Potential Vorticity Advection Research Articles

Evidence of a dual African and Australian biomass burning influence on the vertical distribution of aerosol and carbon monoxide over the southwest Indian Ocean basin in early 2020

Abstract. During the 2020 austral summer, the pristine atmosphere of the southwest Indian Ocean (SWIO) basin experienced significant perturbations. This study examines the variability of aerosols and carbon monoxide (CO) over this remote oceanic region and investigates the underlying processes in the upper troposphere–lower stratosphere (UT-LS). Aerosol profiles in January and February 2020 revealed a multi-layer structure in the tropical UT-LS. Numerical models – the FLEXible PARTicle dispersion model (FLEXPART) and the Modèle Isentropique de transport Mésoéchelle de l'Ozone Stratosphérique par Advection (MIMOSA) – indicated that the lower-stratospheric aerosol content was influenced by the intense and persistent stratospheric aerosol layer generated during the 2019–2020 extreme Australian bushfire events. A portion of this layer was transported eastward by prevailing easterly winds, leading to increased aerosol extinction profiles over Réunion on 27 and 28 January. Analysis of advected potential vorticity revealed isentropic transport of air masses containing Australian biomass burning aerosols from extratropical latitudes to Réunion at the 400 K isentropic level on 28 January. Interestingly, we found that biomass burning (BB) activity in eastern Africa, though weak during this season, significantly influenced (contributed up to 90 % of) the vertical distribution of CO and aerosols in the upper troposphere over the SWIO basin. Ground-based observations at Réunion confirmed the simultaneous presence of African and Australian aerosol layers. This study provides the first evidence of African BB emissions impacting the CO and aerosol distribution in the upper troposphere over the SWIO basin during the convective season.

Track Deflection of Typhoon Chanthu (2021) near Taiwan as Investigated Using a High-Resolution Global Model

Abstract The Model for Prediction Across Scales (MPAS) with variable resolution (60-15-1 km) is used to investigate the track deflection of Typhoon Chanthu (2021) near Taiwan. Chanthu exhibited a rightward track deflection as it approached southeast Taiwan and underwent a leftward deflection when moving northward offshore of northeast Taiwan. Numerical experiments are conducted to identify the physical processes for the track deflection. The rightward deflection of the northbound typhoon is induced by the recirculating flow resulting from the effect of Taiwan’s topography. A wavenumber-one potential vorticity (PV) budget analysis indicates that horizontal PV advection dominates the earlier rightward deflection, while the later leftward deflection is mainly in response to stronger asymmetric cloud heating at low levels at the offshore quadrant of the typhoon. A pair of cyclonic and anticyclonic gyres in the wavenumber-one flow difference is induced by Taiwan’s topography. These rotate counterclockwise to drive the track deflection, most often in westbound typhoons. Idealized WRF simulations are also conducted to explore the track deflection under different northbound conditions. The simulations confirm the track deflection mechanism with similar PV dynamics to the MPAS simulations for Chanthu and illustrate the variabilities of the track deflection for different steering conditions and vortex origins. The rightward deflection of northbound typhoons is essentially determined by a reduced ratio of R/LE where R is the vortex size and LE is the effective length of the mountain range.

Numerical simulation of size contraction of Typhoon Cempaka (2021)

In 2021, Cempaka, a tiny tropical cyclone, made landfall in China. As the TC intensified prior to landfall, the tropical cyclone size measured with precipitation decreased significantly. A numerical simulation was conducted to examine the possible processes modulating the storm size. Azimuthally mean potential vorticity (PV) was found to decrease mainly in the middle to upper troposphere between 50- and 80-km radii. The PV budget results indicate that the advection and generation of mean PV associated with asymmetric processes, rather than the symmetric processes, primarily contributed to the decrease in mean PV. These asymmetric processes leading to a negative PV tendency were likely associated with inactive outer rainbands. In contrast, the tangential winds simultaneously expanded radially outward, possibly related to inner-core diabatic heating. The findings here emphasize the importance of outer rainband activity in tropical cyclone size change.摘要2021年台风“查帕卡”增强过程中风场向外扩张, 但卫星云图显示降水范围减小. 高分辨率数值模拟结果显示, “查帕卡”降水范围减小对应着50-80km半径处对流层中层位涡的减小. 位涡诊断结果表明, 非对称过程是导致上述位涡减小的关键物理过程, 而非对称过程主要同外雨带活动减弱有关. 上述结果强调了外雨带活动对热带气旋尺度变化的潜在重要影响.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 2: A Lagrangian analysis

Abstract. Future changes in extratropical cyclone structure and dynamics may lead to important impacts but are not yet fully understood. In the first part of this study, we have applied a composite approach together with potential vorticity (PV) inversion to study such changes in the dynamics of North Atlantic cyclones. Here, this is complemented with the help of a Lagrangian perspective, making use of air parcel trajectories to investigate the causes of altered PV anomalies as well as the role that cyclone airstreams play in shaping these changes. Intense cyclones in the extended winter seasons of two periods, 1990–2000 and 2091–2100, are studied in Community Earth System Model Large Ensemble (CESM-LE) simulations, and backward trajectories are calculated from the cyclone area as a basis to construct cyclone-centered composites of Lagrangian tendencies and their projected future changes. Our results show that diabatic processes on a timescale of 24 h shape the cyclones' low-level PV distribution and corroborate that the increasing moisture content along with enhanced ascent in warm conveyor belts leads to amplified latent heat release and larger low- and mid-level PV anomalies near the cyclone center in a warmer climate. In contrast, projected upper-level PV changes are due to a combination of several processes. These processes include cloud diabatic PV changes, anomalous PV advection, and likely also radiative PV generation in the lower stratosphere above the cyclone center. For instance, enhanced poleward advection is the primary reason for a projected decrease in upper-level PV anomalies south of the cyclone center. Warm conveyor belt outflow regions are projected to shift upward, but there is not robust change in the associated upper-level PV anomalies due to compensation between enhanced low-level PV generation and upper-level PV destruction. In summary, our two-part study points to future changes in the relative importance of different processes for the dynamics of intense North Atlantic cyclones in a warming climate, with important consequences for the near-surface wind pattern. In particular, a larger role of cloud diabatic processes is projected, affecting the cyclones through PV production in the lower troposphere. The role of other mechanisms, in particular radiative changes near the tropopause, should be investigated in more detail in future studies.

Different mechanisms of air temperature anomalies in the Yangtze River between early and late summer

The middle and lower reaches of the Yangtze River (MLRYR) exhibit notable interannual variability in seasonal-mean air temperature amid ongoing global warming. This study elucidates the specific mechanisms governing anomalous air temperatures during early (June) and late summer (July to August) in the MLRYR. Early summer temperatures arise from anomalous sinking driven by negative anomalies in moist enthalpy advection in the lower troposphere, as a response to warm sea surface temperature anomalies (SSTA) in the tropical western Pacific. This induces a low-level cyclone anomaly over Southeast Asia, reducing specific humidity and determining anomalous vertical motion over the MLRYR. In contrast, late summer temperatures are induced by mid-upper tropospheric descending anomalies over the MLRYR. The combined influence of SSTA in the western Pacific and North Atlantic sustains warmer air by reinforcing a barotropic anticyclone over North China through a mid-latitude wave train. Subsequent anomalous negative potential vorticity advection in the upper troposphere strengthens sinking motion over the MLRYR. The seasonal evolution of the East Asian summer monsoon and SSTA is pivotal in the distinct mechanisms governing warmer air temperatures over the MLRYR. While numerical experiments capture general features, the regulation of SSTA is subject to uncertainty due to the atmospheric intrinsic patterns. This research distinguishes predictability sources of air temperature anomalies over the MLRYR between early and late summer, emphasizing the combined influence of SSTA beyond the ENSO domain on East Asian climate.

Mediterranean tropical-like cyclone forecasts and analysis using the ECMWF ensemble forecasting system with physical parameterization perturbations

Abstract. Mediterranean tropical-like cyclones, called medicanes, present a multi-scale nature, and their track and intensity have been recognized as highly sensitive to large-scale atmospheric forcing and diabatic heating as represented by the physical parameterizations in numerical weather prediction. Here, we analyse the structure and investigate the predictability of medicanes with the aid of the European Centre for Medium-Range Weather Forecast (ECMWF) Integrated Forecast System (IFS) ensemble forecasting system with 25 perturbed members at 9 km horizontal resolution (compared with the 16 km operational resolution). The IFS ensemble system includes the representation of initial uncertainties from the ensemble data assimilation (EDA) and a recently developed uncertainty representation of the model physics with perturbed parameters (stochastically perturbed parameterizations, SPP). The focus is on three medicanes, Ianos, Zorbas, and Trixie, among the strongest in recent years. In particular, we have carried out separate ensemble simulations with initial perturbations, full physics SPP, with a reduced set of SPP, where only convection is perturbed to highlight the convective nature of medicanes and an operational ensemble combining the SPP and the initial perturbations. It is found that compared with the operational analysis and satellite rainfall data, the forecasts reproduce the tropical-like features of these cyclones. Furthermore, the SPP simulations compare to the initial-condition perturbation ensemble in terms of tracking, intensity, precipitation, and, more generally, in terms of ensemble skill and spread. Moreover, the study confirms that similar processes are at play in the development of the investigated three medicanes, in that the predictability of these cyclones is linked not only to the prediction of the precursor events (namely the deep cutoff low) but also to the interaction of the upper-level advected potential vorticity (PV) streamer with the tropospheric PV anomaly that is diabatically produced by latent heat.

Impacts of the Surface Potential Vorticity Circulation over the Tibetan Plateau on the East Asian Monsoon in July

Based on the definition of potential vorticity substance (W) and its equation, an index “iPV” representing the leading mode of the surface potential vorticity circulation (PVC) over the Tibetan Plateau is defined to characterize the orographic potential vorticity (PV) forcing on the atmospheric general circulation. The relationships between the iPV index and the East Asian monsoon in July, as well as the Silk Road pattern in Eurasia, are investigated on an interannual time scale. Results show that the iPV in July is closely related to the interannual variability of the East Asian monsoon. Corresponding to the positive phase of iPV with negative (positive) PVC over the north (south) of the plateau, strong positive PV anomalies and westerly flows develop in the troposphere over the plateau. Consequently, in the downstream region, the zonal PV advection increases with height just above the Jianghuai Meiyu front, which is conducive to the generation of upward movement. Over the East Asian area, the upper troposphere is controlled by the eastward shifted South Asian High. In the lower troposphere, the southwesterly flow anomaly on the northwestern side of the strengthened western Pacific subtropical high transports abundant water vapor to the north, forming a convergence in the Jianghuai area, leading to the formation of large-scale precipitation along the Meiyu front. Results from partial correlation analysis also demonstrate that the link between the variability of the East Asian monsoon in July and the plateau PV forcing is affected very little by the Silk Road pattern, whereas the plateau PV forcing plays a key “bridging” role in the influence of the Silk Road pattern on the East Asian monsoon.

Characteristics of Nepartak (2021), a subtropical cyclone controlled by an upper‐tropospheric cutoff low

AbstractThis study investigated the structural and track change of cyclone Nepartak occurred over the western North Pacific in July 2021, a cyclone appeared at the surface level, using ERA5 data, focusing on the relationship with an upper‐tropospheric cutoff low (COL). The COL was shown to dominate the structural change of Nepartak which developed as a subtropical cyclone with a hybrid structure. As the surface cyclone approached the COL, it was overlapped by the COL and became a cyclone with a two‐storied structure, resulting in its abrupt track change. An analysis of the potential vorticity (PV) tendency showed that the movement of Nepartak toward the COL ceased because of the counter‐balance of the horizontal/vertical PV advection with the PV anomaly generated by diabatic heating, whereas the surface cyclone northward motion was primarily due to the horizontal PV advection associated with the COL. Nepartak is a case in which changes in the structure and track of a cyclone are controlled by a COL.

Synoptic-Scale Effects of Atmospheric Heat Source over Key Areas of the Tibetan Plateau on Its Downstream Rainfall in Summer

Abstract This study demonstrates the synoptic-scale relationship between the atmospheric heat source over the Tibetan Plateau (TP) (TP<Q1>) and summer rainfall in eastern China. The results show that the summer TP<Q1> has three key areas [eastern, western, and northeastern TP (TPE, TPW, TPNE, respectively)] and 3–8-day dominant oscillation periods on the synoptic time scale for each subregion. Analogously, on the synoptic scale, the downstream rainfall of the TP in China has six sensitive regions (zone I to zone VI), each with 3–8-day dominant periods. The TPE<Q1>, TPW<Q1>, and TPNE<Q1> lead rainfall by 4 days with a southeastward propagation pathway, by 5 days with both southeastward and northeastward propagation routes, and by 4 days with an eastward route, respectively. Mechanistically, when TP<Q1> lies in the positive-to-negative transition phase, the rainfall moves off the TP and reaches the maximum in its most inactive phase; the jet cores embedded in the subtropical westerly jet band and the trough from the region of the TP<Q1> lead the eastward movement of the rainfall by the propagation of the divergence center and generalized moist potential vorticity. Besides, the southwest vortex and northwest vortex generated at the southeastern and northeastern flank of the TP are the main cause for the southeastward and northeastward or eastward movement of the precipitation. Further internal atmospheric dynamic structures indicate that both the horizontal moisture advection and generalized moist potential vorticity advection act on the entire propagation processes of the synoptic-scale signals for each TP<Q1> key area. In conclusion, the study has obvious theoretical significance and application value for the development of plateau synoptics.

El Niño‐Related Stratification Anomalies Over the Continental Slope Off Oregon in Summer 2014 and 2015: The Potential Vorticity Advection Mechanism

AbstractOver the continental slope off Oregon at the US West Coast, at 44.6°N, vertical stratification is found to be anomalously weak in July–August of 2014 and 2015 both in a regional ocean circulation model and conductivity–temperature–depth (CTD) profile observations. To understand the responsible mechanism, we focus on the layer between the isopycnal surfaces σθ = 26.5 and 26.25 kg m−3 that is found between depths 100 and 300 m and represents material properties characteristic of the slope poleward undercurrent and shelf‐slope exchange. This layer thickness, about 50 m on average, can be twice as large during the above‐mentioned periods. In the 2009–2018 model analysis, this anomaly is revealed over the continental slope only in summers 2014 and 2015 and only off the Oregon and Washington coasts (40°–47°N). The stratification anomaly is explained as the effect of advection of the seasonal along‐slope potential vorticity (PV) gradient by an anomalously strong poleward slope current. In the annual cycle, the zone of strong along‐slope PV gradient is found between 40° and 47°N, supported by the local upwelling that results in the injection of the large PV in the bottom boundary layer over the shelf followed by its offshore transport in the slope region. The positive along‐slope current anomaly propagates to Oregon with coastally trapped waves as part of the El Niño oceanic response and can be up to 0.1 m s−1. Advection by this anomalous poleward current results in transporting the seasonal PV gradient earlier in the season than on average.

Remote dynamic and thermodynamic effects of typhoons on Meiyu–Baiu precipitation in Japan assessed with bogus typhoon experiments

To advance comprehensive knowledge of typhoon-induced remote precipitation and anomalous atmospheric circulations during the early summer rainy season (Meiyu-Baiu), we performed numerical simulations in which different bogus typhoons are imbedded in the July climatological mean field as ideal initial and lateral boundary conditions using a regional atmospheric model, and we compared experiments with and without the bogus typhoon. The dominance of an anomalous local high (ALH) remotely induced by a typhoon leads to a poleward shift of the primary Meiyu–Baiu frontal zone in the form of the enhancement of the North Pacific subtropical high (NPSH). Typhoon-induced negative potential vorticity advection plays a dominant role in forming the ALH around Japan. Despite the shift of the frontal zone, heavy precipitation in excess of 100 mm per day occurs along the Pacific coast of western Japan, which is triggered by two primary moisture inflows. One is the low-level moisture inflow along the moisture conveyor belt (MCB), which is an appreciable low-level moisture flux zone stretching from the Indian Ocean to a typhoon over the western North Pacific, due to the intensified horizontal pressure gradient between the typhoon and the NPSH. The MCB is more robust in the experiment in which the typhoon is close to the South China Sea where the monsoon westerlies prevail. The parcels along the MCB bring moisture into western Japan even though moisture losses due to condensation occur on the way, to some degree. The other is the moisture inflow through the atmospheric boundary layer along the southwestern periphery of the NPSH. The parcels along the NPSH periphery easily receive moisture from the underlying ocean as they approach western Japan. Both inflows are enhanced by the ALH formation over Japan. The combined effect of the typhoon and its induced ALH is responsible for the prominence of the two major moisture inflows into western Japan, leading to local heavy precipitation in that region.

Subseasonal processes of triple extreme heatwaves over the Yangtze River Valley in 2022

Historical extreme heatwaves struck the Yangtze River Valley (YRV) in the boreal summer of 2022, severely impacting agricultural production, electricity supply, and resident health in China. This study shows that extreme heatwaves recurred over the YRV with distinct subseasonal processes. In early summer, from 14 June to 18 July, two extreme heatwaves were embedded in a mid-latitude 10–25-day intraseasonal oscillation (ISO), and hot-and-wet anomalies persisted over the YRV. A self-sustaining mechanism between the “cold vortex” over Northeast China and the “heat dome” around the YRV maintained this bi-weekly ISO. It fueled the heatwaves by modulating the meridional advection of upper-tropospheric potential vorticity, which firstly warmed the air via adiabatic processes and then later by diabatic heating. In late summer, from 30 July to 29 August, a 30–50-day ISO impacted the heatwave and drought over the YRV. This ISO originated from monsoon convection in the tropics, which regulated the meridional monsoonal circulation and enhanced the heatwave by intensifying the descending air motion and adiabatic heating over the YRV. The alternation of the two ISOs accompanied the northward migration of the subtropical westerly jet over East Asia. The combination of the above-normal 10–25-day ISO and the moderate 30–50-day ISO led to the three consecutive extreme YRV heatwaves in 2022.

Exploring the Evolution of Typhoon Lekima (2019) Moving Offshore Northeast of Taiwan with a Multi-Resolution Global Model

Typhoon Lekima occurred in early August 2019 and moved northwestward toward Taiwan. During offshore passage, the typhoon underwent rapid intensification, with a northward deflected track, moving closer to northeastern Taiwan. A global model, MPAS, at a multi-resolution of 60-15-3 km, is utilized with ensemble forecasts to investigate the dynamic processes causing the track deflection and intensity change as well as identify the track uncertainty to initial perturbed conditions under the topographic effects of the Central Mountain Range (CMR). For spinning up the typhoon vortex in ensemble forecasts, dynamic vortex initialization has been enforced with a 3 km resolution targeted at the Taiwan area. As one specific member track is similar to the best track, the track deflection is significantly reduced in the absence of the Taiwan terrain, highlighting the role of the topographic effects of the CMR. For these tracks with similar deflection, the northward deflection is caused by the induced strong flow to the east of the typhoon center in response to the re-circulating flow around southern Taiwan, which produces the wavenumber-one gyre in the asymmetric flow difference to drive the vortex northward. The typhoon translation around the Taiwan terrain is dominated by the changing wavenumber-one horizontal potential vorticity (PV) advection during the track deflection in the ensemble forecasts. The formation of an intense PV tongue along the upper eyewall is a facilitation precondition of RI, while RI can be significantly enhanced in the presence of an intense lower-stratospheric PV core near the upper eye, which is produced by the radial inflow of the developed transverse vortex circulation over the upper-level outflow layer.

The Sensitivity of Downstream Ridge Building Forecasts to Upstream Warm Conveyor Belt Forecast Uncertainty Using MPAS

Abstract One potential way to improve the skill of medium-range weather forecasts is to improve the evolution of Rossby waves, which largely modulate extratropical weather. Recent research has hypothesized that the predictability of downstream Rossby waves may be limited by forecast uncertainty linked to upstream diabatic processes such as latent heat release within the warm conveyor belt (WCB) of extratropical cyclones. This hypothesis is evaluated using Model for Prediction Across Scales (MPAS) ensemble forecasts for two events characterized by highly amplified flow over the North Atlantic associated with cyclogenesis. The source of variability in ridge forecasts is diagnosed using the ensemble-sensitivity technique and a potential vorticity (PV) tendency budget, which quantifies the contribution from individual physical processes toward subsequent ridge amplification. Before the onset of ridge amplitude differences for both events, ensemble forecasts with a more amplified ridge are associated with greater negative PV advection by the irrotational wind. The importance of PV advection by the irrotational wind suggests that PV changes are modulated by diabatic heating, which is confirmed by the sensitivity of ridge amplitude to earlier diabatic heating and lower-tropospheric moisture within an upstream WCB. After the onset of ridge amplitude differences, PV advection by the nondivergent wind becomes the primary driver of downstream forecast differences. Initial condition perturbations within the sensitive areas of the WCB confirm that increasing the initial lower-tropospheric moisture results in a more amplified ridge. This suggests that more accurate initial conditions near the WCB could lead to better downstream forecasts.

A comparative study on initial developments between explosive and nonexplosive cyclones off the East Asian coast in winter

Explosive cyclones (ECs) pose serious challenges for weather forecasting and significant threats to human life and property. In searching for the key points that make a cyclone go through explosive deepening off the East Asian coast, we present a comparative analysis of ECs and nonexplosive cyclones (or ordinary cyclones; OCs) using 10 years of ERA5 reanalysis data with high temporal and spatial resolutions. Their differences in synoptic backgrounds are shown, and mechanisms of the initial developments are compared quantitatively from the perspective of potential vorticity (PV). Among the identified 135 cyclones, 72 went through explosive growth and 37.5/36.1/20.8/5.6% of these ECs are ranked as weak/medium/strong/super ECs. ECs feature stronger low-level baroclinicity and higher PV than OCs. The decomposition of the local PV tendency shows the dominant role of the PV advection (with a correlation coefficient of 0.8). During the initial development, ECs have an average meridional temperature contrast 4 K larger than OCs within 20 latitudes at the low troposphere in the upstream, due to a stronger cold advection. The upstream colder air increases the horizontal temperature gradient and thus produces steeper isentropic surfaces inclining to the west. Since the PV intrusion is mainly along the isentropic surfaces, the increase in their slope significantly enhances the downward transport of PV from upper air. The importance of the horizontal gradient of potential temperature is further proved by numerical experiments with the Weather Research and Forecast (WRF) model on typical winter ECs. In sensitivity experiments, the low troposphere meridional temperature contrast decreasing by the average difference between ECs and OCs significantly decreases PV and stops the cyclones from explosive deepening. Despite the importance of diabatic processes in the deepening of mid-latitude cyclones emphasized by many studies, this study shows that the PV intrusion dominated by cold air mass is the key cause of winter explosive cyclogenesis in this region.

A Numerical Study for Tropical Cyclone Atsani (2020) Past Offshore of Southern Taiwan under Topographic Influences

Tropical Cyclone Atsani occurred in late October 2020 and moved westward offshore south of Taiwan. During its offshore passage, the cyclone deflects northward as it closes to the southern end of Taiwan. A global model MPAS at a multi-resolution of 60-15-3-km is applied to explore the track responses of Atsani and identify the topographic effects of the Central Mountain Range (CMR) on the cyclone circulation and the associated track deflection. With a 3-km resolution targeted at the Taiwan area, the cyclone track deflection can be reasonably simulated, with more sensitivity to physics schemes and dynamic vortex initialization and less sensitivity to initial environmental perturbations. When the Taiwan terrain is removed, the cyclone indeed deflects more northward earlier, in particular for simulations with a stronger cyclone that tends to generate stronger east-west wind asymmetry in the absence of the terrain. Idealized simulations with a regional model WRF at 3-km resolution are also utilized to contrast the track deflection of different departing cyclones, similar to the real case. It was found that northward deflection will be induced near south of the CMR-like terrain for both stronger and weaker westbound cyclones departing at different latitudes south of the terrain. We have explained why a further northward track at earlier stages is induced in the absence of the terrain effects in regard to model initial states. In both real and idealized cases, the track deflection of the cyclone moving around the terrain is dominated by the wavenumber-one horizontal potential vorticity (PV) advection that is somewhat offset by both vertical PV advection and differential diabatic heating.

Impact of Radio Occultation Data on the Prediction of Typhoon Haishen (2020) with WRFDA Hybrid Assimilation

FORMOSAT-7/COSMIC-2 (FS7/C2) satellite was successfully launched in June 2019. The satellite provides about 5000 radio occultation (RO) soundings daily over the tropical and partial subtropical regions. Such plentiful RO soundings with high accuracy and vertical resolution could be used to improve model initial analysis for typhoon prediction. In this study, assimilation experiments with and without the RO data were conducted with the WRFDA hybrid system for the prediction of Typhoon Haishen (2020). The experimental results show a remarkable improvement in typhoon track prediction with RO data assimilation, especially when using a nonlocal refractivity operator. Results in cycling DA and forecast are analyzed and verified for the RO data impact. Diagnostics of potential vorticity (PV) tendency budget helps explain the typhoon translation induced by different physical processes in the budget. The typhoon translation is essentially dominated by horizontal PV advection, but the track deviation can increase due to the vertical PV advection with opposite effects in the absence of RO data. Sensitivity experiments for different model initial times, physics schemes, and RO observation amounts show positive RO data impacts on typhoon prediction, mainly contributed from FS7. Complementary, an improved forecast of Typhoon Hagupit (2020) is also illustrated for the RO data impact.

Forced, Balanced, Axisymmetric Shallow Water Model for Understanding Short-Term Tropical Cyclone Intensity and Wind Structure Changes

A minimal modeling system for understanding tropical cyclone intensity and wind structure changes is introduced: Shallow Water Axisymmetric Model for Intensity (SWAMI). The forced, balanced, axisymmetric shallow water equations are reduced to a canonical potential vorticity (PV) production and inversion problem, whereby PV is produced through a mass sink (related to the diabatic heating) and inverted through a PV/absolute–angular–momentum invertibility principle. Because the invertibility principle is nonlinear, a Newton–Krylov method is used to iteratively obtain a numerical solution to the discrete problem. Two versions of the model are described: a physical radius version which neglects radial PV advection (SWAMI-r) and a potential radius version that naturally includes the advection in the quasi-Lagrangian coordinate (SWAMI-R). In idealized numerical simulations, SWAMI-R produces a thinner and more intense PV ring than SWAMI-r, demonstrating the role of axisymmetric radial PV advection in eyewall evolution. SWAMI-R always has lower intensification rates than SWAMI-r because the reduction in PV footprint effect dominates the peak magnitude increase effect. SWAMI-r is next demonstrated as a potentially useful short-term wind structure forecasting tool using the newly added FLIGHT+ Dataset azimuthal means for initialization and forcing on three example cases: a slowly intensifying event, a rapid intensification event, and a secondary wind maximum formation event. Then, SWAMI-r is evaluated using 63 intensifying cases. Even though the model is minimal, it is shown to have some skill in short-term intensity prediction, highlighting the known critical roles of the relationship between the radial structures of the vortex inertial stability and diabatic heating rate. Because of the simplicity of the models, SWAMI simulations are completed in seconds. Therefore, they may be of some use for hurricane nowcasting to short-term (less than 24 h) intensity and structure forecasting. Due to its favorable assumptions for tropical cyclone intensification, a potential use of SWAMI is a reasonable short-term upper-bound intensity forecast if the storm intensifies.

Emergence of representative signals for sudden stratospheric warmings beyond current predictable lead times

Abstract. Major sudden stratospheric warmings (SSWs) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex and are considered an important source of predictability of tropospheric weather on subseasonal to seasonal timescales over the Northern Hemisphere midlatitudes and high latitudes. However, SSWs themselves are difficult to predict, with a predictability limit of around 1 to 2 weeks. The predictability limit for determining the type of event, i.e., wave-1 or wave-2 events, is even shorter. Here we analyze the dynamics of the vortex breakdown and look for early signs of the vortex deceleration process at lead times beyond the current predictability limit of SSWs. To this end, we employ a mode decomposition analysis to study the potential vorticity (PV) equation on the 850 K isentropic surface by decomposing each term in the PV equation using the empirical orthogonal functions of the PV. The first principal component (PC) is an indicator of the strength of the polar vortex and starts to increase from around 25 d before the onset of SSWs, indicating a deceleration of the polar vortex. A budget analysis based on the mode decomposition is then used to characterize the contribution of the linear and nonlinear PV advection terms to the rate of change (tendency) of the first PC. The linear PV advection term is the main contributor to the PC tendency at 25 to 15 d before the onset of SSW events for both wave-1 and wave-2 events. The nonlinear PV advection term becomes important between 15 and 1 d before the onset of wave-2 events, while the linear PV advection term continues to be the main contributor for wave-1 events. By linking the PV advection to the PV flux, we find that the linear PV flux is important for both types of SSWs from 25 to 15 d prior to the events but with different wave-2 spatial patterns, while the nonlinear PV flux displays a wave-3 wave pattern, which finally leads to a split of the polar vortex. Early signs of SSW events arise before the 1- to 2-week prediction limit currently observed in state-of-the-art prediction systems, while signs for the type of event arise at least 1 week before the event onset.

Recurvature and movement processes of tropical cyclones over the Bay of Bengal

AbstractActive bimodal tropical cyclones (TCs) are embedded in nonuniform flows over the Bay of Bengal (BoB). More than half of the TCs that occurred from 1990 to 2019 recurved from their original path due to the influence of 500–300 hPa steering flows. Premonsoon TCs have a tendency to turn right and make landfall on the right‐side coastal areas of the BoB. On the other hand, most of the TCs that occurred in the postmonsoon season crossed the left coast or east coast of India, despite being left or right from their path. The simulation shows that Cyclone Roanu (2016) was driven by a warm, moist, unstable southwesterly that was blocked and diverted by the hot, dry stable air masses of northerlies and northwesterlies. As a result, the TC moved along the dry–moist boundary, keeping a constant distance of ∼400 km between the eye and dryline. Conversely, the combined steering of moist southeasterly and southward‐moving upper‐level westerly jets controlled the track of postmonsoon Cyclone Madi (2013), encompassing a left turn of ∼150°. In addition to the primary steering mechanism, TC vortex propagation can be explained by the strong vertical shear (>30 m·s−1) offered by the dryline during the premonsoon period and near the monsoon trough in the postmonsoon period. This vertical shear causes the upper‐level anticyclone to tilt downshear, resulting in the forward motion of the TC vortex. The deep convection and associated mesovortices, especially those that are more prominent along the dryline, additionally influence asymmetric diabatic heating, contributing to the advection of TC potential vorticity.