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Kara Hartig
and
Eli Tziperman

Abstract

In spite of the mean warming trend over the last few decades and its amplification in the Arctic, some studies have found no robust decline or even a slight increase in wintertime cold air outbreaks over North America. But fossil evidence from warmer paleoclimate periods indicates that the interior of North America never dropped below freezing even in the depths of winter, which implies that the maintenance of cold air outbreaks is unlikely to continue indefinitely with future warming. To identify key mechanisms affecting cold air outbreaks and understand how and why they will change in a warmer climate, we examine the development of North American cold air outbreaks in both a pre-industrial and a roughly 8×CO2 scenario using the Community Earth System Model, CESM2. We observe a sharp drop-off in the wintertime temperature distribution at the freezing temperature, suppressing below-freezing conditions in the warmer climate and above-freezing conditions in the pre-industrial case. The disappearance of Arctic sea ice and loss of the near-surface temperature inversion dramatically decrease the availability of below-freezing air in source regions. Using an air parcel trajectory analysis, we demonstrate a remarkable similarity in both the dynamics and diabatic effects acting on cold air masses in the two climate scenarios. Diabatic temperature evolution along cold air outbreak trajectories is a competition between cooling from longwave radiation and warming from boundary layer mixing. Surprisingly, while both diabatic effects strengthen in the warmer climate, the balance remains the same, with a net cooling of about −6 K over 10 days.

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Arthur Coquereau
,
Florian Sévellec
,
Thierry Huck
,
Joël J.-M. Hirschi
, and
Antoine Hochet

Abstract

As well as having an impact on the background state of the climate, global warming due to human activities could affect its natural oscillations and internal variability. In this study, we use four initial-condition ensembles from the CMIP6 framework to investigate the potential evolution of internal climate variability under different warming pathways for the twenty-first century. Our results suggest significant changes in natural climate variability and point to two distinct regimes driving these changes. The first is a decrease in internal variability of surface air temperature at high latitudes and all frequencies, associated with a poleward shift and the gradual disappearance of sea ice edges, which we show to be an important component of internal variability. The second is an intensification of the interannual variability of surface air temperature and precipitation at low latitudes, which appears to be associated with El Niño–Southern Oscillation (ENSO). This second regime is particularly alarming because it may contribute to making the climate more unstable and less predictable, with a significant impact on human societies and ecosystems.

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Timothy A. Coleman
,
Richard L. Thompson
, and
Gregory S. Forbes

Abstract

Recent articles have shown that the long-portrayed “tornado alley” in the central plains is not an accurate portrayal of current tornado frequency over the United States. The greatest tornado threat now covers parts of the eastern United States. This paper shows that there has been a true spatial shift in tornado frequency, dispelling any misconceptions caused by the better visibility of tornadoes in the Great Plains versus the eastern United States. Using F/EF1+ tornadoes (the dataset least affected by increasing awareness of tornado locations or by changing rating methods), a 1° × 1° grid, and data for the two 35-yr periods 1951–85 and 1986–2020, we show that since 1951, by critical measures (tornadogenesis events, tornado days, and tornado pathlength), tornado activity has shifted away from the Great Plains and toward the Midwest and Southeast United States. In addition, tornadoes have trended away from the warm season, especially the summer, and toward the cold season since 1951. Annual trends in tornadoes by season (winter, spring, summer, and autumn) confirm this. All of the increase in F/EF1+ tornadoes in the eastern United States is due to an increase in cold season tornadoes. Tornadoes in the western United States decreased 25% (from 8451 during 1951–85 to 6307 during 1986–2020), while tornadoes in the eastern United States. increased 12% (from 9469 during 1951–85 to 10 595 during 1986–2020). The cities with the largest increases and decreases in tornado activity since 1951 are determined.

Significance Statement

This paper quantifies in many ways (tornadoes, tornado days, and pathlength) the geographical shift in tornadoes from the central to the eastern United States and from the warm season to the cold season, since 1951. Where and when tornadoes most frequently occur is significant not only for the research and operational meteorology communities but also for public perception and risk awareness. Some research studies have shown that tornado casualties are more likely in the eastern United States and the cold season because of preconceived notions of a “tornado alley” in the Great Plains and a “tornado season” in the spring. Publication of the results of this research might help ameliorate this problem.

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Francesco De Martin
,
Silvio Davolio
,
Mario Marcello Miglietta
, and
Vincenzo Levizzani

Abstract

The Po Valley in northern Italy is a hotspot for tornadoes in Europe in spite of being surrounded by two mountain ridges: the Alps in the north and the Apennines in the southwest. The research focuses on the case study of 19 September 2021, when seven tornadoes (four of them rated as F2) developed in the Po Valley in a few hours. The event was analyzed using observations and numerical simulations with the convection-permitting Modello Locale in Hybrid Coordinates (MOLOCH) model. Observations show that during the event in the Po Valley, there were two surface boundaries that created a triple point: an outflow boundary generated by convection triggered in the Alpine foothills and a dryline generated by downslope winds from the Apennines, while warm and moist air advected westward from the Adriatic Sea east (ahead) of the boundaries. Tornadoes developed about 20 km northeast of the triple point. Numerical simulations with 500-m grid spacing suggest that the development of supercells and drylines in the Po Valley was sensitive to the elevation of the Apennines. Simulated vertical profiles show that the best combination of instability and wind shear for the development of tornadoes was attained within a narrow area located ahead of the dryline. A conceptual model for the development of tornadoes in the Po Valley is proposed, and the differences between tornado environments over a flat terrain and over a region with complex terrain are discussed.

Significance Statement

The Po Valley is a highly populated area where some of the most violent tornadoes in Europe have developed. We investigated a tornado outbreak that occurred on 19 September 2021 in this region, in order to identify its main environmental characteristics. High-resolution numerical simulations revealed that values of instability and wind shear were compatible with the development of several tornadoes only in a narrow area close to the intersection of two surface boundaries (a triple point). Moreover, the atmospheric environment during the tornado outbreak was strongly influenced by the presence of mountain ridges surrounding the plain. We have summarized our results in a conceptual model that can potentially be used for forecasting applications.

Open access
Sining Ling
and
Riyu Lu

Abstract

The climatological western North Pacific summer monsoon onset, so called convection jump, occurs around 41th pentad, corresponding to an abrupt northeastward extension of strong convection. This study investigates the process of convection jump from a local perspective. Composite analyses are performed based on the onset dates that are identified in individual years. The results show that the convective inhibition (CIN) decreases dramatically around the onset dates, while the convective available potential energy (CAPE) reaches its maximum long before the onset, suggesting that the CIN, rather than CAPE, plays a dominant role in triggering convection. Further analysis indicates that the reduction of CIN is induced by the increased low-lever relative humidity, which is the result of enhanced water vapor convergence. The moisture transportation is primarily contributed by the wind transfer from easterlies to southeasterlies or southerlies along the southern boundary of convection jump region, in accordance with the monsoon trough establishment. The present observational results may be used to evaluate climate models in simulating stepwise evolution of summer monsoon.

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Dylan J. Dodson
and
William A. Gallus Jr.

Abstract

Ten bow echo events were simulated using the Weather Research and Forecasting (WRF) Model with 3- and 1-km horizontal grid spacing with both the Morrison and Thompson microphysics schemes to determine the impact of refined grid spacing on this often poorly simulated mode of convection. Simulated and observed composite reflectivities were used to classify convective mode. Skill scores were computed to quantify model performance at predicting all modes, and a new bow echo score was created to evaluate specifically the accuracy of bow echo forecasts. The full morphology score for runs using the Thompson scheme was noticeably improved by refined grid spacing, while the skill of Morrison runs did not change appreciably. However, bow echo scores for runs using both schemes improved when grid spacing was refined, with Thompson runs improving most significantly. Additionally, near storm environments were analyzed to understand why the simulated bow echoes changed as grid spacing was changed. A relationship existed between bow echo production and cold pool strength, as well as with the magnitude of microphysical cooling rates. More numerous updrafts were present in 1-km runs, leading to longer intense lines of convection which were more likely to evolve into longer-lived bow echoes in more cases. Large-scale features, such as a low-level jet orientation more perpendicular to the convective line and surface boundaries, often had to be present for bow echoes to occur in the 3-km runs.

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Yu-Chieng Liou
,
Tzu-Jui Chou
,
Yu-Ting Cheng
, and
Yung-Lin Teng

Abstract

This study presents a sequential procedure formulated by combining a multiple-Doppler radar wind synthesis technique with a thermodynamic retrieval method, which can be applied to retrieve the three-dimensional wind, pressure, temperature, rainwater mixing ratio, and moisture over complex terrain. The retrieved meteorological state variables are utilized to reinitialize a high-resolution numerical model, which then carries out time integration using four different microphysical (MP) schemes, including the Goddard Cumulus Ensemble (GCE), Morrison (MOR), WRF single-moment 6-class (WSM6), and WRF double-moment 6-class (WDM6) schemes. It is found that through this procedure, the short-term quantitative precipitation forecast (QPF) skill of a numerical model over mountainous areas can be significantly improved up to 6 h. The moisture field plays a crucial role in producing the correct rainfall forecast. Since no specific microphysical scheme outperforms the others, a combination of various rainfall scenarios forecasted by different MP schemes is suggested in order to provide a stable and reliable rainfall forecast. This work also demonstrates that, with the proposed approach, radar data from only two volume scans are sufficient to improve the rainfall forecasts. This is because the unobserved meteorological state variables are instantaneously retrieved and directly used to reinitialize the model, thereby the model spinup time can be effectively shortened.

Open access
Yaodeng Chen
,
Hong Zheng
,
Tao Sun
,
Deming Meng
,
Luyao Qin
, and
Jinfang Yin

Abstract

On 20–21 July 2021, a record-breaking rainfall event occurred in Henan Province, China, and a maximum hourly accumulated precipitation of 201.9 mm was recorded at Zhengzhou Meteorological Station. To improve the prediction of such extreme rainfall and to better understand the impacts of the radar reflectivity assimilation on forecasting, we assimilated radar reflectivity data using the hydrometeor background error covariance (HBEC) that includes vertical and multivariate correlations and then diagnosed the dynamic, thermal, and microphysical forecasts of this event. The results show that the radar reflectivity assimilation based on the HBEC properly transferred the observed radar reflectivity to the analysis of hydrometeors and other model states, and clearly improved the heavy rainfall forecast. The diagnosis of the dynamic and thermal forecasts indicated that the reflectivity assimilation based on the HBEC improved the convective environments of the precipitation systems, with stronger cold pools near the surface and deeper and wetter updrafts near Zhengzhou station, when compared with the experiment that did not assimilate radar reflectivity and the experiment that assimilated radar reflectivity without using the HBEC. The diagnosis of the microphysical forecasts further shows that assimilating reflectivity data using HBEC contributed to higher conversion rates of water vapor and cloud water to graupel and higher conversion rates of graupel and cloud water to rainwater, when compared with the other experiments. These improvements of both convective environments and microphysical processes within the convections ultimately enhanced the forecasts of this extreme rainfall event.

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Hanzhao Yu
,
Tianjun Zhou
, and
Linqiang He

Abstract

The zonal wavenumber-5 circumglobal teleconnection pattern (CGT) is one of the most critical atmospheric teleconnection patterns during boreal summer over the Northern Hemisphere (NH). CGT can exert significant climatic impact across NH including Europe, East Asia and North America but how reliable coupled climate models simulate the characteristics of CGT is poorly understood. Here, twenty coupled models with their respective versions in Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6 are selected to evaluate their performance on CGT simulation. We find that while both CMIP5 and CMIP6 models are able to capture the basic features of CGT in multi-model mean (MMM), there are large inter-model discrepancies in the simulation of CGT pattern among CMIP5 and CMIP6 models. High-skill models exhibit strong action center over west-central Asia, coinciding with the pattern derived from reanalysis, while the corresponding action center in low-skill models are weaker. Further analyses demonstrate that high-skill models are capable of simulating more realistic Indian Summer Monsoon (ISM) precipitation anomalies related to CGT. The resultant anomalous upper-tropospheric divergence over west-central Asia, acting as a Rossby wave source, can therefore excite the above-mentioned action center. This high- and low-skill model difference on CGT-ISM relationship is consistent in both CMIP5 and CMIP6. It is also found that high-skill models tend to simulate more realistic CGT-ENSO relationship. The relationship between simulation skills of CGT-ENSO correlation and CGT spatial pattern is attributed to the remote impact of ENSO on CGT wavetrain through affecting ISM precipitation anomalies.

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Matthew C. Brown
,
Geoffrey R. Marion
, and
Michael C. Coniglio

Abstract

Observational and modeling efforts have explored the formation and maintenance of mesovortices, which contribute to severe hazards in quasi-linear convective systems (QLCS). There exists an important interplay between environmental shear and cold pool-induced circulations which, when balanced, allow for upright QLCS updrafts with maximized lift along storm outflow boundaries. Numerical simulations have primarily tested the sensitivity of squall lines to zonally-varying low-level (LL) shear profiles (i.e., purely line-normal, assuming a north-south oriented system), but observed near-storm environments of mesovortex-producing QLCSs exhibit substantial LL hodograph curvature (i.e., line-parallel shear). Therefore, previous QLCS simulations may fail to capture the full impacts of LL shear variability on mesovortex characteristics. To this end, this study employs an ensemble of idealized QLCS simulations with systematic variations in the orientation and magnitude of the ambient LL shear vector, all while holding 0–3-km line-normal shear constant. This allows for a nuanced examination of how line-parallel shear modulates system structure, as well as mesovortex strength, size, and longevity. Results indicate that hodographs with LL curvature support squall lines with prominent bowing segments and wider, more intense rotating updrafts. Shear orientation also impacts mesovortex characteristics, with curved hodographs favoring cyclonic vortices that are stronger, wider, deeper and longer-lived than those produced with straight-line wind profiles. These results provide a more complete physical understanding of how LL shear variability influences the generation of rotation in squall lines.

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