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Jason Naylor and Aaron D. Kennedy

Abstract

This study analyzes the frequency of strong, isolated convective cells in the vicinity of Louisville, Kentucky. Data from the Severe Weather Data Inventory (SWDI) are used to compare the frequency of convective activity over Louisville to the observed frequency at nearby rural locations from 2003–2019. The results show that Louisville experiences significantly more isolated convective activity compared to the rural locations. The difference in convective activity between Louisville and the rural locations is strongest during summer, with peak differences occurring between May and August. Compared to the rural locations, Louisville experiences more isolated convective activity in the afternoon and early evening, but less activity after midnight and into the early morning. Isolated convective events over Louisville are most likely during quiescent synoptic conditions, while rural events are more likely during active synoptic patterns.

To determine if these differences can be attributed primarily to urban effects, two additional cities are shown for comparison—Nashville, Tennessee and Cincinnati, Ohio. Both Nashville and Cincinnati experience more isolated convective activity than all five of their nearby rural comparison areas, but the results for both are statistically significant at four of the five rural locations. In addition, the analysis of Cincinnati includes a sixth comparison site that overlaps the urbanized area of Columbus, Ohio. For that location, differences in convective activity are not statistically significant.

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Xiaohe An, Bo Wu, Tianjun Zhou, and Bo Liu

Abstract

Interdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Oscillation (AMO), two leading modes of decadal climate variability, are not independent. It was proposed that ENSO-like sea surface temperature (SST) variations play a central role in the Pacific responses to the AMO forcing. However, observational analyses indicate that the AMO-related SST anomalies in the tropical Pacific are far weaker than those in the extratropical North Pacific. Here, we show that SST in the North Pacific is tied to the AMO forcing by convective heating associated with precipitation over the tropical Pacific, instead of by SST there, based on an ensemble of pacemaker experiments with North Atlantic SST restored to the observation in a coupled general circulation model. The AMO modulates precipitation over the equatorial and tropical southwestern Pacific through exciting an anomalous zonal circulation and an interhemispheric asymmetry of net moist static energy input into the atmosphere. The convective heating associated with the precipitation anomalies drive SST variations in the North Pacific through a teleconnection, but remarkably weaken the ENSO-like SST anomalies through a thermocline damping effect. This study has implications that the IPO is a combined mode generated by both AMO forcing and local air-sea interactions, but the IPO-related global-warming acceleration/slowdown is independent of the AMO.

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Yingkai Sha, David John Gagne II, Gregory West, and Roland Stull

Abstract

We present a novel approach for the automated quality control (QC) of precipitation for a sparse station observation network within the complex terrain of British Columbia, Canada. Our QC approach uses Convolutional Neural Networks (CNNs) to classify bad observation values, incorporating a multi-classifier ensemble to achieve better QC performance. We train CNNs using human QC’d labels from 2016 to 2017 with gridded precipitation and elevation analyses as inputs. Based on the classification evaluation metrics, our QC approach shows reliable and robust performance across different geographical environments (e.g., coastal and inland mountains), with 0.927 Area Under Curve (AUC) and type I/type II error lower than 15%. Based on the saliency-map-based interpretation studies, we explain the success of CNN-based QC by showing that it can capture the precipitation patterns around, and upstream of the station locations. This automated QC approach is an option for eliminating bad observations for various applications, including the pre-processing of training datasets for machine learning. It can be used in conjunction with human QC to improve upon what could be accomplished with either method alone.

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Kieran M. R. Hunt, Andrew G. Turner, and Reinhard K. H. Schiemann

Abstract

Interactions over South Asia between tropical depressions (TDs) and extratropical storms known as western disturbances (WDs) are known to cause extreme precipitation events, including those responsible for the 2013 floods over northern India. In this study, existing databases of WD and TD tracks are used to identify potential WD–TD interactions from 1979–2015; these are filtered according to proximity and intensity, leaving 59 cases which form the basis of this paper. Synoptic charts, vorticity budgets, and moisture trajectory analyses are employed to identify and elucidate common interaction types among these cases. Two broad families of interaction emerge. Firstly, a dynamical coupling of the WD and TD, whereby either the upper- and lower-level vortices superpose (a vortex merger), or the TD is intensified as it passes into the entrance region of a jet streak associated with the WD (a jet-streak excitation). Secondly, a moisture exchange between the WD and TD, whereby either anomalous moisture is advected from the TD to the WD, resulting in anomalous precipitation near the WD (a TD-to-WD moisture exchange), or anomalous moisture is advected from the WD to the TD (a WD-to-TD moisture exchange). Interactions are most common in the post-monsoon period as the subtropical jet, which brings WDs to the subcontinent, returns south; there is a smaller peak in May and June, driven by monsoon onset vortices. Precipitation is heaviest in dynamically-coupled interactions, particularly jet-streak excitations. Criteria for automated identification of interaction types are proposed, and schematics for each type are presented to highlight key mechanisms.

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Cara L. Cuite, Rebecca E. Morss, Julie L. Demuth, and William K. Hallman

Abstract

Both hurricanes and nor’easters can be destructive and deadly. The current study investigates whether, when all other features of a storm warning message are held constant, people perceive the risks posed by nor’easters and hurricanes differently and whether these differences affect their attitudes and decisions about taking protective action. We conducted an online experiment involving 1,700 Americans residing in Northeastern coastal ZIP codes to test the effects of storm type (hurricane vs. nor’easter). Participants were told that their area was under an evacuation order due to either a predicted hurricane or nor’easter. Reported message comprehension and perceived relevance were similar across storm type; however, storm type had small but significant effects on other dependent measures. Those in the hurricane condition were more likely to believe the storm would be severe (p =. 007). They were also more likely to say that it is important to evacuate, that they would evacuate their homes, and that they would recommend to their neighbors that they evacuate (ps <.001). Additional analysis demonstrated that the effect of storm type on evacuation likelihood is mediated, at least in part, by perceived severity. These findings provide evidence that people perceive hurricanes as more severe and more likely to require taking protective action than nor’easters, even when other attributes of the storms remain the same. Forecasters, broadcast meteorologists, and emergency management professionals should consider these small but important differences in perceptions when communicating about these types of storms.

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Lei Zhou, Ruomei Ruan, and Raghu Murtugudde

Abstract

Madden-Julian Oscillations (MJOs) are a major component of tropical intraseasonal variabilities. There are two paths for MJOs across the Maritime Continent; one is a detoured route into the Southern Hemisphere and the other one is around the equator across the Maritime Continent. Here, it is shown that the detoured and non-detoured MJOs have significantly different impacts on the South Pacific convergence zone (SPCZ). The detoured MJOs trigger strong cross-equatorial meridional winds from the Northern Hemisphere into the Southern Hemisphere. The associated meridional moisture and energy transports due to the background states carried by the intraseasonal meridional winds are favorable for reinforcing the SPCZ. In contrast, the influences of non-detoured MJOs on either hemisphere or the meridional transports across the equator are much weaker. The detoured MJOs can extend their impacts to the surrounding regions by shedding Rossby waves. Due to different background vorticity during detoured MJOs in boreal winter, more ray paths of Rossby waves traverse the Maritime Continent connecting the southern Pacific Ocean and the eastern Indian Ocean, but far fewer Rossby wave paths traverse Australia. Further studies on such processes are expected to contribute to a better understanding of extreme climate and natural disasters on the rim of the southern Pacific and Indian Oceans.

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Hossein A. Kafiabad, Jacques Vanneste, and William R. Young

Abstract

Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initialwave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.

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Yuntao Jian, Marco Y. T. Leung, Wen Zhou, Maoqiu Jian, Song Yang, and Xiaoxia Lin

Abstract

In this study, the interdecadal variability of the relationship between ENSO and winter synoptic temperature variability (STV) over the Asian-Pacific-American region is investigated based on observational data from 1951 to 2018. An interdecadal shift in the ENSO-STV relationship occurred in the 1980s over Eastern China, changing from significant in Period 1 (P1, 1951-1987) to insignificant in Period 2 (P2, 1988-2018). But the ENSO-STV relationship is significantly stable over North America for the whole period. In addition, a possible reason for this interdecadal shift in the ENSO-STV relationship over Eastern China is also investigated. During P1, the ENSO pattern is significantly correlated to the temperature gradient over Northeast Asia, which is the key region influencing the intensification of extratropical eddies. The intensification of extratropical eddies over Northeast Asia is directly associated with the magnitude of STV over Eastern China. But in P2, the ENSO pattern is not related to the temperature over Northeast Asia. Therefore, the change in the ENSO pattern from P1 to P2 contributes to the interdecadal shift in the ENSO-STV relationship in the 1980s over Eastern China by influencing the temperature gradient over Northeast Asia, while ENSO can influence the temperature gradient over North America for the whole period. Furthermore, the possible role of the ENSO patterns in P1 and P2 is also examined by using an atmospheric general circulation model, highlighting that the pattern of SST variation is a determining factor in regulating STV in different regions.

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Basanta Raj Adhikari

Abstract

The lightning hazard is one of the devastating hazards in Nepal due to a large amount of atmospheric water vapor coming from the Indian Ocean and a large orographic lifting of this moist air. In 2019, a total of 2884 people were affected with loss of 110982 USD and the fatality was highest (94) in reported lightning events in reported lightning events since 1971. The long-term analysis of this hazard is very scanty in Nepal. Therefore, this study analyzes lightning fatality events, fatality rates, and economic loss from 1971 to 2019 collected from Desinventar dataset and the Disaster Risk Reduction portal of the Government of Nepal using Statistical Package for Social Sciences (SPSS) and Geographical Information System (ArcGIS) tools. The analysis shows that the overall countrywide lightning fatality rate of the entire period is 1.77 per million per year. The district lightning fatality rates range from 0.10 to 4.83 per million people per year and the Bhaktapur district has the highest fatality density (0.067). Furthermore, there were altogether 2501 lightning fatality events where 1927 people lost their lives and 20569 people were affected. The increase in lightning fatality events in recent years is due to internet penetration and other measures of information gathering that results in lightning fatality reports reaching agencies collecting information. The high and low concentration of loss and damage are mainly due to geographic distribution, population density, and economic activities. This study recommends the establishment of Lightning Early Warning Systems in the Nepal Himalaya to save life and property.

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Eric Simonnet, Joran Rolland, and Freddy Bouchet

Abstract

We demonstrate that turbulent zonal jets, analogous to Jovian ones, which are quasi-stationary, are actually metastable. After extremely long times, they randomly switch to new configurations with a different number of jets. The genericity of this phenomenon suggests that most quasi-stationary turbulent planetary atmospheres might have many climates and attractors for fixed values of the external forcing parameters. A key message is that this situation will usually not be detected by simply running the numerical models, because of the extremely long mean transition time to change from one climate to another. In order to study such phenomena, we need to use specific tools: rare event algorithms and large deviation theory. With these tools, we make a full statistical mechanics study of a classical barotropic beta-plane quasigeostrophic model. It exhibits robust bimodality with abrupt transitions. We show that new jets spontaneously nucleate from westward jets. The numerically computed mean transition time is consistent with an Arrhenius law showing an exponential decrease of the probability as the Ekman dissipation decreases. This phenomenology is controlled by rare noise-driven paths called instantons. Moreover, we compute the saddles of the corresponding effective dynamics. For the dynamics of states with three alternating jets, we uncover an unexpectedly rich dynamics governed by the symmetric group S3 of permutations, with two distinct families of instantons, which is a surprise for a system where everything seemed stationary in the hundreds of previous simulations of this model. We discuss the future generalization of our approach to more realistic models.

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