Diffluent Flow Research Articles

Three-Dimensional Structure and Evolution of the Moisture Field in the MJO

Abstract The large-scale circulation features that determine the structure and evolution of MJO-related moisture and precipitation fields are examined using a linear analysis protocol based on daily 850- minus 150-hPa global velocity potential data. The analysis is augmented by a compositing procedure that emphasizes the structural features over the Indo-Pacific warm pool sector (60°E–180°) that give rise to the eastward propagation of the enhanced moisture and precipitation. It is found that boundary layer (BL) convergence in the low-level easterlies to the east of the region of maximum ascent produces a deep but narrow plume of equatorial ascent that moistens the midtroposphere, while weakly diffluent flow above the BL spreads moisture away from the equator. Vertical advection of moisture from this plume of ascent accounts for the eastward propagation of the positive moisture anomalies across the Maritime Continent into the western Pacific. When the convection is first developing over the Indian Ocean, horizontal moisture advection contributes to both the eastward propagation and the amplification of the positive moisture anomalies along the equator to the east of the region of enhanced convection. Neither horizontal advection nor the net moistening from vertical advection and the apparent moisture sink exhibit significant westward tilt with height in the equatorial plane, but when they are superposed they explain the westward tilt of the moisture field. The strong spatial correlation between relative humidity and vertical velocity underscores the important role of equatorial wave dynamics in shaping the structure and evolution of the MJO.

Ice stream influence on West Greenland Ice Sheet dynamics during the Last Glacial Maximum

AbstractThis paper investigates the processes governing bedrock bedform evolution in ice sheet and ice stream areas in central West Greenland, and explores the evidence for a cross‐shelf ice stream at the Last Glacial Maximum (LGM). To the east of Sisimiut the formation of streamlined bedforms with high elongation ratios and high bedform density has been controlled by geological structure and topography in slow‐flowing ice sheet areas. At the coast, the effects of regional flow convergence, caused by coastal fjord orientation, routed ice into the Sisimiut/Itilleq area where it formed an ice stream onset zone. This funnelled ice into an offshore trough (Holsteinsborg Dyb), resulting in a southwesterly regional ice flow direction and the formation of a topographically routed ice stream (Holsteinsborg Isbrae). To the south of this, striae and bedform evidence show that local valley glaciers initially flowed east to west across the coast, but were later redirected by the Itilleq Fjord ice which turned southwestward due to diffluent flow and deflection by Holsteinsborg Isbrae. Roches moutonnées in this area have low elongation ratios and high bedform density, but do not provide unequivocal support for ice streaming, as they are a product of both bedrock structure and changes in ice flow direction, rather than enhanced flow velocities. Cosmogenic surface exposure ages limit maximum ice sheet surface elevation to ca. 755–810 m above sea level in this region. Such ice thickness enabled Holsteinsborg Isbrae to reach the mid/outer continental shelf during the LGM, and to contribute to the formation of a trough mouth fan and the Outer Hellefisk moraines. Initial deglaciation across this region was driven by rising sea level and increasing air temperatures prior to the Bølling Interstadial at ca. 14.5 cal. ka BP. Between 12 and 10 cal. ka BP both increased air and ocean temperatures post the Younger Dryas, and peak sea‐level rise up to the marine limit, caused accelerated thinning and marginal retreat through calving, although dating evidence suggests ice streams remained along the inner shelf/coast boundary until at least ca. 10 cal. ka BP, their longevity maintained by increased ice thickness and ice discharge. Copyright © 2009 John Wiley & Sons, Ltd.

Interaction between planetary-scale diffluent flow and synoptic-scale waves during the life cycle of blocking

In this paper, a new transient forced quasi-resonant triad interaction theory in a beta channel is proposed to investigate the interaction between planetary-scale diffluent flow composed of zonal wavenumbers 1–3 and synoptic-scale waves produced continuously by a synoptic-scale vorticity source fixed upstream of an incipient blocking region during the life cycle of blocking. It is shown that the superposition of initial three Rossby waves for zonal wavenumbers 1 (monopole), 2 (dipole), and 3 (monopole), which permit triad quasi-resonance, can represent an incipient blocking event. The synoptic-scale eddies may act to amplify the incipient blocking and to excite a blocking circulation with a strong meander, whose flow pattern depends on the initial amplitudes of the planetary waves and both the intensity and location of preexisting synoptic-scale waves. The onset (decay) of the planetary-scale split-flow blocking is mainly represented by a strong increase (decrease) in the amplitude of the zonal wavenumber 2 component, having a dipole meridional structure related to the preexisting synoptic-scale eddies. The typical persistence time of the model blocking was of about 20 days, consistent with observations of blocking patterns.

Vortex-Driven Sensitivity in Deformation Flow

Abstract A sensitivity mechanism for the interaction of two vortices in a two-dimensional deformation background flow is explored. A nonlinear model describing the vortex interaction up to a critical merging distance is developed. This model shows that in a confluent or diffluent background flow, two vortices can obtain a global minimum distance by counterclockwise turning and a local minimum distance by clockwise turning about each other. In a major portion of the phase space, the global minimum distance is smaller than the local minimum distance. Therefore, vortex merger is more likely to occur during the process in which the two vortices approach their global smallest distance. Analysis of the governing equations shows that there exists a “most sensitive line” of initial vortex positions. When an initial state is on one side of the most sensitive line, the two vortices can obtain their global smallest distance and thus have a higher chance for merger. Conversely, when an initial state is on the other side relative to the most sensitive line, the two vortices can only obtain their local smallest distance. Consequently, they have a lower chance for merger. When an initial state is close to the most sensitive line, a very small initialization error can put this initial state on the wrong side of the most sensitive line, and the forecast evolution of the pair of vortices may be very different from the observed evolution. Numerical simulations that support these theoretical results are shown.

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

This study investigates the marginal subglacial bedrock bedforms of Jakobshavns Isbrae, West Greenland, in order to examine the processes governing bedform evolution in ice stream and ice sheet areas, and to reconstruct the interplay between ice stream and ice sheet dynamics. Differences in bedform morphology (roche moutonnee or whaleback) are used to explore contrasts in basal conditions between fast and slow ice flow. Bedform density is higher in ice stream areas and whalebacks are common. We interpret that this is related to higher ice velocities and thicker ice which suppress bed separation. However, modification of whalebacks by plucking occurs during deglaciation due to ice thinning, flow deceleration, crevassing and fluctuations in basal water pressure. The bedform evidence points to widespread basal sliding during past advances of Jakobshavns Isbrae. This was encouraged by increased basal temperatures and melting at depth, as well as the steep marginal gradients of Jakobshavns Isfjord which allowed rapid downslope evacuation of meltwater leading to strong ice/bedrock coupling and scouring. In contrast to soft-bedded ice stream bedforms, the occurrence of fixed basal perturbations and higher bed roughness in rigid bed settings prevents the basal ice subsole from maintaining a stable form which, coupled with secondary plucking, counteracts the development of bedforms with high elongation ratios. Cross-cutting striae and double-plucked, rectilinear bedforms suggest that Jakobshavns Isbrae became partially unconfined during growth phases, causing localised diffluent flow and changes in ice sheet dynamics around Disko Bugt. It is likely that Disko Bugt harboured a convergent ice flow system during repeated glacial cycles, resulting in the formation of a large coalesced ice stream which reached the continental shelf edge.

Interpreting the Opposition between Two Block-Onset Forcing Mechanisms

The opposition between two block-onset forcing mechanisms, previously identified in midtropospheric analyses over the Southern Hemisphere midlatitudes, is analytically interpreted with an idealized model. These mechanisms are the interaction (Finter) between deformation and potential vorticity and the advection (Fadv) of meridionally varying potential vorticity. Weather systems of concern, primarily consisting of planetary- and synoptic-scale waves, mostly fall into two regimes of zonal and meridional wavenumber space in which the opposition between the two block-onset forcing mechanisms is analytically derived. A synoptic interpretation of this opposition is schematically presented within the framework of barotropic dynamics. It is found that whether blocking occurs in diffluent or confluent flow depends upon the critical wavelength associated with the geostrophic flow. Blocking tends to take place in the diffluent flow of long waves in which Finter dominates over Fadv. In addition, blocking also tends to occur in the confluent flow of relative short waves in which Fadv prevails over Finter. An investigation of Rossby wave phase speeds in one diagnosed case reveals a lengthening with time of the dominant wave until it reaches the stationary wavelength on the block-onset day. In this context blocking may be understood as a stationarity and thus persistence of one of the two block-onset forcing mechanisms.

A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part IV: Block Activity and Its Linkage with a Sheared Environment

Abstract In this paper, a basic-state flow that has a linear, weak meridional shear is used as a mean flow condition to see if the blocking activity is related to the state of the zonal mean flow based upon the envelope soliton theory proposed in Part I. It is found that an isolated coherent structure similar to a dipole block can arise from the resonant interaction between preexisting planetary- and synoptic-scale waves, but its asymmetry, intensity, and persistence depend strongly upon the horizontal shear of the basic-state flow prior to block onset. The cyclonic shear of the basic-state flow not only plays an important role in preventing the eastward spread of block energy, but also provides a strong diffluent flow as a precursor of block flow. In such an environment, a high amplitude dipole anomaly is maintained and the deformed synoptic-scale eddies tend to deflect northward. But the anticyclonic shear of a basic-state flow plays a reverse role, which causes deformed eddies to deflect southward. It is also found that positive westerly wind anomalies at both high and low latitudes and negative anomalies at middle latitudes are maintained by synoptic-scale eddies through the self-interaction of block. In this case, the zonal mean westerly wind is accelerated at high and low latitudes, and decelerated at middle latitudes. Only for a basic state with cyclonic shear is the meridional profile of the zonal mean wind during the onset of a dipole block in agreement with observations. In addition, it can be shown by computing scatter diagrams of potential vorticity against streamfunction in different sheared environments that an eddy-induced isolated block can exhibit a local free-mode characteristic. Two approximately linear functional relationships between potential vorticity and streamfunction are found to be probably attributed to synoptic eddies.

Contraction Rate and Its Relationship to Frontogenesis, the Lyapunov Exponent, Fluid Trapping, and Airstream Boundaries

Abstract Although a kinematic framework for diagnosing frontogenesis exists in the form of the Petterssen frontogenesis function and its vector generalization, a similar framework for diagnosing airstream boundaries (e.g., drylines, lee troughs) has not been constructed. This paper presents such a framework, beginning with a kinematic expression for the rate of change of the separation vector between two adjacent air parcels. The maximum growth rate of the separation vector is called the instantaneous dilatation rate and its orientation is called the axis of dilatation. Similarly, a maximum decay rate is called the instantaneous contraction rate and its orientation is called the axis of contraction. These expressions are related to the vector frontogenesis function, in that the growth rate of the separation vector corresponds with the scalar frontogenesis function, and the rotation rate of the separation vector corresponds with the rotational component of the vector frontogenesis function. Because vorticity can rotate air-parcel pairs out of regions of deformation, the instantaneous dilatation and contraction rates and axes may not be appropriate diagnostics of airstream boundaries for fluid flows in general. Rather, the growth rate and orientation of an airstream boundary may correspond better to the so-called asymptotic contraction rate and the asymptotic axis of dilatation, respectively. Expressions for the asymptotic dilatation and contraction rates, as well as their orientations, the asymptotic dilatation and contraction axes, are derived. The asymptotic dilatation rate is related to the Lyapunov exponent for the flow. In addition, a fluid-trapping diagnostic is derived to distinguish among adjacent parcels being pulled apart, being pushed together, or trapped in an eddy. Finally, these diagnostics are applied to simple, idealized, steady-state flows and a nonsteady idealized vortex in nondivergent, diffluent flow to show their utility for determining the character of air-parcel trajectories and airstream boundaries.

A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part II: Role of Westward-Traveling Planetary Waves

Abstract The role of westward-traveling planetary waves in the block onset and the deformation of eddies during the interaction between synoptic-scale eddies and an incipient block is first examined by constructing an incipient block that consists of a stationary dipole wave for zonal wavenumber 2 and a westward-traveling monopole wave with constant amplitude (C wave) for zonal wavenumber 1 or 2. It is shown that the C-wave can affect the onset and strength of blocking through influencing the preblock (diffluent) flow even though it does not affect the amplification of the dipole wave associated with the synoptic-scale eddies. Whether the storm tracks organized by the deformed eddies deflect northward depends upon the zonal wavenumber, amplitude, and phase of the C wave relative to the stationary dipole wave. A typical retrograde blocking anticyclone can arise through the interaction of an incipient block with synoptic-scale perturbations when the C-wave ridge with zonal wavenumber 1 shifts westward from the east of the dipole wave in an incipient block. In this process, a slight northward deflection of organized storm tracks is also observed, particularly under the condition of a large-amplitude C wave. In addition, the interaction between a diffluent flow, consisting of a coupled dipole and monopole waves, and upstream synoptic-scale eddies is investigated. It is found that the eddy forcing tends to not only periodically amplify the dipole soliton and to retard its eastward movement, but to make the monopole wave break up. The breaking of the traveling monopole wave will suppress the eddy-induced blocking ridge that exhibits a surf zone structure where the negative meridional gradient of planetary-scale potential vorticity exists and cause the planetary-scale blocking field to lose its closed circulation compared to that without coupling.

A diagnostic study on the environmental influence of a mesoscale convective system over southern China in Meiyu season

¶During the Post-TAMEX forecast experiment of Taiwan in 1992, a mesoscale convective system (MCS) developed on June 5–6 over southern China. As this system matured, it produced readily apparent cirrus outflow on satellite imageries while the upper level flow also exhibited a diffluent pattern. The purpose of the current study is to examine the possible changes in its environment associated with the development of this MCS. By using 12-h data from 1200 UTC June 5 to 1200 UTC June 6, objective analyses were performed for a 1°×1° latitude/longitude grid using sounding data and a low-pass filter. To facilitate the diagnosis, a band-pass filter was further applied to separate mesoscale features from macroscale ones, while the apparent heat source and apparent moisture sink defined by Yanai et al (1973) were also calculated. Results suggest that the MCS exerted clearly discernable effects on its environment. The latent heat release led to the development of a warm core and mesoscale high-pressure disturbance at upper levels when the system matured. Ageostrophic winds and diffluent flow patterns together with strong anticyclonic vorticity at 200 hPa near the MCS were associated with the mesohigh. After the mature stage, weak cooling occurred above 350 hPa, likely due to radiative emission from the cloud top. However, a mid-level cyclonic vortex, often present in MCSs over the North America, was not apparent here due to weak environmental vorticity and small Coriolis parameter f. The level of maximum divergence was initially located at 500 hPa, but rose to 200 hPa as the MCS matured. In response, the upward motion not only intensified, but the level at which strongest rising occurred also ascended from 700 to 350 hPa. Results from the apparent heat source and moisture sink calculation suggest that this slow ascent of maximum heating was partially due to vertical transport of sensible heat by updrafts. During the MCS’s mature stage, under the stratiform clouds to the west of the strongest convection, a cold mesohigh formed at the surface due to evaporative cooling in downdrafts, and a gust front appeared along the leading edge of the outflow boundary. A trailing mesolow was also observed, likely due to near-adiabatic warming in drier downdrafts since no precipitation was associated with it.

Upper-Level Frontogenesis Associated with the Birth of Mobile Troughs in Northwesterly Flow

Previous studies have shown that 500-hPa mobile trough births (or genesis) occur preferentially in northwesterly flow during upper-level frontogenesis, and that cold advection assists in, and is a product of, mobile trough intensification. This study focuses on the synoptic environments and thermal-advection patterns of upperlevel fronts associated with mobile trough births in northwesterly flow. A climatology of 186 such events, derived from an earlier study by Sanders, shows that most births tend to occur within uniform or diffluent flow and that most tend to be associated with relatively weaker 500-hPa thermal advection. Most mobile trough births in diffluence, however, tend to be associated with increasing 500-hPa cold advection, typically indicated by a cyclonic rotation of isentropes, whereas, most mobile trough births in confluence tend to be associated with weaker 500-hPa thermal advection. Two cases of upper-level frontogenesis associated with mobile trough genesis—one in diffluence and one in confluence—are compared to determine the processes acting to produce the differing thermal-advection patterns at 500 hPa. A thermal-advection tendency equation is developed and shows that the magnitude of the temperature advection can be changed by accelerating the advecting wind speed or by changing the temperature gradient (i.e., vector frontogenesis). The latter can be accomplished either by changing the magnitude of the temperature gradient (the frontogenetical component Fn, also known as scalar frontogenesis) or by rotating the direction of the temperature gradient relative to the flow (the rotational component Fs). The dominant processes acting on Fn for the diffluence and confluence cases are tilting and deformation frontogenesis, respectively. The dominant process acting on Fs for the diffluence case is rotation of the isentropes due to the vorticity term, whereas rotation of the isentropes due to the vorticity and tilting terms are both important for the confluence case. The rotational component of frontogenesis is cyclonic downstream of the vorticity maximum for both cases, favoring increasing cold advection downstream of the vorticity maximum. For both cases, the rate of rotation of the isentropes at a point due to horizontal advection is large and that due to vertical advection is negligible. Since advection can only transport the existing isentrope angle and cannot change the isentrope angle, the rotational component of frontogenesis normalized by the temperature gradient is the only term that can increase the isentrope angle following the flow. This term dominates in the diffluence case but is small in the confluence case. This diagnosis suggests the following reasoning. In diffluent flow, the vorticity associated with the incipient trough is compacted into a more circular shape and intensifies. The potent vorticity maximum leads to robust isentrope rotation. In confluent flow, however, the vorticity is deformed into an elongated maximum, inhibiting both strong isentrope rotation and increasing cold advection. Thus, the rotational frontogenesis component explains the rotation of the isentropes that is responsible for the differing thermal-advection patterns. Diagnosis of these cases supports the results from the climatology indicating a strong relationship between the synoptic environment and the upper-tropospheric thermal-advection pattern. Nevertheless, current conceptual models of upper-level frontogenesis do not fully explain the variety of these features in the real atmosphere. In particular, mobile trough genesis and its associated upper-level frontogenesis can occur in weak 500-hPa thermaladvection patterns, in contrast to the confluence and cold advection that have been previously identified as important to upper-level frontal intensification. This result provides further support for the possibility that generation and intensification of mobile troughs can occur by barotropic processes.

Rapid cyclogenesis over Poland on 28 March 1997

A small wave on the polar front crossed the Atlantic and experienced strong explosive cyclogenesis on 28 March 1997 when passing over the North Sea. It then progressed eastwards over northern Poland causing severe damage and the death of at least 11 people. A phenomenological study of the event is presented based on a potential vorticity (PV) approach supplemented by a synoptic analysis. Results confirm previous findings about the impact of a dry stratospheric intrusion on the process of explosive cyclogenesis and fit well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. Also apparent is the role of potential vorticity advection, which is associated with the rapid change of flow pattern in the upper atmosphere. The careful analysis of potential vorticity fields every 100 m suggests that further research is required into fine mesh analysis and wind field retrieval. Copyright © 1999 Royal Meteorological Society

The Southern Margin of the Late Cainozoic Ice Cap on the Central Plateau of Tasmania

The easternmost extremity of the ice cap that developed in the Tasmanian Central Highlands during the time of most extensive Late Cainozoic glaciation lay on the doleritecapped Central Plateau east and north-east of Lake St Clair. During the Last Glacial Maximum (LGM), the more restricted ice cover included a small discrete ice cap (probably less than 250-300 m thick) that formed on the Central Plateau. The LGM ice limits on the southern part of the Central Plateau, including all five southern outlet valleys, are reported here. Earlier ice limits have been identified in two of these valleys, but on the plateau proper earlier glacial deposits have been generally extensively reworked beyond the LGM limit, such that confirmation of a glacial origin for diamictons on slopes is difficult. South of the plateau, the oldest deposits flooring lower reaches of two outlet valleys indicate that ice flowed southwards directly from the plateau, but later deposits indicate diffluent flow from the Derwent Glacier.

Predictability of the Onset of Blocking and Strong Zonal Flow Regimes

Abstract Flows with high and low sensitivity with respect to the initial conditions for onset of blocking (BL) and strong zonal flow (SZF) regimes have been analyzed. The author has considered BL and SZF regimes at 20°W (Atlantic region) and at 150°W (Pacific region). The BL and SZF regimes are characterized by the same dipolelike anomaly pattern but with opposite signs. Experiments have been performed with a three-level quasigeostrophic model triangularly truncated at wavenumber 21 (T21QG) and its tangent linear and adjoint versions. The sensitivity is calculated by perturbing the reference flow with perturbations that optimally trigger the onset of a BL or SZF regime after a prescribed forecast time. For forecast times larger than 3 days an iterative technique is used to take into account nonlinear growth of the perturbations. The flows with a high sensitivity show an intensified jetstream to the west of a diffluent flow. The strong jetstream by itself results in large perturbation growth. The presence ...

Nonlinear adjustment of a rotating homogeneous atmosphere to zonal momentum forcing

Idealized numerical simulations using a simple shallow water model are performed to study ageneralized Rossby adjustment problem which focuses on the nonlinear response of a rotating, uniform, homogeneous, barotropic zonal flow to meso-α and β scale zonal momentum forcing. The prescribed forcings propagate downstream at a speed (c) which is less than the basic stateflow speed (U), and represent the local effects of momentum deposition/redistribution attributableto a variety of physical processes. For small Rossby number flow and t≤ τ =2a/(U — c), the near-field response to meso-α scale forcing in the moving frame of reference is to producelocalized zonal jets of finite longitudinal and latitudinal extent whose geometries are similar tothe imposed forcing structure. The perturbation mass (height) field adjusts to the wind fieldassociated with these disturbances. Although the free surface vertical motion is dominated bytransient inertia-gravity waves at early times, well-defined localized vertical motions also formin the vicinity of the forcing center. For isolated forcing, ascending and descending verticalmotion occurs south and north of the forcing center, respectively. For dipole forcing, a fourcellpattern of vertical motion characterized by ascent in the southwest and northeast quadrantsand descent in the northwest and southeast quadrants flanks the forcing center where a pairof easterly and westerly jets form. For t >t, the exit region of the localized zonal jet producedby isolated forcing is advected downstream, carrying portions of the meridional perturbationwinds and free surface displacement fields with it. The long term asymptotic response is azonally elongated, synoptic scale jet due to the temporally continuous relative vorticity generatedby the zonal momentum forcing. A divergent cross-stream ageostrophic flow in the jetentrance region produces an isolated region of ascending vertical motion which is compensatedby weaker regions of descent to the east and west of the forcing center. The easterly jet producedby flow deceleration in the exit region of the dipole forcing is advected downstream during thesame time period. A four-cell pattern of vertical motion accompanies this easterly jet. Theresponse in the vicinity of the forcing center is an isolated meso-α scale westerly jet, withmeridionally confluent flow in its entrance region and meridionally diffluent flow in its exitregion. The ageostrophic circulation produces rising motion in the jet entrance region andsinking motion in the jet exit region. For moderately large Rossby number flow and meso-bscale dipole forcing, a mesoscale cyclone forms in response to fluid parcels being displacedsouthward into deeper fluid around a ridge in the height field. The moderately strong meso-bscale zonal wind maximum which is produced has associated vertical motions whose geometryis similar to those produced by larger meso-α scale dipole forcing. Stronger nonlinear advectionallows the meso-β scale jet to form four times sooner than the westerly jet produced by smallerRossby number meso-α scale dipole forcing.

North atlantic blocking during January 1979: Linear theory

AbstractThe second half of January 1979 was a time of persistent and large‐scale blocking in the North Atlantic region. In this study we examine the instability properties of the flow during this period and relate the growing modes to the development that occurs. We have analysed the growth rates, phase frequencies and structures of the ten fastest‐growing modes, within a five‐level quasi‐geostrophic model incorporating spherical geometry, on each day for the period between 10 and 30 January 1979.Large‐scale equivalent barotropic dipole or multipole instability modes are prevalent in the North Atlantic during the second half of January. These modes are stationary or slowly propagating and grow rapidly with efolding times as short as 1.7 days in some cases. We find that there is a close connection between these modes and the dynamical developments observed subsequently a few days later. Associated with the early stages of block formation, there are large‐scale eastward propagating westward tilting onset‐of‐blocking wave‐train modes. the onset‐of‐blocking modes also appear to be associated with rapid cyclogenesis off the east coast of North America.The formation of the North Atlantic blocking system and diffluent flow also produces corresponding changes in the storm tracks. During the early parts of this period the fastest‐growing monopole cyciogenesis instability modes have regions of preferential development in the climatological storm tracks over the Pacific, Atlantic and Siberian regions. Subsequently, however, the Atlantic cyclogenesis modes split into wave trains poleward and equatorward of the block.In the developments during this period, we also examine the roles of the adjoint eigenmodes and of optimal perturbations which produce the largest initial tendency of the L2 norm squared of the streamfunction.

Atmospheric conditions during the spring and fall transitions in the coastal ocean off western United States

We examine large‐scale atmospheric behavior around the time of the spring and fall transitions in the coastal ocean off the west coast of North America. Records of adjusted sea level (ASL), coastal wind stress, sea level atmospheric pressure (SLP), and 500‐mbar heights for the years 1971–1975 and 1980–1983 are analyzed. The records cover periods of 91 days, centered on the dates of the spring and fall transitions as determined from coastal adjusted sea level data. Empirical orthogonal functions are obtained from the ASL, coastal wind stress, and SLP records. The two dominant modes of the ASL and coastal wind stress are similar around the times of both the spring and fall transitions, and the time series for these modes are highly correlated with one another. Transitionlike behavior is evident in the time series of the first modes in both spring and fall, but the spring transition is more pronounced. Principal estimator patterns, formed from the dominant empirical orthogonal functions show the spatial patterns of SLP which force the ASL and coastal wind stress during the transitions. The SLP pattern which coincides with the spring transition is the formation of a high‐pressure system centered at 45°N and 140°W along with the development of a low‐pressure cell over the southwest continental United States. Inspection of the 500‐mbar height composites for 91 days surrounding the spring transition for the 9 years reveals the formation of a ridging pattern and diffluent flow over the western United States at the time of the transition; following the transition, the ridging relaxes but the diffluent flow over the continent remains for the duration of the 45 days examined here. The fall transition is characterized by a rise in ASL, particularly north of 40°N, and a change from southward to northward wind stress. The SLP pattern which coincides with the fall transition involves the appearance of a low‐pressure system off western North America centered at 50°N and 140°W, representing the passage of synoptic storms through the region. Prior to the fall transition, the 500‐mbar heights are somewhat diffluent and show a trough over the southwestern United States; after the transition the 500‐mbar flow over the northeast Pacific and North America is nearly zonal.

On cyclogenesis in the lee of the Canadian Rocky Mountains

Cyclogenesis on the lee slope of the Canadian Rocky Mountains is examined in the light of current theories of cyclone initiation and development. Cyclogenesis is closely associated with vorticity advection in themiddle and upper troposphere, but the basic cause of lee cyclone initiation is net mass divergence in the overlying air. In the course of this study it was observed that most lee cyclones formed initially under the eastern margin of a 500-mb, orographically intensified, diffluent cross-barrier flow, superimposed on a zone of low-level convergence and orographic descent. However, sudden intensification of mobile cyclones usually commenced with the onset of positive vorticity advection, in accordance with Petterssen's hypothesis.

COMPARISON OF RESULTS OF THE HURRICANE DEBBIE (1969) MODIFICATION EXPERIMENTS WITH THOSE FROM ROSENTHAL'S NUMERICAL MODEL SIMULATION EXPERIMENTS

Comparison of the wind-speed profiles, the outer wind envelopes, pressure changes, and possibly the temperature differences that characterized hurricane Debbie before, during, and after the Project STORMFURY seeding experiments of August 18 and 20, 1969, with those calculated in the model simulation experiments of Rosenthal reveals many similarities. Some of the excessive diminution of wind speed noted following the experiment of August 18 may have been due to the vigor and proximity of a synoptic scale trough that appeared to interfere with the upper level diffluent flow from the hurricane.