Vertical Motion Field Research Articles

Energetics of Large-Scale Motion in the Tropics During GATE at 250 mb

Use is made of a mixture of observing bases to provide a data set for the global tropics. This data set allows a reasonably good delineation of the tropical motion field at three levels, 200. 250 and 300 mb. An iterative procedure, used to initialize the Florida Side University multilevel primitive equation prediction model, is used to obtain potential fields which provide a temperature field at 250 mb. The kinematic method is used to obtain vertical motion fields. Computations have been carried out for the conversion of available potential energy to kinetic energy for the first 15 wavenumbers at 250 mb. The largest conversions occurred for wavenumbers 1 and 2. Computations were also carried out for the wave-wave and wave-zonal-trasnfers of available potential energy and kinetic energy at same level. When these waves are divided into the three groups, long, medium and short, the results show that in the intergroup interactions for APE, the long waves receive energy from the shorter scale waves, whereas in the KE interactions, the long waves supply energy to the shorter scale waves. In the wave-zonal interactions all of the shorter scale waves received energy from the zonal flow both for APE and KE.

Mesoscale Wind Fields for a Severe Storm Situation Determined from SMS Cloud Observations

Abstract An attempt is made to obtain fully three-dimensional mesoscale wind fields from satellite cloud displacement data for an area over the continental United States. A method to derive such fields and their likely accuracy is discussed prior to the presentation of a test case for 30 October 1974. The computed divergence and vertical motion fields are consistent with features of the observed mesoscale weather systems, particularly the locations of subsequent severe convective storms.

The Structure and Energetics of Transient Disturbances in the Northern Hemisphere Wintertime Circulation

Abstract The hemispheric distributions of wintertime circulation statistics derived from the forecast fields of vertical motion are presented. The dominant features in the pattern for time-mean vertical velocity are consistent with the existence of thermally direct meridional circulations over the entrance regions of the principal jet streams, and thermally indirect circulations over the jet exit regions. Rising motions in the transient disturbances are seen to display positive temporal correlations with temperature and geopotential height over the major oceanic storm track regions. On the other hand, the western portion of the continents and the adjacent oceanic areas are characterized by downward eddy transports of geopotential energy at 850 and 500 mb, as well as much reduced temporal correlations between the vertical motion and temperature fields. The vertical phase structure of the transient disturbances at various geographical locations is studied by performing a cross-spectral analysis of the time ...

The application of non‐linear normal mode initialization to an operational forecast model

Abstract The non‐linear normal mode initialization technique used in shallow water equation models by Baer (1977) and Machenhauer (1977) is now applied to a full baroclinic primitive equations forecast model. The initialization procedure is shown to be capable of completely removing high frequency oscillations from model integrations, even in the presence of topography. The procedure also produces a consistent and physically realistic initial vertical motion field.

The Observed Structure of Tropospheric Stationary Waves and the Local Balances of Vorticity and Heat

The physical structure and the associated transport properties of stationary waves in the troposphere are described using circulation statistics compiled from twice-daily hemispheric analyses covering 11 winters. The distributions of standing eddy meridional transports in middle latitudes are characterized by momentum flux convergence and equatorward geopotential energy transports in the upper troposphere, and by poleward heat fluxes at the lower levels. The contributions of steady and transient motions to the local, time-averaged budget of vorticity at 300 mb are evaluated. The dominant terms in the time-averaged vorticity equation are the local advection of relative vorticity by the stationary flow and the divergence term. The advection of planetary vorticity by the mean flow (the β-effect) and the convergence of vorticity fluxes by transient eddies appear to be of secondary importance. The hemispheric distributions of the principal terms in the vorticity balance are closely related to topographical features at the lower boundary. The hemispheric field of stationary flow divergence at various levels is determined as a residual in the time-averaged vorticity balance. This diagnosed divergence field is used to 1) demonstrate the feasibility of retrieving essential stationary flow features in the upper troposphere through solution of the linearized vorticity equation with a prescribed divergence forcing; 2) deduce the velocity potential field; and 3) compute the mean vertical motion field through vertical integration of the continuity equation. Standing eddy statistics involving vertical motions are described. Mean vertical motions in middle latitudes are found to be positively correlated with mean meridional motions and with mean temperature. The distributions of meridional and vertical transports of geopotential energy and westerly momentum in the meridional plane are presented in a vectorial format. The pattern depicting geopotential energy fluxes suggests that the enhanced standing wave kinetic energy over the subtropics is maintained by geopotential energy transports which originate from higher latitudes. A diagnosis of the local, time-averaged balance of heat at 1000 and 700 mb is performed. The heat transports by the transient eddies in the lower troposphere exhibit a strong tendency to destroy the zonally asymmetric component of the stationary temperature field. This dissipative mechanism acts on a time scale of several days. The hemispheric distributions of the diabatic heating deduced from the heat budget are indicative of the central role of geographically fixed influences such as ocean currents and sea-land contrast.

The Eulerian-and Lagrangian-Mean Meridional Circulations in the Stratosphere at the Time of a Sudden Warming

Abstract The Eulerian-mean (usual zonal mean) and the Lagrangian-mean (an average in the zonal direction along a curved “material line,” consisting of definite individual air particles) meridional circulations are investigated, given an upward propagating planetary wave incident on a critical level. The Eulerian-mean circulation is such that an upward motion appears at higher latitudes, a downward motion at lower latitudes, and an equatorward meridional flow near the critical level. The Lagrangian circulation is quite different from the Eulerian one. A strong poleward meridional flow appears concentrated at the critical level. In connection with this meridional flow, a Lagrangian-mean vertical motion occurs which diverges from this level at higher latitudes, i.e., it consists of a downward motion below the critical level and an upward motion above the level. At lower latitudes, vertical motions converge toward the critical level. The vertical motion field of this four-sector structure in the meridional se...

Triple-Doppler Observations of a Convective Storm

Abstract The advantages of three or more radars is illustrated in terms of extended areal coverage, error variance reduction and mitigation of poor sampling effects of divergence fields near the ground. Errors in the horizontal and vertical particle motion fields are illustrated for a three-, four- and five-radar network, and the importance of experimental design is illustrated. A variationally formulated multiple-Doppler radar analysis, which includes data from a surface network, and an illustrative analysis of storm data are presented.

The Life Cycles of GATE Convective Systems

Abstract The life cycles of GATE squall lines and loosely organized cloud clusters are analyzed and documented using radar and composited rawinsonde data. Time variations of the temperature, moisture, wind and vertical motion fields are presented for both types of systems. The convective systems are usually triggered by the approach of a middle-level trough in the easterlies. Low-level convergence increases prior to intensification of convection, possibly by several hours. Once the convection begins, the systems largely cause the changes in their upper and lower level divergence profiles through cumulus-induced and radiational heating. Squall line and cluster systems are found to be essentially similar except for their propagation speeds and their vertical wind shears.

Hemispheric asymmetry of oceanic heat budget in the Equatorial Atlantic and Eastern Pacific

The oceanic heat budget is studied on the basis of ship observations during 1911–70 compiled by one degree square areas. The clockwise turning cross-equatorial surface air flow is conducive to upwelling to the South and downwelling to the North of the Eastern Pacific and Atlantic Equator, particularly during Northern summer. The wind-induced vertical motion field is considered a factor in maintaining the cold water tongues immediately to the South, and the strong temperature increase northward across the Equator. Net radiation is only slightly higher in the Southern hemisphere equatorial zone, but sensible and latent heat flux are greatly reduced in the cold water regions, especially in Northern summer. As a result, the ocean exports heat at the rate of 53 and 62 W m -2 to the South of the Atlantic and Pacific Equator, respectively, compared to an export of only 29 and an import of 3 W m -2 in the Northern hemisphere equatorial zone. DOI: 10.1111/j.2153-3490.1977.tb00764.x

Kinematic analyses over eastern Asia during a subperiod of the air mass transformation experiment 1975

This study examines the large-scale vorticity and vertical motion fields over eastern Asia during a subperiod (13–16 February) of the Air Mass Transformation Experiment (AMTEX) 1975. This represents a period of cyclogenesis and subsequent formation of mesoscale cellular convection (MCC) over the AMTEX region. Vertical motion and vorticity fields reflect the developing cyclone and a short synoptic-scale upper wave which propagated across the computational region. Results for the AMTEX region reveal that during MCC occurrences large-scale downward motion and a change from negative to positive vertical gradient of vorticity in the lower troposphere occur. DOI: 10.1111/j.2153-3490.1977.tb00750.x

The Structure and Energetics of the Tropical Cyclone I. Storm Structure

A composite study of 10 years of northwest Pacific rawinsonde data is used to analyze the large-scale structure of tropical cyclones. The temperature, height, moisture, wind and vertical motion fields are analyzed for various storm regions. Mean soundings for all regions from the eye through 12° radius are presented. Rainfall characteristics of the area inside 4° radius are discussed. Hurricane flight data are used to augment the analyses in the inner regions. Many important features are noted. Strong persistent asymmetries in storm structure exist, particularly at large radii. The storm circulation has very broad horizontal extent and appears to conform to a constant scale regardless of inner core intensity. Inflow in the middle troposphere is substantial from 4° outward. A mean subsidence region is observed from about 4–6° radius. Humidities are extremely high in the inner regions, and conditional instability exists everywhere outside the eye. Significant diurnal variations in rainfall and temperature are observed.

Kinematic Analysis and Air-Sea Heat Flux Associated with Mesoscale Cellular Convection during AMTEX 75

Several excellent examples of open and closed mesoscale cellular convection (MCC) were observed in DMSP satellite imagery during the second field period of the Air Mass Transformation Experiment (AMTEX), conducted 14–28 February 1975 over the East China Sea in the vicinity of the Kuroshio current. The kinematic fields of horizontal divergence, vertical motion and vorticity of the large-scale flow producing the MCC and the associated heat flux at the air-sea interface were determined to starch for observational evidence as to the cause of circulation direction in the convection cells. None of these kinematic fields had features unique to open and closed cells, with predominantly downward vertical motion and diverging horizontal flow in the convecting layer. From the study of the kinematic fields determined for 14 cases of open cells and 20 cases of closed Cells a physical depiction has been made of the distribution of the vertical motion in the vertical plane of a mesoscale cell during its formation. Critical values of daily averaged air-sea temperature difference and surface wind speed as necessary conditions for the occurrence of MCC due to heating from below have been determined to be 1) sea surface temperature at least 5°C greater than air temperature and 2) surface wind speed in excess of 5 m s−1. Calculations of the sensible heat flux yielded a threshold value of ∼70 W m−2 for the onset of MCC. Similarly, the threshold value for the total heat flux was ∼200 W m−2. Calculations also showed that both open and closed cells occurred in the cold air outbreak when the sea to air energy flux (latent plus sensible) was either strong (∼1200 W m−2) or weak (∼200 W m−2).

Kinematic analyses over eastern Asia during a subperiod of the air mass transformation experiment 1975

This study examines the large-scale vorticity and vertical motion fields over eastern Asia during a subperiod (13–16 February) of the Air Mass Transformation Experiment (AMTEX) 1975. This represents a period of cyclogenesis and subsequent formation of mesoscale cellular convection (MCC) over the AMTEX region. Vertical motion and vorticity fields reflect the developing cyclone and a short synoptic-scale upper wave which propagated across the computational region. Results for the AMTEX region reveal that during MCC occurrences large-scale downward motion and a change from negative to positive vertical gradient of vorticity in the lower troposphere occur. DOI: 10.1111/j.2153-3490.1977.tb00750.x

Some Aspects of Cloud and Precipitation Features Associated with a Mid-Latitude Cyclone

Abstract A multi-level diagnostic model is used to study the physical and dynamical mechanisms that produce low-level clouds and light precipitation within the cold air sector on the west side of a Midwest cyclone. Computational results show frictional effects to be the primary mechanism in the absence of secondary upper level trough mechanisms or intense heating from the underlying surface. Also, this study shows that the vertical motion field may be substantially modified by secondary effects when the usually dominant contributions by vorticity and thermal effects are weak.

Probable Cause of a Significant Precipitation Episode in the Western United States

Abstract This synoptic study for the period 22 September to 3 October 1974 involves a case analysis of an unforeseen major precipitation episode. This apparently resulted from the merging of an inactive propagating short–wave trough that moved cast-northeastward out of the subtropics and an inactive extratropical low moving southeastward. Prior to the amalgamation of the two systems, weak upward vertical motion fields allowed only specks of high clouds to be associated with each. Almost immediately with the merging of the two systems, a rapid increase in upward vertical motion around the upper-level extratropical system took place, as evidenced on VHRR satellite imagery by the sudden development of rather organized middle and high cloudiness. Within 24 hours, a distinct vortex comprising all cloud levels was evident on satellite photographs. During the next 24 hours, a major rain-producing storm developed. The major impact of this storm was an abrupt end to the California dry season.

Hemispheric asymmetry of oceanic heat budget in the Equatorial Atlantic and Eastern Pacific

The oceanic heat budget is studied on the basis of ship observations during 1911–70 compiled by one degree square areas. The clockwise turning cross-equatorial surface air flow is conducive to upwelling to the South and downwelling to the North of the Eastern Pacific and Atlantic Equator, particularly during Northern summer. The wind-induced vertical motion field is considered a factor in maintaining the cold water tongues immediately to the South, and the strong temperature increase northward across the Equator. Net radiation is only slightly higher in the Southern hemisphere equatorial zone, but sensible and latent heat flux are greatly reduced in the cold water regions, especially in Northern summer. As a result, the ocean exports heat at the rate of 53 and 62 W m -2 to the South of the Atlantic and Pacific Equator, respectively, compared to an export of only 29 and an import of 3 W m -2 in the Northern hemisphere equatorial zone. DOI: 10.1111/j.2153-3490.1977.tb00764.x

The dynamics and thermodynamics of precipitation loading

A one-dimensional, time-dependent numerical cloud model is used to analyze the factors in the dynamic and thermodynamic equations which lead to a steady-state or nonsteady-state solution for the cloud vertical motion, buoyancy, precipitation, and cloud water fields. ‘Bulk water’ microphysical techniques are used for the cloud, rain, and hail variables. An atmospheric sounding from a severe storm situation is used as initial and environmental conditions, yielding model updrafts of 40 m sec−1 maximum and more than 10 m sec−1 over the entire cloud region.

Vertical motion in the Indian summer monsoon

The vertical motion field in the Indian monsoon region has been studied for mean July, a strong monsoon day (7 July 1933) and a weak monsoon day (19 July 1963). We have used the quasi-geostrophic omega equation for this purpose, In addition to forcings due to thermal and differential voriticty advection, the vertical motion due to orography and friction have been included. The vertical motions due to the different forcing functions have been computed. The cumulative omega field due to all forcing functions and due to individual forcing functions together with their vertical profiles are delineated. Conversion from potential to kinetic energy associated with the vertical circulations have been computed and presented in this paper, It is seen that the meridicnal circulation in the y-p plane is very marked on 7 July with ascent over north India and descent to the south. On 19th, however, uniform descent is seen everywhere. There is conversion from zonal available potential energy to zonal kinetic energy on the day of active monsoon. The conversion is much smaller on the day of weak monsoon. Other energy conversions are also discussed.

Development of Banded Structure in a Numerically Simulated Hurricane

The developments of the propagating and the stationary bands in a three-dimensional model of a hurricane (Isbell, 1964) are investigated. Propagating bands in the vertical motion fields in the middle and the upper troposphere form in the regions of strong heating in the upper troposphere and weak cooling in the middle troposphere. The structures of the wind, temperature and pressure fields in these bands are similar to those observed in the outer radar bands of hurricanes. Strong, nearly stationary bands form close to the center in the intense storm stage. Results of two experiments, one (M1) in which the so-called nonconvective release of latent heat in the upper troposphere is included and the other (M2) in which this heating is not incorporated, are compared. Convective release of latent heat is included in both experiments. The stationary bands in which form in M1, also develop in M2. The propagating bands which form in M1, however, do not develop in M2. The rate of intensification of the simulated storm in M1 is nearly the same as observed in Isbell; it is, however, significantly weaker in M2. It is shown that the inclusion of nonconvective release of latent heat in M1 enhances the upper tropospheric outflow which induces strong zones of convergence in the boundary layer. The resulting increase in the upward motion at the top of the boundary layer augments the convective release of latent heat and leads to a rapid intensification of the disturbance.

The influence of momentum advections on a well‐mixed layer

AbstractThis study examines the influence of weak horizontal advections of momentum on a well‐mixed planetary boundary layer. In the simple flow system examined, the direction and magnitude of flow modifications depend on the Rossby number, the nondimensional surface drag coefficient and the orientation of the geostrophic wind gradient field. It is found that advective modification of the boundary layer vertical motion field involves dependencies on the square of the local vorticity and dependencies on vorticity gradients. The mixed‐layer assumptions are relaxed for a special flow case to show that these results do not depend crucially on the mixed‐layer condition.