Potential Vorticity Anomalies Research Articles

Interannual variations of the Hawaiian Lee Countercurrent induced by potential vorticity variability in the subsurface

Interannual variations of the Hawaiian Lee Countercurrent (HLCC) in the 2000s were investigated using satellite and Argo profiling float observations. The satellite-observed sea surface height shows that the geostrophic eastward current was anomalously strong to the west away from Hawaii in 2003 and 2005. However, the trade winds and the orographic wind curl dipole in the lee of Hawaii that drives the climatological mean HLCC were not particularly strong in these years, suggesting that the accelerations of the HLCC were not caused by the wind stress curl forcing around Hawaii and subsequent Rossby wave propagation. Using Argo observations, we found negative potential vorticity (PV) anomalies in the subsurface north of the HLCC in these 2 years. The pycnocline is lifted northward as low PV waters of different densities stack up in the vertical, and the HLCC is then accelerated via the thermal wind. The intensification and/or southward intrusion of the eastern subtropical mode water and subtropical mode water seem to have induced negative PV anomalies in 2003 and 2005, respectively. Using high-resolution ocean simulations, we confirmed the migrations of PV anomalies and their contributions to the HLCC accelerations. Although the HLCC is located away from the cores of major mode waters, our results suggest that interannual variations of the HLCC are affected by those of mode waters.

A polar low named Vera: the use of potential vorticity diagnostics to assess its development

Abstract20 November 2008 the Norwegian Meteorological Institute issued an extreme weather warning for Trøndelag County. A storm was expected in the afternoon. The storm, a polar low (PL), was named ‘Vera’. Vera was the second of two polar lows that developed along a wedge of warm air at the rear of a synoptic‐scale low that moved northeastwards into northern Norway. The dynamical development may be explained by classic dynamical theory: low‐level warm air seclusion and shallow secondary circulation in a frontal zone that couples to transient upper level disturbances. The study focuses on the potential vorticity (PV) perspective of the PL development and we have modified the initial PV field to study the effect of each individual PV perturbation. Two PV and two low‐level temperature anomalies have been chosen. The PV anomalies retained their structure during the development while the temperature anomalies became less coherent and difficult to define by the end of the cyclogenesis period. By modifying the initial conditions in the upper troposphere over Greenland, it is shown that in this case the upper PV anomaly had the strongest effect on the development, and that this effect remained strong throughout the cyclogenesis until the polar low made landfall. The effect from the low level PV anomaly became large after onset of cyclogenesis, reflecting the positive feedback, although its impact on the development remained smaller than that of the upper PV anomaly. It is also shown that the topography of Greenland was important in determining the correct position of this polar low development. Copyright © 2011 Royal Meteorological Society

Recent changes in the Labrador Sea Water within the Deep Western Boundary Current southeast of Cape Cod

Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the western boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.

Dynamical Response of the South Asian Monsoon Trough to Latent Heating from Stratiform and Convective Precipitation

AbstractSimulation experiments using a simplified atmospheric GCM and supplementary diagnostic analyses of observations are performed to understand how the South Asian monsoon trough (MT) responds dynamically to latent heating from mesoscale convective systems (MCSs). Observations reveal that the MT during active monsoons is characterized by a deep cyclonic vorticity extending from the surface to 350 hPa and organized MCSs covering over 3500–4000 km along the Indo-Gangetic plains. The MCSs during active monsoons are composed of a relatively higher abundance of stratiform-type precipitation (mostly nimbostratus) as compared to the convective type. The results suggest that a stratiform-type heating profile is very effective in promoting upward development of continental-scale cyclonic circulation well above the midtroposphere over the MT region. The vertical development involves a dynamical uplift of the layer of cyclonic circulation and is induced by midlevel (600–500 hPa) convergence and vorticity stretching above 500 hPa. By varying the population of stratiform and convective rain types in the simulation, the horizontal scale of midlevel vorticity response is shown to increase significantly with stratiform population; in contrast, the midlevel response is more localized when the MCS is dominated by deep convective clouds. For large stratiform populations, the midlevel response is found to extend far westward up to the northern flanks of the African ITCZ, indicative of Rossby wave dispersion of PV anomalies that are generated near the level of maximum heating gradient. From the present findings, one can conclude that the vertical deepening of MT during active monsoons is not merely a localized phenomenon; instead it represents a large-scale dynamical response to organized MCSs that exert pivotal influence on the upward development of cyclonic circulation well above the midtroposphere.

Deformation and splitting of baroclinic eddies encountering a tall seamount

The transformation of baroclinic eddies encountering a tall seamount is explored using a three-layer primitive equation model on the β-plane. The topography is finite in that the seamount penetrates the isopycnal layer in which the eddy resides, but does not span the entire fluid depth. In our numerical simulations, the eddies are represented by potential vorticity anomalies in the upper and middle layers, and propagating towards the seamount due to the beta-effect. Circulations created near the topography, both by fluid removed from the seamount and by external fluid stranded over the seamount, play a key role in the drift, deformation, and erosion of the approaching eddies. When the radius of the seamount is small, the deviation of the eddy trajectory is well described by a simple kinematic model that does not take into account deformations of the vortex cores. For wider seamounts, such interactions may result in horizontal and/or vertical splitting of the vortex core, i.e., in increased occurrences of eddy destruction. In particular, an interesting mechanism is found, related to enhancement of topographic circulation by potential vorticity entrained from the vortex core in the middle layer, and resulting in strong deformations and splitting of the upper layer core. Numerical estimates of the transformed eddy structure indicate that topographic interactions provide powerful mechanisms for significantly influencing baroclinic eddy evolution. Our results are summarized in a specific nondimensional parameter space according to the eddy evolution.

The data assimilation method “Latent Heat Nudging” assessed with the Dynamic State Index

In April 2007, the German Weather Service (DWD) added the non-hydrostatic limited area model COSMO-DE to its model chain. COSMO-DE covers mainly Germany and bordering countries and has a horizontal resolution of 2.8 km (0.025°). An advantage of the COSMO-DE is that deep convection is expected to be resolved explicitly. In order to improve the initial state of COSMO-DE an assimilation of radar derived precipitation rates is applied by using 'Latent Heat Nudging' (LHN). The aim of LHN is to adjust the model state so that the model will respond by producing a rain rate close to the observed value. In this study, the influence of LHN on diabatic processes is investigated by evaluation of the Dynamic State Index (DSI). The DSI is calculated on both isentropic (DSI θ ) and model levels (DSI θ ). To analyse the influence of LHN, model analyses with and without LHN are examined for summer 2009 (June to August). The influence of LHN is also examined in more detail in a case study of a heavy rainfall event on July 18, 2009 where a strong rainfall area was developed on the front side of a trough. Results, based on a statistical investigation and the case study, show that the model with LHN can reproduce the observed rainfall better than the runs without LHN. This is also suggested by a higher correlation between DSI and observed precipitation. The release of latent heat enhances the diabatic processes associated with the formation of additional potential vorticity anomalies resulting in stronger DSI-signals.

Displacing Potential Vorticity Structures by the Assimilation of Pseudo-Observations

Abstract Classic formulations of variational data assimilation in amplitude space are not able to directly handle observations that measure the geographical positions of meteorological features like fronts and vortices. These observations can be derived from satellite images, as is already the case for tropical cyclones. Although some advanced data assimilation algorithms have been specifically designed to tackle the problem, a widespread way of dealing with this information is to use so-called bogussing pseudo-observations: user-specified artificial observations are inserted in a traditional data assimilation scheme. At the midlatitudes, there is a relationship between dry intrusions in water vapor images and upper-level potential vorticity structures. Some prior work has also shown that it was possible to automatically identify dry intrusions with tracking algorithms. The difference of positions between model and image dry intrusions could therefore be used as observations of the misplacement of potential vorticity structures. One strategy to achieve the displacement of potential vorticity anomalies is to sample them, and assimilate the values at displaced locations. The uncertainty of these pseudo-observations is left as a tuning parameter to try to make the displacement both effective and robust. A simple one-dimensional assimilation model is used to study the displacement of curves defined by Gaussian humps. The concept is then illustrated in realistic examples from real synoptic systems, where pseudo-observations of potential vorticity are incorporated in a global variational data assimilation scheme. Overall and despite reasonable optimization, the results contain artifacts. This suggests that the use of pseudo-observations to displace identifiable structures is not an effective strategy.

Coupling effect diagnoses of quasi-stationary mesoscale vortex in Guangxi rainstorm process of China

The mesoscale weather system which affected the Guangxi flash-flood-producing rainstorm of China in June 2008 is a quasi-stationary mesoscale vortex. Its genesis and development is closely related to the coupling effects of weather systems in different scales and different latitudes. On the one hand, the coupling of synoptic scale high- and low-level jets provides the environmental conditions for development of vortices and vertical circulations in the mesoscale vortex; On the other hand, the coupling of waves in mid-latitude westerlies and perturbations in low-latitude warm-moist flow under the influence of complex terrain makes the mesoscale vortex circulations strengthened. With the piecewise potential vorticity (PV) inversion method, PV anomalies in different regions are analyzed; also the vortex-vortex interactions and vortex-background flow interactions are diagnosed. Thus, the reasons why the mesoscale is quasi-stationary at first, while developing and deepening later are indicated. Under the condition of coupling effects, the vertical motions accompanied with the mesoscale vortex can be diagnosed with the PV-ω inversion system based on the analysis of quasi-balanced flow.

Ocean turbulence at meso and submesoscales: connection between surface and interior dynamics

High resolution simulations of ocean turbulence with Rossby number of order one have revealed that upper layer dynamics significantly differs from the interior dynamics. As reported before, upper layer dynamics is characterized with shallow velocity spectrum corresponding to kinetic energy distributed over a spectral range from mesoscales to small scales. This dynamics is driven by small-scale frontogenesis related to surface density anomalies. Interior dynamics is characterized by steeper velocity spectrum and is driven by the potential vorticity anomalies set up by the interior baroclinic instability. Impact of the divergent motions related to surface frontogenesis leads to a warming of the upper layers, a cyclone dominance and a negative skewness of the isopycnal displacements. On the contrary, divergent motions in the interior lead to a cooling of the deeper layers, an anticylone dominance and a positive skewness of the isopycnal displacements. These different ageostrophic processes are consistent with an SQG regime extended to Rossby number of order one on one hand and an interior QG regime extended to Rossby number of order one on the other hand, as proposed by previous studies. Synthesis of these characteristics suggest a connection between upper and deeper layers, induced by the divergent motions, through which small scales near the surface interact with mesoscales in the interior.

Multiscale Interactions in the Life Cycle of a Tropical Cyclone Simulated in a Global Cloud-System-Resolving Model. Part II: System-Scale and Mesoscale Processes*

Abstract The life cycle of Tropical Storm Isobel was simulated reasonably well in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), a global cloud-system-resolving model. The evolution of the large-scale circulation and the storm-scale structure change was discussed in Part I. Both the mesoscale and system-scale processes in the life cycle of the simulated Isobel are documented in this paper. In the preconditioned favorable environment over the Java Sea, mesoscale convective vortices (model MCVs) developed in the mesoscale convective systems (MCSs) and convective towers with cyclonic potential vorticity (PV) anomalies throughout the troposphere [model vortical hot towers (VHTs)] appeared in the model MCVs. Multiple model VHTs strengthened cyclonic PV in the interior of the model MCV and led to the formation of an upright monolithic PV core at the center of the concentric MCV (primary vortex enhancement). As the monolithic PV core with a warm core developed near the circulation center, the intensification and the increase in horizontal size of the cyclonic PV were enhanced through the system-scale intensification (SSI) process (the secondary vortex enhancement), leading to the genesis of Isobel over the Timor Sea. The SSI process can be well explained by the balanced dynamics. After its genesis, the subsequent evolution of the simulated Isobel was controlled by both the external influence and the internal dynamics. Under the unfavorable environmental conditions, the development of asymmetric structure reduced the axisymmetric diabatic heating in the inner core and the SSI process became ineffective and the storm weakened. Later on, as the eyewall reformed as a result of the axisymmetrization of an inward-propagating outer spiral rainband, the SSI process became effective again, leading to the reintensification of Isobel. Therefore, the large-scale environmental flow provided the precondition for the genesis of Isobel and the triggering mechanism for subsequent storm-scale structure change as discussed in Part I. The system-scale and mesoscale processes, such as the evolution of MCVs and merging VHTs, were responsible for the genesis, while the eyewall processes were critical to the storm intensity change through the SSI process.

Potential Temperature and Potential Vorticity Inversion: Complementary Approaches

Abstract Given the distribution of one atmospheric variable, that of nearly all others can be derived in balanced flow. In particular, potential vorticity inversion (PVI) selects potential vorticity (PV) to derive pressure, winds, and potential temperature θ. Potential temperature inversion (PTI) starts from available θ fields to derive pressure, winds, and PV. While PVI has been applied extensively, PTI has hardly been used as a research tool although the related technical steps are well known and simpler than those needed in PVI. Two idealized examples of PTI and PVI are compared. The 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) datasets are used to determine typical anomalies of PV and θ in the North Atlantic storm-track region. Statistical forms of PVI and PTI are applied to these anomalies. The inversions are equivalent but the results of PTI are generally easier to understand than those of PVI. The issues of attribution and piecewise inversion are discussed.

Extreme Events and the General Circulation: Observations and Stochastic Model Dynamics

Abstract This study explores the dynamical role of non-Gaussian potential vorticity variability (extreme events) in the zonally averaged circulation of the atmosphere within a stochastic framework. First the zonally averaged skewness and kurtosis patterns of relative and potential vorticity anomalies from NCEP–NCAR reanalysis data are presented. In the troposphere, midlatitude regions of near-zero skewness coincide with regions of maximum variability. Equatorward of the Northern Hemisphere storm track positive relative/potential vorticity skewness is observed. Poleward of the same storm track the vorticity skewness is negative. In the Southern Hemisphere the relation is reversed, resulting in negative relative/potential vorticity skewness equatorward, and positive skewness poleward of the storm track. The dynamical role of extreme events in the zonally averaged general circulation is then explored in terms of the potential enstrophy budget by linking eddy enstrophy fluxes to a stochastic representation of non-Gaussian potential vorticity anomalies. The stochastic model assumes that potential vorticity anomalies are advected by a random velocity field. The assumption of stochastic advection allows for a closed expression of the meridional enstrophy flux: the potential enstrophy flux is proportional to the potential vorticity skewness. There is some evidence of this relationship in the observations. That is, potential enstrophy fluxes might be linked to non-Gaussian potential vorticity variability. Thus, extreme events may presumably play an important role in the potential enstrophy budget and the related general circulation of the atmosphere.

The Detection and Significance of Diurnal Pressure and Potential Vorticity Anomalies East of the Rockies

Abstract Harmonic analysis of pressure, temperature, and precipitation data from 1000 Automated Surface Observing System (ASOS) stations reveals a mix of stationary and east–west moving disturbances east of the Rockies. Optimization of the pressure data using a “temperature-based tide assumption” separates a strong sun-following continentally enhanced tide from a smaller eastward-propagating wave (EPW). The latter signal moves at a similar speed to the previously discovered eastward-moving precipitation systems. Analysis of ASOS summer precipitation data confirms eastward propagation, but east of 90°W it shows nonpropagating diurnal convection at a fixed local time (i.e., 1800 LST). Analysis of winter days still finds the EPW, suggesting that it is the cause and not the result of the propagating precipitation. A possible mechanism for the EPW is developed from the linear Bousinesq equations with heating and wind shear. Solutions show eastward-moving diurnal pulses of potential vorticity (PV) generated by imposed heating over the Rockies. Because of the background shear, these pulses produce vertical motion in the lower troposphere. The PV hypothesis for precipitation propagation was tested with North American Regional Reanalysis (NARR) data. Diurnal drifting thermal and PV anomalies are clearly found near the 500- and 600-hPa levels in both winter and summer. In winter, the PV signal is weaker, moves faster, and does not influence precipitation. The existence of the winter PV signal again suggests that it is the cause, not the effect, of summer propagating precipitation.

대류권계면 접힘에 의한 영동지방 집중호우사례의 특성분석

2006년 10월 22일에서 24일에 걸쳐 한반도 강릉지역에서 강한 집중호우가 발생했다. 이 기간에 대해서 집중호우가 발생하기 전과 강우강도가 가장 강했을 때 나타나는 종관적, 운동학적 특성을 조사하기 위하여 본 연구에서는 지상일기도, 상층일기도, 적외위성영상, 자동기상관측장비(AWS) 자료, NCEP(National Centers for Environmental Prediction) 전구분석자료를 이용하였다. 분석 기간 동안 강릉에서 관측된 총 강수량은 316.5 mm이고, 최대 순간풍속은 <TEX>$63.7m\;s^{-1}$</TEX>이다. 일기도를 분석해보면 집중호우가 시작하기 전 온대저기압이 한반도 중부에서 발달하였으며 한반도 북부에 역전기압골이 형성되어 있다. 또한 300 hPa 상층일기도에서는 서해와 한반도 남부에 제트기류가 위치하며, 위치 소용돌이도 이상과 관련된 절리 저기압이 한반도 북서부에서 발달하고 있다. 강우강도가 가장 강했을 때의 특성을 좀 더 자세히 알아보기 위하여 강릉지역의 위치 소용돌이도 이상과 바람, 위치 소용돌이도의 시간-연직 단면도, 연직운동, 발산장과 수렴장, 역학적 온위의 연직분포에 대해 조사하였다. 종관적, 운동학적 과정을 분석해 본 결과 대류권계면 접힘이 집중호우 발생에 큰 역할을 한 것으로 사료된다. The synoptic and kinematic characteristics of a heavy rainfall that occurred in Gangneung region on 22 to 24 October 2006 were investigated using weather maps, infrared images, AWS observation data and NCEP global final analyses data. The total amount of rainfall observed in the region for the period was 316.5 mm, and the instanteneous maximum wind speed was <TEX>$63.7m\;s^{-1}$</TEX>. According to the analysis of weather maps, before the starting of the heavy rainfall, an extratropical low pressure system was developed in the middle region of the Korean Peninsula, and an inverted trough was formed in the northern region of the peninsula. In addition, a jet stream on the upper charts of 300 hPa was located over the Yellow Sea and the southern boundary of the peninsula. A cutoff low in the cyclonic shear side of the upper jet streak, which was linked to an anomaly of isentropic potential vorticity, was developed over the northwestern part of the peninsula. And there are analyzed potential vorticity and wind, time-height cross section of potential vorticity, vertical air motion, maximums of the divergence and convergence and vertical distribution of potential temperature in Gangneung region. The analyzed results of the synoptic conditions and kinematic processes strongly suggest that the tropopause folding made a significant role of initializing the heavy rainfall.

A Piecewise Potential Vorticity Inversion Algorithm and Its Application to Hurricane Inner-Core Anomalies

Abstract In this study, a piecewise potential vorticity (PV) inversion algorithm for an arbitrary number of PV pieces is developed by extending Wang and Zhang’s PV inversion scheme, and then the nonlinear responses to various types and magnitudes of axisymmetric PV anomalies (PVAs) in hurricane vortices are investigated. Results show that the upper- and lower-level PVAs in the eye help enhance cyclonic flows in the eyewall, but with weak vertical interactions between them. The balanced flows corresponding to the PVAs in the eyewall appear to account for a substantial portion of the warm core and the minimum pressure in the eye. However, the lower-level PVA in the eye inversion layer is more effective in contributing to the hurricane intensity than that at the upper levels. Results also show that the radius of the balanced response of PVAs is more sensitive to the mean vortex intensity than the vertical penetration; the weaker the mean vortex intensity, the larger the radius of the influence will be. Similar behaviors are also observed for the quasi-balanced secondary circulations. That is, given a diabatic heating profile in the eyewall, the weaker the background vortex or the PVAs, the stronger the secondary circulations will be. It is found that the development of an outer eyewall (or spiral rainbands) could be inimical to the inner eyewall in several ways, such as by 1) adding an anticyclonic flow inside to offset the cyclonic rotation of the inner eyewall, 2) enhancing a ring of a lower pressure zone underneath to broaden the inner-core lower pressure region, 3) inducing an inward (outward) radial flow outside (inside) in the PBL (upper level) to block the energy supply to (outflow of) the inner eyewall, and 4) generating subsidence between the two eyewalls to suppress the development of deep convection in the inner eyewall.

The Antecedent Large-Scale Conditions of the “Perfect Storms” of Late October and Early November 1991

Abstract The “Perfect Storms” (PSs) were a series of three high-impact extratropical cyclones (ECs) that impacted North America and the North Atlantic in late October and early November 1991. The PSs included the Perfect Storm in the northwest Atlantic, a second EC over the North Atlantic that developed from the interaction of the PS with Hurricane Grace, and a third EC over North America commonly known as the “1991 Halloween Blizzard.” The PSs greatly impacted the North Atlantic and North America with large waves, coastal flooding, heavy snow, and accumulating ice, and they also provided an opportunity to investigate the physical processes that contributed to a downstream baroclinic development (DBD) episode across North America that culminated in the ECs. Downstream baroclinic development resulted from an amplification of the large-scale flow over the North Pacific that was influenced by anomalous tropical convection, the recurvature and extratropical transition of western North Pacific Tropical Cyclones Orchid, Pat, and Ruth, and the subsequent evolution of the extratropical flow. The progression of DBD occurred following the development of a negative PNA regime and the generation of baroclinic instability over North America associated with equatorward-displaced potential vorticity anomalies and poleward-displaced corridors of high moisture content. An analysis of the eddy kinetic energy tendency equation demonstrated that the resulting baroclinic conversion of eddy available potential energy into eddy kinetic energy during the cyclogenesis process facilitated the progression of DBD across North America and the subsequent development of the ECs.

Disentangling the Forcing Mechanisms of a Heavy Precipitation Event along the Alpine South Side Using Potential Vorticity Inversion

AbstractExtreme precipitation events along the Alpine south side (AS) are often forced by upper-level positive potential vorticity (PV) anomalies over western Europe. These so-called PV streamers go along with a dynamical forcing for upward motion, a reduction of the static stability in the troposphere (hence facilitating convection), and are associated with low-level winds that transport moisture toward the Alps.A case of heavy precipitation is examined using the 40-yr ECMWF Re-Analysis data. Piecewise PV inversion (PPVI) and the limited-area Climate High Resolution Model (CHRM) are used to assess the influences of mesoscale parts of the streamer on the precipitation event. The impacts on the vertical stability are quantified by the convective available potential energy (CAPE) and an index of static stability. Very sensitive areas in terms of the stability are located beneath the southern tip of the streamer; smaller changes in the stability are observed in the Alpine region.The moisture transport toward the Alps is sensitive to the amplitude of the streamer, which influences the amount of water that can be transported along its eastern flank.The impacts of the topography on the flow are assessed by calculating an average inverse Froude number. Whether or not the air parcels are blocked by or lifted over the barrier (going along with suppressed and enhanced precipitation, respectively) depends on the vertical stability and the impinging wind velocity, two parameters that are inherently linked to the PV streamer and its substructure.

Potential Vorticity Anomalies of the Lowermost Stratosphere: A 10-Yr Winter Climatology

Abstract Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space–time characteristics of these anomalies, a combined “identification and tracking” tool is developed that can catalog each individual anomaly’s effective amplitude, location, overall spatial structure, and movement from genesis to lysis. A 10-yr winter climatology of such anomalies in the Northern Hemisphere is derived for the period 1991–2001 based upon the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The climatology indicates that the anomalies are frequently evident above high topography and in a quasi-annular band at about 70°N, are long lived (days to weeks), and that their effective amplitude is typically 2 PV units (PVU) higher than that of the ambient environment. In addition, the derived climatologies and associated composites pose questions regarding the origin of the anomalies, detail their life cycle, and shed light on their dynamics and role as long-lived precursors of surface cyclogenesis.

Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

Abstract Several hypotheses have been put forward for the mechanisms of generation of surface circulation associated with tropical cyclones. This paper examines high-resolution simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during NASA’s Tropical Cloud Systems and Processes Experiment, to investigate the development of low-level circulation and its relationship to the precipitation evolution. Two simulations are examined: one that better matches available observations but underpredicts the storm’s minimum sea level pressure and a second one that somewhat overintensifies the storm but provides a set of simulations that encapsulates the overall genesis and development characteristics of the observed storm. The roles of convective and stratiform precipitation processes within the mesoscale precipitation systems that formed Gert are discussed. During 21–25 July, two episodes of convective system development occurred. In each, precipitation system evolution was characterized by intense and deep convective upward motions followed by increasing stratiform-type vertical motions (upper-level ascent, low-level descent). Potential vorticity (PV) in convective regions was strongest at low levels while stratiform-region PV was strongest at midlevels, suggesting that convective processes acted to spin up lower levels prior to the spinup of middle levels by stratiform processes. Intense vortical hot towers (VHTs) were prominent features of the low-level cyclonic vorticity field. The most prominent PV anomalies persisted more than 6 h and were often associated with localized minima in the sea level pressure field. A gradual aggregation of the cyclonic PV occurred as existing VHTs near the center continually merged with new VHTs, gradually increasing the mean vorticity near the center. Nearly concurrently with this VHT-induced development, stratiform precipitation processes strongly enhanced the mean inflow and convergence at middle levels, rapidly increasing the midlevel vorticity. However, the stratiform vertical motion profile is such that while it increases midlevel vorticity, it decreases vorticity near the surface as a result of low-level divergence. Consequently, the results suggest that while stratiform precipitation regions may significantly increase cyclonic circulation at midlevels, convective vortex enhancement at low to midlevels is likely necessary for genesis.

Assessing links between upper atmospheric vorticity patterns and directional changes in hurricane tracks

Links between hurricane track changes and upper atmospheric potential vorticity (PV) anomaly patterns were identified qualitatively and analytically between 1990 and 2005 in the Western Atlantic. Strong track changes of hurricanes, particularly the constellations that triggered northward acceleration of the storm systems, were associated with upper-air PV patterns characterized by strongly positive anomalies to the northeast in combination with weak PV to the north of the system center. Constellations that triggered eventual eastward acceleration were associated with strongly positive PV anomalies to the northwest in combination with weak PV to the northeast of the system center. These results may assist hurricane forecasters and modelers in identifying possible signatures of future tropical cyclone tracks.