Potential Vorticity Tendency Research Articles

Numerical Investigation of Track and Intensity Evolution of Typhoon Doksuri (2023)

This study utilized the WRF model to investigate the track evolution and rapid intensification (RI) of Typhoon Doksuri (2023) as it moved across the Luzon Strait and through the South China Sea (SCS). The simulation results indicate that Doksuri has a smaller track sensitivity to the use of different physics schemes, while having a greater intensity sensitivity. Sensitivity numerical experiments with different physics schemes can well capture its northwestward movement in the first two days, but they predict less westward track deflection as the typhoon moves across the Luzon Strait and through the SCS. Moreover, all the experiments successfully simulated Doksuri’s RI, albeit with quite different rates and a time lag of 12 h. Among different combinations of physics schemes, there exists an optimal set of cumulus parameterization and cloud microphysics schemes for track and intensity predictions. Doksuri’s track changes as the typhoon moved across the Luzon Strait and through the SCS were influenced by the topographic effects of the terrain of the Philippines and Taiwan, to different extents. The track changes of Doksuri are explained by the wavenumber-one potential vorticity (PV) tendency budget from different physical processes, highlighting that the horizontal PV advection dominates the PV tendency throughout most of the simulation time due to the offset of vertical PV advection and differential diabatic heating. In addition, this study applies the extended Sawyer–Eliassen (SE) equation to compare the transverse circulations of the typhoon induced by various forcing sources. The SE solution indicates that radial inflow was largely driven in the lower-tropospheric vortex by strong diabatic heating, while being significantly enhanced in the lower boundary layer due to turbulent friction. All other physical forcing terms were relatively insignificant for the induced transverse circulation. The coordinated radial inflow at low levels may have led to the eyewall development in unbalanced dynamics. Intense diabatic heating thus was vital to the severe RI of Doksuri under a weak vertical wind shear.

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.

Examining Atmospheric River Life Cycles in East Antarctica

AbstractDuring atmospheric river (AR) landfalls on the Antarctic ice sheet, the high waviness of the circumpolar polar jet stream allows for subtropical air masses to be advected toward the Antarctic coastline. These rare but high‐impact AR events are highly consequential for the Antarctic mass balance; yet little is known about the various atmospheric dynamical components determining their life cycle. By using an AR detection algorithm to retrieve AR landfalls at Dumont d'Urville and non‐AR analogs based on 700 hPa geopotential height, we examined what makes AR landfalls unique and studied the complete life cycle of ARs reaching Dumont d'Urville. ARs form in the mid‐latitudes/subtropics in areas of high surface evaporation, likely in response to tropical deep convection anomalies. These convection anomalies likely lead to Rossby wave trains that help amplify the upper‐tropospheric flow pattern. As the AR approaches Antarctica, condensation of isentropically lifted moisture causes latent heat release that—in conjunction with poleward warm air advection—induces geopotential height rises and anticyclonic upper‐level potential vorticity tendencies downstream. As evidenced by a blocking index, these tendencies lead to enhanced ridging/blocking that persist beyond the AR landfall time, sustaining warm air advection onto the ice sheet. Finally, we demonstrate a connection between tropopause polar vortices and mid‐latitude cyclogenesis in an AR case study. Overall, the non‐AR analogs reveal that the amplified jet pattern observed during AR landfalls is a result of enhanced poleward moisture transport and associated diabatic heating which is likely impossible to replicate without strong moisture transport.

Different Propagation Mechanisms of Deep and Shallow Wintertime Extratropical Cyclones over the North Pacific

Abstract Extratropical cyclones (ETCs) are three-dimensional synoptic systems in the middle and high latitudes. Previous studies on ETC propagation have typically focused on cyclones identified at a single level. However, more recent studies have found that ETCs have diverse vertical structures and cyclones with different vertical extents always exhibit distinct characteristics and surface impacts. In this work, we study the movement of wintertime (December–February) extratropical cyclones by classifying North Pacific ETCs into deep cyclones, shallow low-level cyclones, and shallow upper-level cyclones, based on reanalysis data from 1979 to 2019. Applying a Lagrangian perspective, we track the cyclones at different vertical levels to investigate the different characteristics and mechanisms for the propagation of deep and shallow ETCs. A potential vorticity (PV) tendency analysis of cyclone-tracking composites reveals that, for deep cyclones, the diabatic heating at 850 hPa and the horizontal advection by the stationary flow at 500 hPa are the main contributors to the poleward movement. For shallow cyclones, the nonlinear advection terms play a dominant role in their meridional motion, advecting shallow low-level cyclones poleward but shallow upper-level cyclones equatorward. A piecewise PV inversion analysis suggests that the nonlinear advection by winds induced from upper-level PV anomalies is responsible for the different performance of nonlinear advection terms for shallow low-level and upper-level cyclones. These findings further our understanding of the mechanisms and variations of cyclone propagation. Significance Statement Extratropical cyclones (ETCs) can be identified at different levels in the troposphere. These mobile low pressure cyclonic storms are the main sources of synoptic variability in the extratropics and often bring severe or even disastrous weather. Previous studies on ETC movement have typically been restricted to cyclones identified at a single level. Our study, by identifying ETCs at multiple levels, classifies cyclones into deep, shallow low-level, and shallow upper-level cyclones. For deep cyclones, their poleward movement is found to be a result of diabatic heating at lower levels and dominated by the stationary circulation at upper levels. For shallow cyclones, nonlinear advection determines whether they propagate poleward or equatorward. These findings further our understanding of the mechanisms of cyclone propagation and imply that the movement of deep and shallow cyclones may undergo different changes with different weather and climate impacts in the future, given the enhanced diabatic heating under global warming, which deserves further investigation.

The role of boundary layer processes in summer-time Arctic cyclones

Abstract. Arctic cyclones are the most energetic weather systems in the Arctic, producing strong winds and precipitation that present major weather hazards. In summer, when the sea ice cover is reduced and more mobile, Arctic cyclones can have large impacts on ocean waves and sea ice. While the development of mid-latitude cyclones is known to be dependent on boundary layer (BL) turbulent fluxes, the dynamics of summer-time Arctic cyclones and their dependence on surface exchange processes have not been investigated. The purpose of this study is to characterise the BL processes acting in summer-time Arctic cyclones and understand their influence on cyclone evolution. The study focuses on two cyclone case studies, each characterised by a different structure during growth in the Arctic: (A) low-level-dominant vorticity (warm-core) structure and (B) upper-level-dominant vorticity (cold-core) structure, linked with a tropopause polar vortex. A potential vorticity (PV) framework is used to diagnose the BL processes in model runs from the ECMWF Integrated Forecasting System model. Both cyclones are associated with frictional Ekman pumping and downward sensible heat fluxes over sea ice. However, a third process, the frictional baroclinic generation of PV, acts differently in A and B due to differences in their low-level temperature structures. Positive PV is generated in Cyclone A near the bent-back warm front, like in typical mid-latitude cyclones. However, the same process produces negative PV tendencies in B, shown to be a consequence of the vertically aligned axisymmetric cold-core structure. This frictional process also acts to cool the lower troposphere, reducing the warm-core anomaly in A and amplifying the cold-core anomaly in B. Both cyclones attain a vertically aligned cold-core structure that persists for several days after maximum intensity, which is consistent with cooling from frictional Ekman pumping, frictional baroclinic PV generation, and downward sensible heat fluxes. This may help to explain the longevity of isolated cold-core Arctic cyclones with columnar vorticity structure.

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.

Similarity and variability of blocked weather-regime dynamics in the Atlantic–European region

Abstract. Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. Due to their role in weather extremes and atmospheric predictability, regimes that feature a blocking anticyclone are of particular interest. This study investigates the dynamics of these “blocked” regimes in the North Atlantic–European region from a year-round perspective. For a comprehensive diagnostic, wave activity concepts and a piecewise potential vorticity (PV) tendency framework are combined. The latter essentially quantifies the well-established PV perspective of mid-latitude dynamics. The four blocked regimes (namely Atlantic ridge, European blocking, Scandinavian blocking, and Greenland blocking) during the 1979–2021 period of ERA5 reanalysis are considered. Wave activity characteristics exhibit distinct differences between blocked regimes. After regime onset, Greenland blocking is associated with a suppression of wave activity flux, whereas Atlantic ridge and European blocking are associated with a northward deflection of the flux without a clear net change. During onset, the envelope of Rossby wave activity retracts upstream for Greenland blocking, whereas the envelope extends downstream for Atlantic ridge and European blocking. Scandinavian blocking exhibits intermediate wave activity characteristics. From the perspective of piecewise PV tendencies projected onto the respective regime pattern, the dynamics that govern regime onset exhibit a large degree of similarity: linear Rossby wave dynamics and nonlinear eddy PV fluxes dominate and are of approximately equal relative importance, whereas baroclinic coupling and divergent amplification make minor contributions. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset. The retrograde pathway is dominated by nonlinear PV eddy fluxes, whereas the upstream pathway is dominated by linear Rossby wave dynamics. Importantly, there is a large degree of cancellation between the two pathways for some of the mechanisms before regime onset. The physical meaning of a regime-mean perspective before onset can thus be severely limited. Implications of our results for understanding predictability of blocked regimes are discussed. Further discussed are the limitations of projected tendencies in capturing the importance of moist-baroclinic growth, which tends to occur in regions where the amplitude of the regime pattern, and thus the projection onto it, is small. Finally, it is stressed that this study investigates the variability of the governing dynamics without prior empirical stratification of data by season or by type of regime transition. It is demonstrated, however, that our dynamics-centered approach does not merely reflect variability that is associated with these factors. The main modes of dynamical variability revealed herein and the large similarity of the blocked regimes in exhibiting this variability are thus significant results.

Importance of self‐induced vertical wind shear and diabatic heating on the Fujiwhara effect

AbstractThis study hypothesises that the motion of binary tropical cyclones (TCs) can be modified by diabatic heating (DH) asymmetry associated with self‐induced vertical wind shear. To demonstrate this hypothesis in the quiescent environment, a set of the idealised f‐plane numerical experiments such as identical (varied in the initial separation distance) and non‐identical (varied in TC intensity and size) experiments are conducted. Results show that the binary TC tracks are affected by the initial separation distance, and TC intensity and size, that is, TC influence radius. Identical experiments show that the critical distance for separating, merging or repelling motion is 8° (where 1° ≃ 111 km). The binary TC interaction shows that single large‐scale cyclonic and anticyclonic circulations can develop as a result of the superposition of the circulation of the binary TCs in the lower and upper troposphere. These two different large‐scale circulations result in a vertically sheared environment in each TC. Potential vorticity (PV) tendency diagnosis shows that the TC motion is consistent with a positive region of the local tendency of PV. To quantify the contribution of each PV diagnostic term to the TC motion, we calculate each vector of the PV tendency diagnostic equation. In the merger case, the horizontal advection vector (HADV) appears to be greater than the DH vector (DHV) due to attraction. In the repulsion case, the DHV is largely compensated by the HADV. Specifically, the strong shear‐induced DH generated in the lower troposphere and downshear or downshear‐left quadrant can modify the TC motion in collaboration with the transport of PV to the middle and upper troposphere by offsetting the PV imbalance via the HADV. Overall, these results validate our hypothesis, which we refer to as the three‐dimensional Fujiwhara effect.

Scale interactions between the meso‐ and synoptic scales and the impact of diabatic heating

AbstractFor both the meso‐ and synoptic scales, reduced mathematical models give insight into their dynamical behaviour. For the mesoscale, the weak temperature gradient approximation is one of several approaches, while for the synoptic scale the quasigeostrophic theory is well established. However, the way these two scales interact with each other is usually not included in such reduced models, thereby limiting our current perception of flow‐dependent predictability and upscale error growth. Here, we address the scale interactions explicitly by developing a two‐scale asymptotic model for the meso‐ and synoptic scales with two coupled sets of equations for the meso‐ and synoptic scales respectively. The mesoscale equations follow a weak temperature gradient balance and the synoptic‐scale equations align with quasigeostrophic theory. Importantly, the equation sets are coupled via scale‐interaction terms: eddy correlations of mesoscale variables impact the synoptic potential vorticity tendency and synoptic variables force the mesoscale vorticity (for instance due to tilting of synoptic‐scale wind shear). Furthermore, different diabatic heating rates—representing the effect of precipitation—define different flow characteristics. With weak mesoscale heating relatable to precipitation rates of , the mesoscale dynamics resembles two‐dimensional incompressible vorticity dynamics and the upscale impact of the mesoscale on the synoptic scale is only of a dynamical nature. With a strong mesosocale heating relatable to precipitation rates of , divergent motions and three‐dimensional effects become relevant for the mesoscale dynamics and the upscale impact also includes thermodynamical effects.

Causation of the heavy rainfall in Kyushu in early July 2020: A perspective of the depression that originated over the Tibetan Plateau

Heavy rain fell in Kyushu, Japan, in early July 2020, which caused huge life and economic losses. The present work reports that a Tibetan Plateau vortex (TPV) generated over the Tibetan Plateau transformed into a trough at the eastern edge of the Tibetan Plateau, and the trough continued to move eastward and was responsible for the heavy rainfall in Kyushu. Accordingly, the maintenance and eastward movement of the trough and the influencing mechanism of the trough on the rainfall in Kyushu are explored based on the final analysis data (FNL) from NCEP and the JRA-55 reanalysis from JMA. Diagnoses of the potential vorticity (PV) tendency equation indicate that the horizontal PV flux convergence east of the trough is the primary contributor to the eastward movement and maintenance of the trough. Furthermore, the trough is proved to have important effects on the moisture condition and the ascending motion around Kyushu. That is, the trough increases the moisture in Kyushu by intensifying the eastward moisture transportation to Kyushu; the positive vorticity and warm center, as well as the wind perturbation related to the trough, are in favor of stronger ascending motion in Kyushu. The findings in this work provide extended knowledge on the causation of rainfall in Japan, which is beneficial for further precipitation prediction.

Origin of low-tropospheric potential vorticity in Mediterranean cyclones

Abstract. Mediterranean cyclones are extratropical cyclones, typically of smaller size and weaker intensity than other cyclones that develop over the main open ocean storm tracks. Nevertheless, Mediterranean cyclones can attain high intensities, even comparable to the ones of tropical cyclones, and thus cause large socioeconomic impacts in the densely populated coasts of the region. After cyclogenesis takes place, a large variety of processes are involved in the cyclone’s development, contributing with positive and negative potential vorticity (PV) changes to the lower-tropospheric PV anomalies in the cyclone center. Although the diabatic processes that produce these PV anomalies in Mediterranean cyclones are known, it is still an open question whether they occur locally within the cyclone itself or remotely in the environment (e.g., near high orography) with a subsequent transport of high-PV air into the cyclone center. This study introduces a Lagrangian method to determine the origin of the lower-tropospheric PV anomaly, which is applied climatologically to ERA5 reanalysis and to 12 monthly simulations, performed with the integrated forecasting system (IFS) model. We define and quantify so-called “cyclonic” and “environmental” PV and find that the main part of the lower-tropospheric PV anomaly (60 %) is produced within the cyclone, shortly prior (−12 h) to the cyclones' mature stage. Nevertheless, in 19.5 % of the cyclones the environmental PV production near the mountains surrounding the Mediterranean Basin plays a significant role in forming the low-tropospheric PV anomaly and therefore in determining the intensity of these cyclones. The analysis of PV tendencies from the IFS simulations reveals that the major PV production inside the cyclone is typically due to convection and microphysics, whereas convection and turbulent momentum tendencies cause most of the positive PV changes in the environment.

Importance of Diabatic Heating for the Eastward-Moving Heavy Rainfall Events along the Yangtze River, China

Abstract This study highlights the importance of the diabatic process in the heavy rainfall events (HREs) that are initiated on the eastern slope of the Tibetan Plateau and move to the lower reaches of the Yangtze River basin. These HREs, which cause significant socioeconomic losses in the Yangtze River basin, are typically maintained for 3 days. They develop when a large amount of moisture converges on the eastern slope of the Tibetan Plateau. By solving the quasigeostrophic (QG) omega equation, it is revealed that the vertical motion of HREs is organized by both dynamic and diabatic forcings, with the latter being dominant. The stationary boundary forcing on the eastern slope of the Tibetan Plateau also contributes to the initial organization of the HREs. While the dynamic vertical motion does not change much and the boundary forcing becomes negligible after the initial organization, diabatic vertical motion becomes more dominant in QG vertical motion (∼80%) as HREs develop and move downstream. The potential vorticity (PV) tendency budget analysis reveals that the development and eastward movement of the HRE-related surface cyclone is primarily associated with diabatic PV production to the east of the cyclone where a large amount of moisture converges. This result implies that the long-traveling HREs along the Yangtze River basin are highly self-maintaining in nature.

A potential vorticity budget view of the atmospheric circulation climatology over the Tibetan Plateau

AbstractTo aid the understanding of atmospheric circulation climatology over the Tibetan Plateau (TP), this study investigated the comprehensive potential vorticity (PV) budget over the TP using model‐level reanalysis data. Our findings demonstrate the effects of diabatic heating, friction, gravity wave drag, advection, and convection in determining circulation over the TP, from the PV budget perspective. In summer, the diabatic heating‐generated positive (negative) PV tendency facilitates cyclonic (anticyclonic) circulation near the surface (in the middle troposphere). Low PV in the upper troposphere, pertaining to the Asian monsoon anticyclone and monsoonal overturning circulation is induced by the upward transportation of diabatic heating‐generated low PV in the middle troposphere. Horizontal advection further spreads the low PV—that is vertically inputted from the middle troposphere—to a wide area around the TP in the upper troposphere. In winter, the diabatic heating‐generated positive PV near the surface is balanced by the sinking motion‐carried low PV. In addition, gravity wave drag‐generated PV is prominent in the upper troposphere in winter and can affect the downstream climate through advection. We further revealed the crucial role of the diurnal cycle in shaping the near‐surface cyclonic circulation in summer by regulating the vertical structure of diabatic heating.

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.

Application of potential vorticity tendency diagnosis method to high-resolution simulation of tropical cyclones

As the grid spacing in the numerical simulation decreases to ∼1 km, the potential vorticity (PV) structure of the simulated tropical cyclone (TC) is an annular tower of high PV with low PV in the eye and the resulting reversal of the radial PV gradient in the inner core is subject to dynamic instability, leading to complicated small-scale features in the PV field. While the PV tendency (PVT) method has been successfully used to diagnose TC motion in numerical simulations with relatively coarse resolution (∼10 km), it has been found that the PVT diagnosis method fails in the TC simulation with grid spacing of ∼1 km. This study reveals that the failure of the PVT diagnosis method in the high-resolution simulation with grid spacing of ∼1 km arises from the induced small-scale features in the PV field. The high localized PV features do not affect TC motion, but make it difficult to calculate the gradient of azimuthal mean PV when the time interval of the model output is ∼1 h. It is suggested that the PVT method can be applied to high-resolution simulations by increasing the time interval of the model output and/or smoothing the model output to reduce the influence of small-scale PV features.

Systematic assessment of the diabatic processes that modify low-level potential vorticity in extratropical cyclones

Abstract. Diabatic processes significantly affect the development and structure of extratropical cyclones. Previous studies quantified the dynamical relevance of selected diabatic processes by studying their influence on potential vorticity (PV) in individual cyclones. However, a more general assessment of the relevance of all PV-modifying processes in a larger ensemble of cyclones is currently missing. Based on a series of twelve 35 d model simulations using the Integrated Forecasting System of the European Centre for Medium-Range Weather Forecasts, this study systematically quantifies the diabatic modification of positive and negative low-level PV anomalies along the cold front, warm front, and in the center of 288 rapidly intensifying extratropical cyclones. Diabatic PV modification is assessed by accumulating PV tendencies associated with each parametrized process along 15 h backward trajectories. The primary processes that modify PV typically remain temporally consistent during cyclone intensification. However, a pronounced case-to-case variability is found when comparing the most important processes across individual cyclones. Along the cold front, PV is primarily generated by condensation in half of the investigated cyclones in the cold season (October to March). For most of the remaining cyclones, convection or long-wave radiative cooling is the most important process. Similar results are found in the warm season (April to September); however, the fraction of cyclones with PV generation by convection as the most important process is reduced. Negative PV west of the cold front is primarily produced by turbulent mixing of momentum, long-wave radiative heating, or turbulent mixing of temperature. The positive PV anomaly at the warm front is most often primarily generated by condensation in the cold season and by turbulent mixing of momentum in the warm season. Convection is the most important process only in a few cyclones. Negative PV along the warm front is primarily produced by long-wave radiative heating, turbulent mixing of temperature, or melting of snow in the cold season. Turbulent mixing of temperature becomes the primary process in the warm season, followed by melting of snow and turbulent mixing of momentum. The positive PV anomaly in the cyclone center is primarily produced by condensation in most cyclones, with only few cases primarily associated with turbulent mixing or convection. A composite analysis further reveals that cyclones primarily associated with PV generation by convection exhibit a negative air–surface temperature difference in the warm sector, which promotes a heat flux directed into the atmosphere. These cyclones generally occur over warm ocean currents in the cold season. On the other hand, cyclones that occur in a significantly colder environment are often associated with a positive air–surface temperature difference in the warm sector, leading to PV generation by long-wave radiative cooling. Finally, long-wave radiative heating due to a negative air–surface temperature difference in the cold sector produces negative PV along the cold and warm front, in particular in the cold season.

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.

A Numerical Study of the Sensitivity of Typhoon Track and Convection Structure to Cloud Microphysics

AbstractTyphoon Saola affected Taiwan from July 30, 2012, to August 03, 2012. It started in a northwestward track, and it was weakening to Category 1 storm and had a sharp cyclonic deflection track when it was about to made landfall in Taiwan on August 02. After the deflection, the typhoon moved northward and then northwestward across northern Taiwan. The cyclonic southward deflection and the northward motion constituted 12 h of looping motion which increase both the typhoon duration time and the rainfall in northeastern Taiwan. This research is to study the looping motion by the Weather Research and Forecasting model for this very weak typhoon. The model is able to reproduce the looping motion similar to that of observation. The model shows a strong asymmetry of precipitation and cross‐mountain downslope wind near the typhoon center while the typhoon track is looping toward the southwest. The potential vorticity (PV) tendency diagnosis confirms that diabatic heating and downslope wind vertical stretching are the major contributors to tropical cyclone southwestward motion, rather than the commonly referred channeling effect. The asymmetric rainfall disappears when the tropical cyclone starts moving northward in its end‐stage of looping. The northward motion is dominated by horizontal advection of PV tendency. The results highlight the influence of convection and downslope wind near steep terrain on the initial southward motion in the looping track. The model sensitivity study indicates that a large part of track uncertainty resides in the interaction of model internal dynamics and model cloud microphysics.

A Comparative Modeling Study of Supertyphoons Mangkhut and Yutu (2018) Past the Philippines with Ocean-Coupled HWRF

The ocean-coupled Hurricane Weather Research and Forecasting (HWRF) system was used to investigate the evolution of Supertyphoons Mangkhut and Yutu (2018) over the Philippines Sea and near landfall in the northern Philippines. The simulation results indicate that Mangkhut at a deepening stage has a smaller track sensitivity to the use of different physics schemes but greater intensity sensitivity, which becomes reversed for Yutu at a weakening stage. When both upstream tracks are well simulated with some specific suite of physics schemes, sensitivity experiments indicate that both track deviations near the northern Philippines are only weakly modified by the air–sea interaction (ocean-coupled or uncoupled processes), the topographic effects of the Philippines terrain (retained or not), and the initial ocean temperature change along both typhoon tracks. The interactions between the internal typhoon vortex and the large-scale flow play an important role in the overall movement of both typhoons, which were explored for their structural and convective evolutions near the terrain. The wavenumber-one potential vorticity (PV) tendency budget of the typhoon vortex was analyzed to explain the induced typhoon translation from different physical processes. The west-northwestward translation for the stronger Mangkhut near the northern Philippines is primarily induced by both horizontal and vertical PV advection but with the latter further enhanced to dominate the northward deflection when closing in to the terrain. However, the northwestward translation and track deflection near landfall for the weaker Yutu are driven by the dominant horizontal PV advection. Differential diabatic heating is relatively less important for affecting the movement of both typhoons near landfall.