique. Results from the PSS method of obtaining the wind vector are compared to the DBS results, which are assumed to be an accurate representation of the atmospheric wind. This study suggests that aspect sensitivity causes an underestimate of the horizontal wind using PSS. Corrections were made using the aspect sensitivity functions obtained from the DBS experiment and the corrected PSS velocities compare well with the DBS velocities. The vertical velocity estimate using a vertically pointing beam is shown to possess a gravity wave characteristic which is due solely to contamination from the horizontal wind.

The North Atlantic circulation: Combining simplified dynamics with hydrographic data

We estimate the time-averaged velocity field in the North Atlantic from observations of density, wind stress and bottom topography. The flow is assumed geostrophic, with prescribed Ekman pumping at the surface, and no normal component at the bottom. These data and dynamics determine velocity to within an arbitrary function of (Coriolis parameter)/(ocean depth), which we call the “dynamical free mode.” The free mode is selected to minimize mixing of potential density at mid-depth. This tracer-conservation criterion serves as a relatively weak constraint on the calculation. Estimates of vertical velocity are particularly sensitive to variations in the free mode and to errors in density. In contrast, horizontal velocities are relatively robust. Below the thermocline, we predict a strong 0(1 cm/set) westward flow across the entire North Atlantic, in a narrow range of latitude between 25N and 32N. This feature supports the qualitative (and controversial) conjecture by Whist (1935) of flow along the “Mediterranean Salt Tongue.” Along continental margins and at the Mid-Atlantic Ridge, predicted bottom velocity points along isobaths, with shallow water to the right. These flows agree with many long-term current measurements and with notions of the circulation based on tracer distributions. The results conflict with previous oceanographic-inverse models, which predict mid-depth flows an order of magnitude smaller and often in opposite directions. These discrepancies may be attributable to our relatively strong enforcement of the bottom boundary condition. This involves the plausible, although tenuous, assertion that the flow “feels” only the large-scale features of the bottom topography. Our objective is to investigate the consequences of using this hypothesis to estimate the North Atlantic circulation.

Estimation of divergent wind from OLR data for use in objective analysis over the Indian region

A scheme is formulated for the use of OLR data in the estimation of vertical velocity; divergence and then the divergent part of the wind over Indian region. In this scheme, ascending motion over cloudy region is estimated from an empirical relation between the cloud top temperature and descending motion over cloud-free region is estimated from the thermodynamic energy equation and both are blended. From this blended vertical velocity field, aivergence, velocity potential and divergent winds at all standard levels from 4 to 8 July 1979 at 00 UTC are computed. These fields are compared with satellite cloud pictures, rainfall etc and they are found to be realistic in depicting the synoptic conditions. Total wind is computed as the sum of the estimated divergent component and rotational component computed from observed wind field. For assessment of the scheme, this total wind field at 850 hPa is used as initial. guess field in univariate optimum interpolation scheme and analyses were made for the period 4 to 8 July 1979. Results show that scheme is able to produce realistic analyses which included divergent part of the wind.

Drifter observations of a cold filament off Point Arena, California, in July 1988

We use the position fixes and temperature readings from 56 Tristar‐II mixed‐layer drifters, in conjunction with advanced very high resolution radiometer images, to provide a description of the mesoscale variability of the flows associated with cold‐water filaments in the California Current system off northern California in July 1988. We find that the northern, offshore temperature front of the filament is convergent and is closely associated with the core of a high‐speed (>100 cm/s) jet that has a broader spatial scale than the cold‐water filament. Mesoscale features are observed to be related to all developments of the cold‐water filament on a variety of time scales (1–30 days). In one well‐observed feature, estimates of vertical velocity (w) based on a vorticity budget are −20 m/d over an area of 150 km2 and are associated with a gain of vorticity. Estimates of w based on a local heat budget following a drifter are about 3 times larger over an area of 60 km2. These intense downwelling features are also places where horizontal stirring occurs.

Groundwater flow, velocity, and age in a thick, fine-grained till unit in southeastern Wisconsin

Piezometer nests were installed at study sites in each of five north-south-trending end moraines of the late Pleistocene Oak Creek Formation in southeastern Wisconsin. The formation is composed primarily of a fine-grained glacial diamicton (till) and laterally continuous and discontinuous, coarse-grained lake and meltwater stream sediment. It overlies the Silurian dolomite aquifer, which is a source of drinking water to rural areas. The average vertical linear velocity and age of ground water in the Oak Creek Formation were estimated by three methods: Darcy's Law, environmental isotopes including 3H, δ 2H, δ 18O, and 14C (dissolved inorganic carbon), and solute transport modeling of 18O. The F-1 and Metro sites in the Tinley moraine showed excellent agreement among the three estimates of vertical velocity and showed the lowest velocity values (0.3–0.5 cm year −1 downward), which suggests that diffusion controls vertical mass transport at these sites. Although the extrapolated maximum age of ground water is 35 000 years, ground water below 75 m at these sites is probably not older than 15 000 years, which is the maximum age of the formation. Estimates of velocity showed less agreement at study sites in the Lake Border moraine system to the east and ranged from about 0.2 to 20.7 cm year −1; maximum groundwater age could range from 213 to 6000 years. Higher and more variable velocities, perhaps owing to thinner and more heterogeneous sediment in these areas, suggest that diffusion may not dominate vertical mass transport. Heterogeneity and fractures may also promote the development of groundwater flow systems dominated by lateral flow. Because of the uncertainty about the nature of groundwater flow, velocity, and age in the formation east of the Tinley moraine, future waste-disposal activity in southeastern Wisconsin should be confined to the thickest parts of the Tinley moraine near the present F-1 and Metro sites.

Spatial and Frequency Domain Interferometry Using the MU Radar: A Tutorial and Recent Developments.

In this paper, we attempt to give an overview of the general concepts of spatial and frequency domain interferometry. The emphasis is placed on fundamentals, so that the uninitiated reader will be able to grasp the basic ideas behind these subjects. In addition, recent research using the MU radar is reviewed as logical results of these fundamental concepts. In particular, the spatial domain interferometer has been shown to be useful in full two-dimensional steering of the antenna beam, after the data has been stored in a multi-receiver configuration. This is an improvement over previously published research, which was limited to one dimension. Also, a new technique for estimating the three-dimensional wind vector using spatial domain interferometry has been developed. It has been shown that this method is well suited for the estimation of the vertical wind component. The improvement in the vertical velocity estimate is based on the fact that this technique eliminates the effect of tilted refractivity layers on VHF echoes. It has been shown that the vertical wind estimate based on the Doppler technique is biased by the horizontal wind. In addition, the basic concepts of frequency domain interferometry are presented, with examples of recent developments in tracking of thin, high-reflectivity layers within the resolution volume of the radar.

A study of the vertical motion field near the high-latitude summer mesopause during MAC/SINE

We present the results of high-resolution observations of the vertical velocity field obtained with the EISCAT and SOUSY VHF radars near the high-latitude summer mesopause during the MAC/SINE campaign in northern Norway in 1987. The data reveal an energetic motion field with maximum amplitudes of ~ 10 m/s and characteristic periods of ~5–30 min. Motions exhibit a high degree of vertical coherence and a quasi-periodic structure, with typical durations of 5–10 cycles. Estimates of the mean vertical velocity are downward at lower levels and are near zero or positive at greater heights. The mean vertical velocity variance is found to be ~5 m 2/s 2, consistent with other high-latitude measurements. Frequency spectra computed for each radar are found to exhibit considerable variability, while vertical wavenumber spectra are seen to be somewhat variable in amplitude and to have slopes approaching −3 at lower wavenumbers. These results are suggestive of an energetic spectrum of gravity wave motions near the mesopause that has a large vertical flux of wave energy, that may have observed wave frequencies differing significantly from intrinsic frequencies due to Doppler shifting by large horizontal winds, and that is consistent with the separability of the frequency and wavenumber dependence of the motion spectrum and with gravity wave saturation at sufficiently small vertical scales.

Uncertainties in estimates of the mean vertical velocity from MST radar observations

Mean values of the vertical velocity based on time series of clear‐air Doppler radar observations are subject to uncertainties due to several factors. This paper examines the statistical uncertainties due to estimating the population means using only finite periods of record and the uncertainties that are introduced by systematic gaps in the series of observations. The statistical uncertainty, the standard error of the mean, depends on the variance of the data and on the effective sample size. The largest change in the variance of vertical velocity is about a factor of 3 and is seen between clear and cloudy sky conditions. Very little change in the effective sample size is seen under any conditions. The averaging time required to estimate the mean with a standard error of the mean of 2 cm/s is found to be about 6 hours under typical conditions at Flatland. The uncertainty of the mean due to systematic gaps in the data increases with the length of the gaps, even though the total observation period remains constant. The root‐mean‐square value of this uncertainty is about 2 cm/s; about the size of the standard error of the mean and about half as large as typical expected subsynoptic scale motions. These results suggest that future sampling programs should be designed to eliminate systematic gaps in the data.

Neotectonism along the Atlantic passive continental margin: A review

An extensive body of geologic data including the modern state of stress, historical seismicity, surface and subsurface stratigraphy, numerical models of crustal deformation, surficial geomorphic systems, and historical precise leveling and tidal gauge records constrain the style and rate of neotectonic deformation for the Appalachians and Atlantic passive continental margin. There are two major styles of neotectonism in the eastern United States. The northeastern United States is dominated by isostatic uplift and northward migration of peripheral bulge collapse in response to deglaciation. This locally rapid, but decreasing rate of deformation is superimposed upon slower, long-term deformation along the Atlantic margin. Most of the long-term, continental margin deformation is attributed to lithospheric flexuring in response to sediment loading in sedimentary basins (especially the Baltimore Canyon Trough and Carolina Trough), isostatic deformation in response to continental denudation and water loading of the shelf, and stress from far-field plate tectonic sources. Significant deformational features include an uplift anomaly near Cape Fear, N.C.; northward and southward tilting of the Coastal Plain into the Salisbury and Southeast Georgia Embayments respectively; seaward tilting of the Coastal Plain/Piedmont, and a complex pattern of postglacial uplift and later subsidence in the northeast. Estimates of vertical crustal velocities for similar locations vary over several orders of magnitude. Measurement interval bias and systematic leveling errors may account for some of the discrepancies. Evidence for periodic deformation in the eastern United States in substantial and it is possible that historic data indicate a period of accelerated deformation along the Atlantic continental margin.