Vertical Velocity Structure Research Articles

Classification of Precipitating Clouds in the Tropics Using 915-MHz Wind Profilers

Abstract An algorithm has been developed that classifies precipitating clouds into either stratiform, mixed stratiform/convective, deep convective, or shallow convective clouds by analyzing the vertical structure of reflectivity, velocity, and spectral width derived from measurements made with the vertical beam of a 915-MHz Doppler wind profiler. The precipitating clouds classified as stratiform and convective clouds match the physical and radar properties deduced by Doppler weather radars in the GATE and EMEX programs. The mixed stratiform/convective cloud category is a hybrid regime containing a melting-layer signature associated with stratiform clouds yet is turbulent above the melting level similar to convective clouds. Shallow convective clouds have hydrometeors confined entirely below the melting level implying that warm rain processes are occurring exclusively. The algorithm is illustrated by classifying precipitating clouds from 10 months of observations at Manus Island (2°S, 147°E) in the western...

Vertical structure of the upper ocean during the Marine Light‐Mixed Layers experiment

The Marine Light‐Mixed Layers (MLML) experiments took place in the subarctic North Atlantic Ocean, approximately 275 miles south of Reykjavik, Iceland, during 1989 and 1991. The 1991 field program took place from April 30 to September 6 and included a central surface mooring to document the temporal evolution of physical, biological, and optical properties. In this paper we describe the physical variability observed at the 1991 mooring site, concentrating on the vertical structure of temperature and velocity in the upper 300 m of the water column and their changes in response to heat and momentum fluxes at the sea surface. The deployment period included the spring transition, when upper ocean re stratification was initiated after deep winter mixing, and the fall transition, when mixed layer deepening began again. The dominant signal in temperature was seasonal variation, with a 6°C increase observed at the sea surface from May to August. Prior to development of the seasonal stratification, a period dominated by near‐surface temperature variability was observed in association with a 15‐day mean flux of only 20 W m−2 into the ocean. Pronounced day/night oscillations of heat flux during this period resulted in alternating development and destruction of stratification and intense diurnal cycling of the mixed layer depth. A qualitative comparison of the observed temperature structure to the prediction of a one‐dimensional mixed layer model showed that local processes dominated during the initiation of restratification and during most of the summer warming period. Nonlocal processes were important after the fall transition.

On the downward bias in vertical velocity measurements by VHF radars

EISCAT VHF (224 MHz) radar observations of the vertical velocity under Polar Mesosphere Summer Echo (PMSE) conditions were performed with 2 s time and 300 m height resolution and velocity uncertainties better than 0.1 ms−1. We show that upward velocities that persist for ∼ 10 min or more lead to greater variance in the motion field and tend to make the PMSE weaker. As smaller radar cross section also correlates with larger velocity uncertainty, any vertical mean velocity composed of thresholded radar data tends to have a downward (negative) bias. We argue that this phenomenon can contribute to the discrepancy between the mean upwelling accounting for the summer mesopause thermal structure and radar observations of the mean vertical velocity.

Turbulent mixing at an inversion layer

A series of laboratory experiments was carried out to examine the interaction between stratification and turbulence at an inversion layer, with the objective of gaining insight into certain wave–turbulence encounters in the atmosphere. A three-layer stratified fluid system, consisting of a (thick) strongly stratified inversion layer, sandwiched between an upper turbulent layer and a lower non-turbulent weakly stratified layer, was employed. Oscillating-grid-induced shear-free turbulence was used in the upper layer. During the experiments, a fourth (interfacial) layer developed in the region between the inversion and the turbulent layer, and most of the wave–turbulence interactions were confined to this layer. Detailed measurements of the vertical velocity structure, internal-wave parameters and mixing characteristics were made in the stratified layers and, whenever possible, the results were compared to available theoretical predictions.

Development and structure of a maritime continent thunderstorm

The evaluation of a maritime continent thunderstorm complex (Hector) occurring over Bathurst and Melville Islands north of Darwin, Australia (12° S, 131° E) is investigated primarily using Doppler radar data. Thunderstorm formation follows the development of sea breeze circulations and a period of shallow non-precipitating convection. Evidence exists for initiation of long-lived and organised convection on the sea breeze fronts, although short-lived, scattered convection is apparent earlier in the day. Merging of the convective systems is observed in regions of enhanced low-level convergence related to sea breeze circulations. The merged convective complex is initially aligned in an almost east-west direction consistent with the low-level forcing. The merged complex results in rapid vertical development with updraughts reaching 40 m s− and echo tops reaching 20 km height. Maximum precipitation production occurs during this merger phase. On the perimeter of the merged convective complex, evidence exists for front-to-rear updraughts sloped over lower-level downdraughts with rear-to-front relative flow and forward propagating cold pools. The mature phase is dominated by this convection and the complex re-orientates in the prevailing easterly vertical shear to an approximate north-south direction, then moves westward off the islands with the classic multicellular squall-like structure. The one-dimensional cloud model of Ferrier and Houze (1989) used with a four class ice formulation reproduced the cloud top height, updraught structure and echo profile very well. To test the importance of ice physics upon thunderstorm development, several sensitivity tests were made removing the effects of the ice phase. All of these model clouds reached nearly 20 km, although simulations without the effects of ice had updraughts reduced from about 40 m s−1 to 30 m s−1. The simulated convection was more sensitive to changes in environmental conditions and parameterised cloud dynamics. The strong intensity of the convection was largely accounted for by increasing equivalent potential temperatures due to diurnal heating of the surface layer. The vertical velocity and radar structure of the island thunderstorm has more similarity with continental rather than oceanic convection. Maximum vertical velocities, in particular are almost an order of magnitude greater than typical of oceanic convection. With the intense updraughts, even in the low shear environment, there is evidence for mesoscale circulations within the convection.

Geothermal heat flux from hydrothermal plumes on the Juan de Fuca Ridge

Estimates of the heat output of hydrothermal vents, identified along the Endeavor and Southern segments of the Juan de Fuca Ridge, are used to evaluate the total heat flux associated with hydrothermal circulation for the ridge segment. A 50‐m array carried by DSV Alvin sampled the temperature and vertical velocity structure of hydrothermal plumes from individual vents. These measurements are used to estimate the thermal flux associated with such plumes. The maximum heat flux calculated for a single vent is 50 MW (1 MW = 1×106 W). The median heat flux per vent is 9 MW and 3 MW, respectively, for the Endeavour Segment (18 vents) and Southern Segment (18 vents). The estimates for any given vent may vary over an order of magnitude. This uncertainty is due mainly to the difficulty of locating the centerline of the plume relative to the point of measurement, although the uncertainties in the constants for the appropriate equations based on laboratory experiments also contribute significantly to the net error. For the Endeavor Segment, the minimum total geothermal heat flux due to hydrothermal circulation exceeds 70 MW. The minimum estimate for the Southern Segment is 16 MW. The maximum estimate is probably closer to the total heat flux from high‐temperature venting (239 MW and 66 MW respectively). High‐temperature hydrothermal venting accounts for only a small fraction of the heat available according to steady state predictions of conductive heat flux; other hydrothermal phenomena (e.g., diffuse flow) probably account for a greater proportion of the total hydrothermal heat flux.

Surface-wave mapping of the crust and upper mantle in the Arctic region

Pure path Rayleigh-wave group velocities for 24 regional grids inside the Arctic have been obtained from 74 mixed-path measurements. The vertical shear-wave velocity structures under these 24 grids were then determined according to their pure-path group-velocity dispersion data. This result indicates that the crustal shear-wave velocity in the Arctic region is generally lower than the worldwide average. In addition, a typical mid-ocean ridge structure was clearly shown under the Nansen Ridge. We also found an abnormally high upper mantle shear-wave velocity under the Alpha Ridge and Makarov Basin, and low velocity beneath the Canadian Arctic Islands. In general, our inversion result agrees with the known tectonics in the Arctic region. To further check the validity of our result, we have computed synthetic Rayleigh waves and a comparison with our observations shows very good agreement.

Integration of Space and In Situ Observations to Study Global Climate Change

The use of model-based global data sets of atmospheric circulation for studying fundamental dynamical and physical processes is discussed, focusing on limitations of the available model-based data sets. Data from the Global Weather Experiment in 1979 were analyzed by two authorized level IIIb data centers in 1980 and in 1981. The analyses led to difference in data-sparse regions such as the tropics. Study areas which can be addressed by an internally-consistent long-term multivariate data set for the atmospheric circulation are considered, including mean climate, forcing for the ocean models, global hydrological cycle, atmospheric energetics, intraseasonal variability, land surface processes, and structure and variability of vertical velocity, divergence, and diabatic heating. It is concluded that the most comprehensive technique for integrating space and in situ observations to produce this type of data set would be a four-dimensional data assimilation system with a realistic physical model of the type employed in operational numerical weather prediction.

Vertical Structure of Low Frequency Variability in the Eastern Equatorial Pacific Ocean

Abstract The vertical structure of low frequency velocity and temperature variability in the eastern equatorial Pacific Ocean is examined using surface and subsurface moored current meter data from 0°, 110°W between 20 and 3027 m depths over the period 30 March 1980 to 2 February 1981. Three methods of analysis are employed: vertical coherence, empirical orthogonal functions, and linear least-squares dynamical mode decompositions. Direct evidence is given for the existence of first baroclinic mode Kelvin waves in the east component of velocity and vertical displacement (estimated from temperature) in that the vertical displacement is coherent and in phase over the water column and the upper-ocean east component of velocity varies out of phase with the vertical displacement over the water column. Near-surface modifications due to advection, nonadiabatic processes, and local forcing are also noted.

Crustal structure in eastern Kazakh, U.S.S.R. from teleseismic receiver functions

Broadband receiver functions determined from teleseismic P‐waveforms recorded at two seismic stations in eastern Kazakh, U.S.S.R., are inverted for the vertical velocity structure beneath the stations. The detailed broadband receiver functions are obtained by stacking source‐equalized radial components of teleseismic P‐waveforms. A time‐domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat‐lying, homogeneous layers. The general features of the inversion results are: a complex shallow crust, velocities less than 6 km/s in the upper crust, a high velocity (6.9‐7.5 km/s) lower crust, and a Moho that varies between 47 and 57 km depth. These results compare favorably with a composite velocity model from Deep Seismic Sounding data, but show lower velocities in the upper crust, and higher velocities in the lower crust. The results indicate that the crust in this region is relatively uniform, however the nature and depth of the Moho changes significantly across the region. The receiver function inversion structure for the Kazakh sites is similar to a published receiver function structure for a site on the Canadian Shield.

Vertical Velocity Structures in an Axisymmetric, Nonhydrostatic Tropical Cyclone Model

Abstract A statistical analysis of several experiments with different microphysical parameterizations in an axisymmetric, nonhydrostatic tropical cyclone model illustrates the impact of icc-phase microphysics on model vertical velocity structure. The parameterizations are designed to illustrate the effects of 1) thermodynamic input through latent heating, 2) vertical sorting of microphysical species by fallspeed, and 3) different rates of the parameterized microphysical conversion processes. The results confirm previous studies on the thermodynamic effect of melting, but they also show that the other factors, namely, fallspeed and microphysical conversion rates, are important in determining model vertical velocity structure and evolution. Statistical summaries of updrafts and downdrafts show distinct increases in the intensity and horizontal scale of downdrafts near the melting level when parameterized snow is included. Model storms without snow show a greater percentage of convective-scale updrafts and d...

Crustal structure of the Adirondacks determined from broadband teleseismic waveform modeling

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of the Department of Energy's Regional Seismic Test Network station RSNY are analyzed to examine the crustal structure beneath the Adirondack Highlands of upstate New York. Radial receiver functions are inverted in the time domain to determine the vertical shear velocity structure at four distinct back azimuths. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. Southeast of RSNY, our model consists of a thick crust, including a broad crust‐mantle transition. The most prominent structure is a high‐velocity zone (shear velocity >4.0 km/s) between 18 and 26 km depth which overlies a lower crust of low average shear velocity (< 3.7 km/s). The extent of the high‐velocity zone correlates in depth with a highly reflective zone in Consortium for Continental Reflection Profiling profiles SE of RSNY, while the deep low‐velocity zone may be correlated with a broad electrical conductivity anomaly in the Adirondacks. This general structure is also seen SW of RSNY, but is more difficult to document at NW and NE azimuths.

Crustal structure at Regional Seismic Test Network stations determined from inversion of broadband teleseismic P waveforms

Abstract The site structure beneath the five broadband stations of the Regional Seismic Test Network (RSTN) has been determined utilizing teleseismic P waveforms. Our teleseismic waveform modeling technique involves inverting the radial component of stacked source-equalized receiver functions in the time domain to estimate the vertical shear velocity structure at the site. The receiver functions at RSNY (Adirondacks, New York), RSON (Red Lake, Ontario), and RSNT (Yellowknife, Northwest Territory) are quite simple compared to previously published RSCP (Cumberland Plateau, Tennessee) data, largely due to the absence of sedimentary surface rocks at these sites. At RSNY, a high-velocity layer at mid-crustal depths (18 to 26 km) in our southeast backazimuth results correlates in depth with a zone of high-amplitude reflections found on COCORP profiles 60 km south of RSNY. A gradational crust-mantle boundary is observed at RSCP and RSNY. The RSON and RSNT sites are characterized by simple crusts with fairly abrupt crust-mantle boundaries. The crust beneath RSON appears to have a clear division between the upper and lower crust at about 18 km depth while this boundary is not well-developed at RSNT. Pronounced azimuthal variations in crustal structure at RSSD (Black Hills, South Dakota) prohibit the determination of velocity using a layered earth model. The crustal thicknesses at each of these sites are: RSCP, 40 to 50 km; RSSD, 47 to 50 km; RSNY, 45 to 50 km; RSNT, 38 km; and RSON, 42 km.

SABRE observations of E-region vertical velocity structures

The variation of backscatter intensity with slant range for the Wick radar of the SABRE system consistently exhibits a minimum at approximately 800 km. Theoretical modelling of the antenna characteristics indicates that this is due to a null in the vertical polar diagram due to interference from ground reflections (Lloyd's mirror effect). By means of this “null” feature, the vertical velocity distribution within the scattering layer can be examined. A sharp discontinuity often exists in the variation of the Doppler velocity with range across the backscatter minimum. These observations can be accounted for in terms of an increase in the drift velocity of about 200 m s −1 with height across the 10 km range of the scattering layer. These results are consistent with the theoretical predictions of St.-Maurice and Schlegel based on the assumption of a two-stream instability.

Determination of large‐scale velocity structure of the crust and upper mantle in the vicinity of Pavlof Volcano, Alaska

Pavlof Volcano is a 2715‐m‐high stratovolcano located at latitude 55.4°N and longitude 161.9°W, near the western end of the Alaska Peninsula. In this study the large‐scale lateral perturbations to a vertical reference velocity structure in the vicinity of Pavlof Volcano are determined using modified versions of standard three‐dimensional inversion techniques. We use as the primary data set P wave arrival times and residuals from shallow (<40 km deep) regional earthquakes located by the Shumagin Islands seismic network. Relative residuals mainly from refracted (e.g., Pn) arrivals allow us to constrain models for a laterally varying crustal velocity structure. Several strong ray path dependent patterns are seen in the residuals; the most pronounced is that rays passing through the crust beneath the volcano are delayed (positive residuals) by up to 1.0 s, while those rays traveling subparallel to and behind the volcanic axis on the retroarc (Bering Sea) side arrive as much as 0.5 s early (negative residuals). The corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with Emmons Caldera (10 km SW of Pavlof). Computed velocity values are about 12–14% lower than in the surrounding crust and are comparable to values found at other volcanic areas. The exact shape and position of the low‐velocity body and its suitability for geothermal power development have not yet been sufficiently determined; for such an assessment a much denser station distribution is required.

An Investigation of the Development of Cumulonimbus Systems over South Florida. Part II: In-Cloud Structure

Abstract The in-cloud structure of radar reflectivity and vertical velocity from Doppler radar measurements are described for two cumulonimbus systems that developed over the FACE-1975 surface mesonetwork area on a case study day, 25 August 1975. Results imply a strong interrelationship between updraft velocity and water loading in convective storm development and show the importance of boundary layer forcing in the development of convective clouds in the Florida environment. Analysis of the temporal evolution of cloud mass at 600-m intervals in the vertical shows that below 3 km the two cumulonimbus systems appeared to evolve to about the same size, with significant differences occurring above that level. The increase cloud mass at some levels above 3 km in the second convective system (System II) were more than twice as large as the increase in cloud mass for the first convective system (System I). The difference in the vertical structure of cloud mass in these systems appears to be caused by the differ...

Seismic evidence for an ancient rift beneath the Cumberland Plateau, Tennessee: A detailed analysis of broadband teleseismic P waveforms

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source‐equalized horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat‐lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high‐velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust‐mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zone between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.

The oceanological application of electromagnetic fields induced by synoptic currents

Summary. The determination of induced magnetic fields of synoptic ocean movements is considered in the case of horizontal uniformity of the bottom conductivity. The problem of calculation of hydrodynamic parameters is solved by means of induced fields. These appear to depend closely on the appearance of barotropic or baroclinic parts in the synoptic ocean movement. The different properties of fields from parts of the above permit us to separate hydrodynamic parameters of complex ocean movements into barotropic and baroclinic parts by means of magnetic measurements. In recent years there has been great interest in the study of synoptic ocean movement. In this paper we investigate the possibility of determining the values of the hydrodynamical parameters of such movement by means of induced magnetic fields. The magnetic field induced by synoptic currents depends on the spatial and time variation of the water velocity and on the Earth's electrical conductivity structure. In the P-plane approximation, the vertical structure of the water velocity is controlled by ocean density stratification (Monin, Kamenkovich & Kort 1974). The vertical structure of the phenomenon outlined above is described by the following boundary-value problem,

On the Vertical Structure of Velocity in the Eastern Equatorial Pacific

We report observations of the vertical structure of horizontal currents on the equator at 110°W in the eastern Pacific Ocean. Profiles indicate high, vertical-mode, deep currents with zonal velocities of up to 20 cm s−1 at a depth of 1500 m. The general similarity between our measurements and those reported in other equatorial regions suggest that such vertical structure is a ubiquitous equatorial phenomenon.

The influence of density variations on estuarine tides and circulations

Numerical experiments are carried out to investigate the influence of density variations on estuarine tides and circulations. The mathematical model, which has been previously published, is outlined. A detailed analysis of the tidal properties and circulations in an estuary is made for two cases. One case involved density variations while the other assumed a constant density. It was found that the discharge through any section, the tidal range, and the tidal phases were independent of the density structure. However, the actual tidal amplitudes, the mean elevation and the vertical structure of longitudinal velocity changed considerably in the various experiments. Both cases assumed the same coefficient of bottom friction and bathymetric schematization.