Surface Cyclone Research Articles

Impact of WindBorne Observation Assimilation on Prediction of a TPV Merger Case From THINICE

AbstractThe impact of assimilating in situ temperature, moisture and wind observations from a WindBorne Systems balloon, with long duration and adjustable ballast, is evaluated using a case study from the 2022 THINICE field campaign. A case is selected wherein the WindBorne balloon directly sampled a jet streak associated with a tropopause polar vortex (TPV). The observed TPV merged with another TPV at the same time as a downstream Arctic cyclone (AC) redeveloped eastward. The case is used to better understand the role of the observed TPV in the evolution of the downstream AC. The assimilation of the WindBorne observations improves the forecast track and amplitude of the TPV during the ∼1 day forecast period that the TPV can be tracked as a distinct feature. The root mean square error of temperature at 350 hPa is improved by the WindBorne assimilation throughout the 2.5 day forecast period. The surface cyclone track forecast is improved by the WindBorne assimilation during the period of eastward redevelopment of the surface cyclone, and the sea level pressure RMSE in the surrounding region is reduced during the first ∼1.5 days of forecast lead time. Results demonstrate the capability of the long‐duration controllable WindBorne balloons to improve the analysis of the TPV associated with the jet streak, leading to improved forecast of both tropopause‐level and surface pressure features. Additionally, the importance of observing the upstream TPV amplitude for improving forecasts of the process of TPV merging and its impact on the evolution and longevity of the mature AC is confirmed.

Regime-Dependent Characteristics and Predictability of Cold-Season Precipitation Events in the St. Lawrence River Valley

Abstract The St. Lawrence River Valley experiences a variety of precipitation types (p-types) during the cold season, such as rain, freezing rain, ice pellets, and snow. These varied precipitation types exert considerable impacts on aviation, road transportation, power generation and distribution, and winter recreation and are shaped by diverse multiscale processes that interact with the region’s complex topography. This study utilizes ERA5 reanalysis data, surface cyclone climatology, and hourly station observations from Montréal, Québec, and Burlington, Vermont, during October–April 2000–18 to investigate the spectrum of synoptic-scale weather regimes that induce cold-season precipitation across the St. Lawrence River Valley. In particular, k-means clustering and self-organizing maps (SOMs) are used to classify cyclone tracks passing near the St. Lawrence River Valley, and their accompanying thermodynamic profiles, into a set of event types that include a U.S. East Coast track, a central U.S. track, and two Canadian clipper tracks. Composite analyses are subsequently performed to reveal the synoptic-scale environments and the characteristic p-types that most frequently accompany each event type. Global Ensemble Forecast System version 12 (GEFSv12) reforecasts are then used to examine the relative predictability of cyclone characteristics and the local thermodynamic profile associated with each event type at 0–5-day forecast lead times. The analysis suggests that forecasted cyclones near the St. Lawrence River Valley develop too quickly and are located left-of-track relative to the reanalysis on average, which has implications for forecasts of the local thermodynamic profile and p-type across the region when the temperature is near 0°C. Significance Statement Diverse precipitation types are observed when near-surface temperatures approach 0°C during the cold season, especially across the St. Lawrence River Valley in southern Québec. This study classifies cold-season precipitation events impacting the St. Lawrence River Valley based on the track of storm systems across the region and quantifies the average meteorological characteristics and predictability of each track. Our analysis reveals that forecasted low pressure systems develop too quickly and are left of their observed track 0–5 days prior to an event on average, which has implications for forecasted temperatures and the type of precipitation observed across the region. Our results can inform future operational forecasts of cold-season precipitation events by providing a storm-focused perspective on forecast errors during these impactful events.

Synoptic-Scale Meteorological Patterns Associated with Heavy Rainfall in the Minnesota Region

Abstract Situated in the Upper Midwest, Minnesota’s midcontinental location places it in a climate transition zone between eastern U.S. humid conditions and western semiarid conditions as well as between warm, moist air from the Gulf of Mexico to the south and drier, polar air to the north. Potential adverse impacts on ecosystems due to changing climate and precipitation patterns, together with ongoing flash flooding risks, indicate that heavy rainfall occurrence and distribution are important considerations for Minnesota. This research used ERA5 reanalysis data with 0.25° grid spacing during May–September 1959–2021 to investigate the synoptic-scale drivers of Minnesota heavy rainfall. The study utilized a neural network, self-organizing map (SOM) technique to identify sea level pressure patterns and precipitation patterns associated with heavy rainfall and used composite analysis to explore the relationships between synoptic-scale conditions and environmental parameters during heavy rain hours. Six sea level pressure patterns were identified, three of which represented advancing surface cyclones and accounted for >70% of the heavy rain hours. The spatial distribution of heavy rainfall was represented by six precipitation patterns. The greatest frequency of heavy rain hours was associated with the northwest precipitation pattern, followed by the southwest and southeast patterns. Analysis of the frequency of pressure and heavy rain precipitation pattern pairs revealed that the top five most frequent pairs were associated with advancing surface cyclones and >26% of the total heavy rain hours. Composite analysis of environmental parameters showed that favorable conditions related to moisture and lift were associated with heavy rainfall.

Investigating Arctic Cyclone–Tropopause Polar Vortex Interactions with Idealized Observing System Simulation Experiments

Abstract Arctic cyclones (ACs) are a primary driver of surface weather in the Arctic, contributing to heat and moisture transport and forcing short-term sea ice variability. Still, our understanding of the processes that drive ACs, particularly their large scales and long lifetimes, is limited. ACs are commonly associated with one or more cyclonic tropopause polar vortices (TPVs), potential vorticity (PV) anomalies in the upper troposphere and lower stratosphere that may spur baroclinic development in the surface system, though the exact processes that link the two have yet to be fully explored. In this study, we investigate physical links between TPVs, especially their mesoscale structure and moisture profiles, and ACs with idealized observing system simulation experiments (OSSEs). Starting with a nature run, we simulate different types of dropsonde observations over a TPV during the nascent phase of a nearby AC. The Model for Prediction Across Scales (MPAS) and the Data Assimilation Research Testbed (DART) ensemble adjustment Kalman filter are then used to run experiments to test the impact of these detailed TPV observations. In addition to a control, five main experiments are conducted, assimilating new observations of temperature and humidity. All experiments reduce forecast errors at the surface and throughout the troposphere. Additional humidity observations alter vertical PV distributions, which in turn impact the development of the AC. Experiments with additional temperature observations exhibit improvements in TPV structure and surrounding PV features and produce stronger surface cyclones with skillful TPV forecasts for up to 36 h longer than the control. Significance Statement Arctic cyclones (ACs) are a weather feature that can produce high winds, precipitation, and changes to sea ice cover in the Arctic. As a result, forecasting these storms accurately is important for human and economic interests in the region; however, there are currently gaps in our understanding of how ACs strengthen and persist. In this study, we explore potential links between ACs and weather features higher up in the atmosphere called tropopause polar vortices (TPVs) using computer modeling experiments. This study shows that there are important connections between the characteristics of TPVs and the development of ACs. These findings will be useful for making more accurate forecasts of future events and advancing our knowledge of how sea ice changes relate to the atmosphere.

Four Storm Surge Cases on the Coast of São Paulo, Brazil: Weather Analyses and High-Resolution Forecasts

The coast of São Paulo, Brazil, is exposed to storm surges that can cause damage and floods. These storm surges are produced by slowly traveling cyclone–anticyclone systems. The motivation behind this work was the need to evaluate high-resolution forecasts of the mean sea-level pressure and 10 m winds, which are the major drivers of the wave model. This work is part of the activity in devising an early warning system for São Paulo coastal storm surges. For the evaluation, four case studies that had a major impact on the coast of São Paulo in 2020 were selected. Because storm surges that reach the coast may cause coastal flooding, precipitation forecasts were also evaluated. The mesoscale Eta model produces forecasts with a 5 km resolution for up to an 84 h lead time. The model was set up in a region that covers part of southeast and south Brazil. The ERA5 reanalysis was used to describe the large-scale synoptic conditions and to evaluate the weather forecasts. The cases showed a region in common between 35° S, 40° S and 35° W, 45° W where the low-pressure center deepened rapidly on the day before the highest waves reached the coast of São Paulo, with a mostly eastward, rather than northeastward, displacement of the associated surface cyclone and minimal or no tilt with height. The winds on the coast were the strongest on the day before the surge reached the coast of São Paulo, and then the winds weakened on the day of the maximum wave height. The pattern of the mean sea-level pressure and 10 m wind in the 36 h, 60 h, and 84 h forecasts agreed with the ERA5 reanalysis, but the pressure was slightly underestimated. In contrast, the winds along the coast were slightly overestimated. The 24 h accumulated precipitation pattern was also captured by the forecast, but was overestimated, especially at high precipitation rates. The 36 h forecasts showed the smallest error, but the growth in the error for longer lead times was small, which made the 84 h forecasts useful for driving wave models and other local applications, such as an early warning system.

Discriminating Factors that Favor the Development of High-Impact Weather Events in Association with Polar–Subtropical Jet Superpositions

Abstract A vertical superposition of the polar and subtropical jet streams constitutes a unique synoptic-scale environment with the potential to induce high-impact weather, including anomalously strong surface cyclones that are accompanied by heavy precipitation and strong winds. Jet superpositions are not always a sufficient condition for the occurrence of high-impact weather, however, so understanding the dynamical and thermodynamic environments that favor the development of high-impact weather in association with jet superpositions is essential for improving sensible weather forecasts during these events. In this study, we pair a climatology of jet superpositions with climatologies of atmospheric rivers and surface cyclones to determine the frequency with which these features accompany jet superpositions. We subsequently construct two subsets of jet superpositions for further analysis. “High-impact” jet superposition cases are defined as those that feature an atmospheric river and a highly anomalous surface cyclone relative to climatology, which can potentially support extreme near-surface winds and precipitation. In contrast, “null” cases are defined as jet superposition cases that are not associated with a surface cyclone and are therefore less likely to support widespread high-impact weather. Composite analyses are then performed to identify discriminating environmental factors between high-impact and null cases, and how these factors influence jet superposition dynamics. We find that stronger environmental baroclinicity and a sufficient moisture supply within the near-jet environment are common characteristics of high-impact cases. These characteristics subsequently support the development of a more vigorous ageostrophic transverse circulation beneath the superposed jet’s exit region during high-impact cases and more intense surface cyclogenesis. Significance Statement A jet superposition event occurs when the normally separate polar and subtropical jets combine to form a single jet. This study aims to understand what factors differentiate jet superposition events that coincide with strong winds and heavy precipitation, or “high-impact weather,” from those that are less likely to coincide with such weather conditions. We identified several important environmental characteristics that tend to precede jet superposition events with a large potential to induce high-impact weather, including increased moisture and a strengthened pole-to-equator temperature gradient. These results provide indicators forecasters may consider when predicting the impacts of a jet superposition event.

Comparison of Intense Summer Arctic Cyclones Between the Marginal Ice Zone and Central Arctic

AbstractArctic cyclone activity is an important component of the local climate, and the frequent occurrence of extreme summer storms has raised widespread scientific interest. In this paper, we investigated the distinctive structural characteristics of intense summer Arctic cyclones by utilizing ERA‐Interim reanalysis data and employing a deep learning algorithm for cyclone detection. We found that the northern edge of Eurasia (i.e., the marginal ice zone (MIZ)) and the Alpha Ridge of Arctic Ocean (AR, i.e. central Arctic) are the two most active regions for intense Arctic cyclone activities in summer (from June to September). However, the surface conditions and coupling frequency between surface cyclone and tropopause polar vortices (TPVs) are distinct over these two regions. By further analysis of 100 intense cyclone activities in these two areas, respectively, we found that cyclones in MIZ are often smaller in size but higher in intensity at their maximum intensity, and their life cycles are generally shorter. MIZ cyclones are typically accompanied by a large Eady growth rate and frontal structure in the lower troposphere and their intensification primarily attributed to the thermal‐baroclinic process. In contrast, cyclones in AR are more frequently associated with higher potential vorticity (PV) values and pronounced PV downward intrusion from the stratosphere, as well as notable “upper warm‐lower cold” structures. The downward intrusion of TPVs and stratosphere vortices contribute to a decrease in the upper and column air mass deficit, leading to the intensification of surface Arctic cyclones in these regions.

Aquaplanet simulations with winter and summer hemispheres: model setup and circulation response to warming

Abstract. To support further understanding of circulation changes in a warming climate, an idealised aquaplanet model setup containing summer and winter hemispheres is presented, and the results of circulation changes under warming are discussed. First, a setup is introduced that enables aquaplanet simulations with a warmer and a colder hemisphere, including realistic-looking summer and winter jet streams, storm tracks, and precipitation patterns that are fairly similar to observations, as well as a more intense and equatorward storm track in the winter compared to the summer hemisphere. The sea surface temperature (SST) distribution used here is inspired by the June–July–August zonal mean SST found in reanalysis data and is flexible to allow control of the occurrence of a single or double intertropical convergence zone (ITCZ). The setup is then used to investigate circulation changes under uniform warming, as motivated by recent research. For example, the stronger poleward shift of the storm tracks during summer compared to winter is reproduced. Furthermore, the jet waviness decreases under warming when compared on isentropes with maximum wind speed or isentropes at similar heights in pressure space. Jet stream waviness increases under warming when compared at similar-valued isentropes but primarily because the corresponding isentrope is closer to the surface in the warmer climate and waviness climatologically increases downwards in the atmosphere. A detailed analysis of the changes in wave amplitude for different wavenumbers confirms that the amplitude of large waves increases with warming, while that of short waves decreases with warming. The reduction in wave amplitude of short synoptic waves is found to dominate in the jet core region, where jet waviness also decreases and is more pronounced on the equatorward side of the jet. Long waves increase in amplitude on the poleward side of the jet and at upper stratospheric levels, which is consistent with increased jet waviness at these levels. The projected increased amplitude of planetary waves and the reduced amplitude of synoptic waves are thus clear in our aquaplanet simulations and do not require zonal asymmetries or regional warming patterns. During so-called high-amplitude wave events, there is no evidence for a preferential phase of Rossby waves of wavenumbers 5 or 7, indicating the crucial role of stationary waves forced by orography or land–sea contrast in establishing previously reported preferential phases. We confirm that feature-based block detection requires significant tuning to the warmer climate to avoid the occurrence of spurious trends. After adjustment for changes in tropopause height, the block detection used here shows no trend in the summer hemisphere and an increase in blocking in the colder hemisphere. We also confirm previous findings that the number of surface cyclones tends to decrease globally under warming and that the cyclone lifetimes become shorter, except for very long-lived cyclones.

Severe Convective Storms in Limited Instability Organized by Pattern and Distribution

Abstract Severe convection occurring in high-shear, low-CAPE (HSLC) environments is a common cool-season threat in the southeastern United States. Previous studies of HSLC convection document the increased operational challenges that these environments present compared to their high-CAPE counterparts, corresponding to higher false-alarm ratios and lower probability of detection for severe watches and warnings. These environments can exhibit rapid destabilization in the hours prior to convection, sometimes associated with the release of potential instability. Here, we use self-organizing maps (SOMs) to objectively identify environmental patterns accompanying HSLC cool-season severe events and associate them with variations in severe weather frequency and distribution. Large-scale patterns exhibit modest variation within the HSLC subclass, featuring strong surface cyclones accompanied by vigorous upper-tropospheric troughs and northward-extending regions of instability, consistent with prior studies. In most patterns, severe weather occurs immediately ahead of a cold front. Other convective ingredients, such as lower-tropospheric vertical wind shear, near-surface equivalent potential temperature (θe) advection, and the release of potential instability, varied more significantly across patterns. No single variable used to train SOMs consistently demonstrated differences in the distribution of severe weather occurrence across patterns. Comparison of SOMs based on upper and lower quartiles of severe occurrence demonstrated that the release of potential instability was most consistently associated with higher-impact events in comparison to other convective ingredients. Overall, we find that previously developed HSLC composite parameters reasonably identify high-impact HSLC events. Significance Statement Even when atmospheric instability is not optimal for severe convective storms, in some situations they can still occur, presenting increased challenges to forecasters. These marginal environments may occur at night or during the cool season, when people are less attuned to severe weather threats. Here, we use a sorting algorithm to classify different weather patterns accompanying such storms, and we distinguish which specific patterns and weather system features are most strongly associated with severe storms. Our goals are to increase situational awareness for forecasters and to improve understanding of the processes leading to severe convection in marginal environments.

Saharan rainfall climatology and its relationship with surface cyclones

The Sahara is the largest and driest of the hot deserts on Earth, with regions where rainfall reaches the surface on average less than once a year. Water resources are scarce, and rainfall tends to occur sporadically in space and time. While rain is a precious resource in the Sahara, heavy precipitation events (HPEs) in the desert have the potential to trigger flash floods on the barren soil. Because of the sparse rainfall monitoring network and the relatively poor performance of global models in representing rainfall over the Sahara, the analysis of Saharan HPEs has primarily relied on case studies. Therefore, general rainfall characteristics of Saharan HPEs are unexplored, and the prevailing weather conditions enabling such rainfall are unknown. To tackle this problem, we utilised satellite-derived precipitation estimations (IMERG) spanning 21 years (2000–2021) to identify ∼42⋅103 small (>103km2) to large (<106km2) HPEs in the Sahara and to extract their rainfall properties, and atmospheric reanalyses (ERA5) to examine the corresponding meteorological conditions in which they develop. Three case studies illustrate the relevance of cyclones for exceptionally large HPEs, including one in the driest region of the Sahara. Saharan HPEs occur, on average, every second day. They are more common in summer than in the other seasons, occur most frequently in the southern Sahara, and exhibit a clear convectively-driven diurnal cycle. Winter events have, on average, larger spatial extent, longer duration, and are characterised by larger areas exhibiting more extreme rainfall in terms of return periods. Autumn HPEs are concentrated in the western Sahara, while events in the north of the desert and in its driest core in the northeast occur mainly in winter and spring. In these regions, north of the Tropic of Cancer, events are highly associated with surface cyclones. HPEs that were associated with cyclones are characterised by larger spatial extent and rainfall volume. Considering that weather and climate models often depict synoptic-scale weather systems more accurately than rainfall patterns, the association of Saharan HPEs with surface cyclones and other synoptic-scale systems can aid in comprehending the effects of climate change in the desert. Furthermore, it underscores the potential for higher predictability of these events.

The monthly evolution of precipitation and warm conveyor belts during the central southwest Asia wet season

Abstract. Understanding the nature of precipitation over central southwest Asia (CSWA), a data-sparse, semi-arid region, is important given its relation to agricultural productivity and the likelihood of hazards such as flooding. The present study considers how daily precipitation and local vertical motion – represented by warm conveyor belts (WCBs) – evolve from November to April over CSWA. First we compare several precipitation datasets, revealing that the seasonality of daily precipitation is consistent across estimates that incorporate satellite information, while total accumulation amounts differ substantially. A common feature across datasets is that the majority of precipitation occurs on the few days when area-averaged accumulation exceeds 4 mm, which are most frequent in February and March. The circulation pattern associated with heavy (&lt; 4 mm d−1) precipitation days evolves within the wet season from a southwest–northeast tilted couplet of circulation anomalies in January and February to a neutrally tilted monopole pattern in April. El Niño conditions are associated with more heavy precipitation days than La Niña conditions, with both enhanced WCB frequency and moisture transport observed during the former. An exception to this is found in January, when precipitation, WCB frequency, and moisture do not increase, despite a similar increase in surface cyclones to other months, suggesting that precipitation changes cannot always be inferred from cyclone frequency changes. Nonetheless, our results generally support prior connections made between the El Niño–Southern Oscillation (ENSO) and seasonal-to-interannual precipitation anomalies and extend this connection to one between the slowly evolving ENSO influence and transient and local vertical motion represented by WCBs.

Ensemble-based analysis of heavy rainfall–snowfall associated with mesoscale precipitation bands within an extratropical cyclone over northeastern China

This paper investigates the key synoptic-scale factors that affected the forecasting of mesoscale rainfall and snowfall and their associated uncertainties in a heavy rain–snow event in northeastern China on 18–20 November 2020, using ensemble-based sensitivity analysis based on global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts. The heavy precipitation event was attributed to an extratropical cyclone and experienced two stages, with the snowfall stage having a better precipitation forecast skill than the rainfall stage. The mesoscale rainfall and snowfall were caused by a mesoscale rainband over Liaoning Province and two mesoscale snowbands over Heilongjiang Province, respectively, and they showed some differences with respect to their forecast skill and related key synoptic-scale factors contributing to the precipitation centers. The precipitation amount in the two different stages was correlated significantly with the midlevel trough and sensitive to the location and intensity of the low-level vortex (surface cyclone), and particularly the low-level jets and the associated water vapor transport. However, some differences were confirmed in the two different stages: the weaker midlevel trough and accompanying weaker low-level temperature trough in the rainfall stage were related to increased precipitation because the midlevel trough was far away from the control area, while the stronger midlevel trough and accompanying stronger low-level temperature trough were associated with increased precipitation in the snowfall stage. In addition to the synoptic-scale low-level jet (SLLJ), the precipitation in the rainfall stage was also affected by a boundary layer jet (BLJ) over the ocean, while only SLLJs were present in the snowfall stage. The uncertainty of the precipitation forecast was derived mainly from the uncertainty in the strength and location of the SLLJs and BLJ. Notably, the intensity of northeasterly winds west of the low-level vortex may affect the predictability of heavy snowfall.

Mesoscale and Synoptic Scale Analysis of Narrow Cold Frontal Rainband During a Landfalling Atmospheric River in California During January 2021

AbstractNarrow cold‐frontal rain bands (NCFR) often produce short‐duration and high‐intensity precipitation that can lead to flooding and debris flow in California (CA). On 27 January 2021, an atmospheric river (AR) associated with an intense surface cyclone made landfall over coastal northern CA, which featured a prominent NCFR. This study uses high‐resolution West Weather Research and Forecasting simulations to accurately resolve the gap and core structure of the NCFR and provide reliable precipitation estimations, compensating for limitations of radar and satellite observations. This NCFR was supported by robust synoptic‐scale quasi‐geostrophic (QG) forcing for ascent and frontogenesis. It propagated southward from Cape Mendocino to Big Sur in 12 hr before stalling and rotating counter‐clockwise in central/southern CA due to upstream Rossby wave breaking and an amplifying upper‐tropospheric trough. With the lower to middle tropospheric flow backed considerably to the south‐southwest over the NCFR, the increase of the vertical wind shear caused the transition from parallel to trailing stratiform precipitation. The stall and pivot of the AR and NCFR led to intense rainfall with a 2‐day precipitation accumulation greater than 300 mm over central CA. In addition, under the potential instability and frontogenesis, a moist absolutely unstable layer between 850 and 700 hPa was captured at the leading edge of the NCFR, which indicated slantwise deep layer lifting and high precipitation efficiency. This study reveals synoptic‐scale and mesoscale drivers of rainfall outside orographic lifting and reaffirms the importance of high‐resolution numerical modeling for the prediction of extreme precipitation and related natural hazards.

Dynamical systems insights on cyclonic compound “wet” and “windy” extremes in the Eastern Mediterranean

AbstractExtreme weather, such as heavy precipitation and strong winds, can co‐occur, resulting in larger socio‐economic impacts than the linear sum of their components. These are termed “compound extremes” and are challenging to forecast even a few days before. With this aim, we investigate the “wet” and “windy” extremes associated with Eastern Mediterranean cyclones in winter. We combine insights from a synoptic classification algorithm, traditional atmospheric analysis techniques, and dynamical systems theory. In phase space, the latter quantifies atmospheric states' co‐recurrence ratio (α) and persistence (Θ−1). We base the analysis on ERA5 at various pressure levels and time lags. Our analysis is two‐fold; first, we define compound dynamical extremes (CDE) based on upper and lower deciles of α and Θ−1, linking them to the synoptic patterns. We find that CDEs are related to the depth and location of the surface cyclone and upper‐level trough. Then, we show that high co‐recurrence and persistence are associated with heavier precipitation and stronger wind speed. Second, we define compound extremes (CE), the upper 5% of precipitation intensity and wind velocity co‐occurring simultaneously. We link these back to the dynamical system metrics anomalies. Finally, we compare CEs with individual extremes and cyclone climatology. We find that CEs display significantly higher co‐recurrence and persistence than individual extremes. Indeed, days with high co‐recurrence and persistence anomalies are ∼2–18 times more likely to be allied with extreme weather, depending on the extreme type, than low co‐recurrence and persistence. Moreover, we show that the upper levels are significantly more persistent than the surface flow during a CE. The mid‐February 2012 “wet” and “windy” CE case‐study strengthened the abovementioned climatological findings. To conclude, the dynamical systems viewpoint is a valuable complement to understanding the dynamics of CEs in the Eastern Mediterranean. We foresee it to be fruitfully applied to other CEs and regions.

Distinct Roles of Cyclones and Anticyclones in Setting the Midwinter Minimum of the North Pacific Eddy Activity: A Lagrangian Perspective

Abstract The North Pacific storm-track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter, which seems inconsistent with linear baroclinic instability theory. This “midwinter minimum” of the storm-track activity has been intensively investigated for decades as a test case for storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we investigate the detailed seasonal evolution of the climatological density of surface migratory anticyclones over the North Pacific, in comparison with its counterpart for cyclones, based on a Lagrangian tracking algorithm. We demonstrate that the frequency of surface cyclones over the North Pacific maximizes in midwinter, whereas that of anticyclones exhibits a distinct midwinter minimum under the upstream influence, especially from the Japan Sea region. In midwinter, it is only on such a rare occasion that prominent weakening of the East Asian winter monsoon allows a migratory surface anticyclone to form over the Japan Sea, despite the unfavorable climatological-mean conditions due to persistent monsoonal cold-air outbreaks and the excessively strong upper-tropospheric westerlies. The midwinter minimum of the North Pacific anticyclone density suggests that anticyclones are likely the key to understanding the midwinter minimum of the North Pacific storm-track activity as measured by Eulerian eddy statistics.

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.

What distinguishes 100-year precipitation extremes over central European river catchments from more moderate extreme events?

Abstract. Historical extreme flooding events in central European river catchments caused high socioeconomic impacts. Previous studies have analysed single events in detail but have not focused on a robust analysis of the underlying extreme precipitation events in general, as historical events are too rare for a robust assessment of their generic dynamical causes. This study tries to fill this gap by analysing a set of realistic daily 100-year large-scale precipitation events over five major European river catchments with the help of operational ensemble prediction data from the ECMWF. The dynamical conditions during such extreme events are investigated and compared to those of more moderate extreme events (20 to 50 year); 100-year precipitation events are generally associated with an upper-level cutoff low over central Europe in combination with a surface cyclone southeast of the specific river catchment. The 24 h before the event is decisive for the exact location of this surface cyclone, depending on the location and velocity of the upper-level low over western Europe. The difference between 100-year and more moderate extreme events varies from catchment to catchment. Dynamical mechanisms such as an intensified upper-level cutoff low and surface cyclone are the main drivers distinguishing 100-year events in the Oder and Danube catchments, whereas thermodynamic mechanisms such as a higher moisture supply in the lower troposphere east of the specific river catchment are more relevant in the Elbe and Rhine catchments. For the Weser and Ems catchment, differences appear in both dynamical and thermodynamic mechanisms.

A Climatology of Easterly Wind Lake-Effect and Lake-Enhanced Precipitation Events over the Western Lake Superior Region

Abstract Lake-effect precipitation is convective precipitation produced by relatively cold air passing over large and relatively warm bodies of water. This phenomenon most often occurs in North America over the southern and eastern shores of the Great Lakes, where high annual snowfalls and high-impact snowstorms frequently occur under prevailing west and northwest flow. Locally higher snow or rainfall amounts also occur as a result of lake-enhanced synoptic precipitation when conditionally unstable or neutrally stratified air is present in the lower troposphere. Although likely less common, lake-effect and lake-enhanced precipitation can also occur with easterly winds, impacting the western shores of the Great Lakes. This study describes a 15-yr climatology of easterly lake-effect (ELEfP) and lake-enhanced (ELEnP) precipitation [conjointly, easterly lake collective precipitation (ELCP)] events that developed in east-to-east-northeasterly flow over western Lake Superior from 2003 to 2018. ELCP occurs infrequently but often enough to have a notable climatological impact over western Lake Superior, with an average of 14.6 events per year. The morphology favors both single shore-parallel ELEfP bands due to the convex western shoreline of Lake Superior and mixed-type banding due to ELEnP events occurring in association with “overrunning” synoptic-scale precipitation. ELEfP often occurs in association with a surface anticyclone to the north of Lake Superior. ELEnP typically features a similar northerly displaced anticyclone and a surface cyclone located over the upper Midwest that favor easterly boundary layer winds over western Lake Superior.

Evaluating Common Characteristics of Antarctic Tropopause Polar Vortices

Abstract Tropopause polar vortices (TPVs) are coherent circulations that occur over polar regions and can be identified by a local minimum in potential temperature and local maximum in potential vorticity. Numerous studies have focused on TPVs in the Arctic region; however, no previous studies have focused on the Antarctic. Given the role of TPVs in the Northern Hemisphere with surface cyclones and other extreme weather, and the role that surface cyclones can play on moisture transport and sea ice breakup, it is important to understand whether similar associations exist in the Southern Hemisphere. Here, characteristics of TPVs in the Antarctic are evaluated for the first time under the hypothesis that their characteristics do not significantly differ from those of the Northern Hemisphere. To improve understanding of Antarctic TPV characteristics, this study examines TPVs of the Southern Hemisphere and compares them to their Northern Hemisphere counterparts from 1979 to 2018 using ERA-Interim data. Common characteristics of TPVs including frequency, locations, lifetimes, strength, and seasonality are evaluated. Results indicate that topography correlates to the geographic distribution of TPVs and the locations of local maxima TPV occurrence, as observed in the Northern Hemisphere. Additionally, TPVs in the Southern Hemisphere exhibit seasonal variations for amplitude, lifetime, and minimum potential temperature. Southern Hemisphere TPVs share many similar characteristics to those observed in the Northern Hemisphere, including longer summer lifetimes. The association of Southern Hemisphere TPVs and surface cyclone frequency is explored, and it appears that TPVs have a precursory role to surface cyclones, as seen in the Northern Hemisphere.

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.