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Flash flooding is an important issue as it has a devastating impact over a short time and in a limited area. However, predicting flash floods is challenging because they are connected to convection systems that rapidly evolve and require a high-resolution forecasting system. In addition, modeling a case study of a mesoscale convective complex (MCC) is the key to improving our understanding of the heavy rainfall systems that trigger flash floods. In this study, we aim at improving modeling skills to simulate a heavy rainfall system related to flash-flood-producing MCCs. We simulated a heavy rainfall event related to a severe flash flood in Luwu, Sulawesi, Indonesia, on 13 July 2020. This flood was preceded by persistent heavy rainfall from 11 to 13 July 2020. In this case, we investigated the role of sea surface temperature (SST) in producing the persistent heavy rainfall over the region. Therefore, we explore the physical and dynamic processes that caused the heavy rainfall using a convection-permitting model with 1 km resolution and an experiment comparing the situation with and without updated SST. The results show that the heavy rainfall was modulated by the development of a pair of MCCs during the night. The pair of MCCs was triggered by a meso-low-pressure system with an anti-cyclonic circulation anomaly over the Makassar Strait and was maintained by the warm front passing between the sea and land over central Sulawesi. This front was characterized by moist–warm and cold–dry low-level air, which may have helped extend the lifetime of the MCCs. The north-westward propagation of the MCCs was due to the interaction between predominantly a south-easterly monsoon and SST anomalies. This study suggested that the long-lived (>10 h) MCCs (>80,000 km2 cloud shield) and persistent precipitation are reproduced well in the updated SST scenario in the WRF model. This relatively simple technique in the running model provides a new strategy for improving flash flood forecasting by better predicting rainfall as an input in the hydrological model. Our findings also indicated a long-lived MCC maintained by back-building mechanisms from night to morning inland as an exceptional MCC, which does not correspond to a previous study.

The aim of this study is to analyze the synergy between environmental factors (pollutant, allergenic birch pollen, weather) and its relationship with asthma hospitalization in Montreal, Canada. The data is stratified into weather types and the study restricted to spring season to limit the impact of multiple confounders. Results shows that the daily count of asthma hospitalization (spring 2006–2008) in situation of warm fronts or trowals (daily average of 3.78 counts, CI 95% 2.95–4.61) was much higher (p < 0.001) than in other situations (2.49 counts, CI 95% 2.37–2.71). Moreover, the explained variance of asthma hospitalization due to air pollution rises from about less than 7% (in the case of no stratification) to about 28% (R = 0.53, p < 0.05 with stratification). Statistical tests for interaction and overall results point towards a synergy between environmental factors which exacerbates asthma. A new concept named frontal asthma is proposed to explain several results found here and in the open literature.

The Eastern inland of Syria has a Mediterranean climate in the north and a tropical desert climate in the south, which results in a dry south and wet north climate feature, especially in winter. The circulation dynamics analysis of 16 winter strong precipitation events shows that the key system is the dry and warm front cyclone. In most cases (81–100% of the 16 cases), the moisture content in the northern part of the cyclone is higher than that in the southern part (influenced by the Mediterranean climate zone). The humidity in the middle layer is higher than that near the surface (uplifting of the dry warm front), and the thickness of the wet layer and the vertical ascending layer obviously expands upward (as shown by the satellite cloud top reflection). These characteristics lead to the moisture thermodynamic instability in the eastern part of the cyclone (dry and warm air at low level and wet and cold air at upper level). The cyclone flow transports momentum to the local humid layer of the Mediterranean climate belt and then causes unstable conditions and strong rainfall. Considering the limitations of the Syrian ground station network, the NCEP/CFSR global reanalysis data and MODIS aqua-3 cloud parameter data are used to build a multi-source factor index of winter precipitation from 2002 to 2016. A decision tree prediction model is then established and the factors index is constructed into tree shapes by the nodes and branches through calculating rules of information entropy. The suitable tree shape models are adjusted and selected by an automated training and testing process. The forecast model can classify rainfall with a forecast accuracy of more than 90% for strong rainfall over 30 mm.

An extreme snowstorm event that occurred over Heilongjiang and Jilin Provinces on 24–26 November 2013 was related to a cyclone characterized by a back-bent occluded front structure. This study investigates the structure of the back-bent occluded front and snowfall mechanism using multiple observations and NCEP/NCAR 1° × 1° reanalysis data in concert with the HYSPLIT model. The main results show that the extreme event was more synoptically governed by the outbreak of the polar vortex and moisture anomaly of the East Sea. The cyclone occurred just ahead of the 500-hPa merged deep trough, and then developed under the effect of the positive vorticity advection ahead of the 500-hPa trough and intense divergence of the upper-level jet. The south-southwest wind strengthened obviously after the merger of the southern and northern branch troughs, which was the main reason behind the cyclone moving northward. The moisture mainly originated from the Sea of Japan, insofar as that dry and cold air in the lower troposphere over the western mainland moistened obviously as it turned southward and passed over the Bohai Sea and the Sea of Japan, supplying abundant moisture for the snowstorm event. The intensity of moisture transport depended on the location and intensity of the cyclone. When the cyclone developed, the dry air continuously intruded into the cyclone’s center, and made a conveyor belt of warm air wrap around it. The dry air gradually changed from descending to ascending motion as it moved ahead of the westerly trough, while the moist air in the northern part of the cyclone moved to the west and south and incorporated into the south of the cyclone center. Warm and moist air was lifted and arrived in the northwestern part of the cyclone after the occluded front’s formation. Frontogenesis within the comma head was enhanced evidently owing to the rotation and deformation. The convergence between the southeast and northeast winds resulted in intense frontogenesis, leading to the enhancement of the front-scale ascent. Strong ascent formed in the comma head of the cyclone, which resulted in intense snowfall.

Exchange flows between estuaries and the coastal ocean are important for land-ocean interactions and ecosystem health. This study is aimed at resolving severe weather-induced exchange flows between the Calcasieu Lake Estuary and Gulf of Mexico. For that purpose, we use data from a long-term deployment of side-looking acoustic Doppler current profilers (ADCPs) and conductivity-temperature-depth sensors (CTDs) as well as flow velocity data from a boat operated survey. Regression between the transport measured from a boat mounted ADCP and the velocity data from a fixed side-looking ADCP is done to calculate a long-term transport along the Calcasieu Pass. Analyses have been done for the hydrodynamic response to 16 cold fronts passing the study area. Effects of six strongest cold fronts are discussed in more detail. Results have confirmed that the hydrodynamics is highly correlated with the frequent cold fronts. The highest correlation coefficient is r ~0.75 between the north wind and along channel transport. In general, winds from the southern quadrants push water into the estuary before each frontal passage; after the passage of the front, a rapid change of wind direction to the northern quadrants produces strong outward flows. A quasi-steady state balance between the wind stress and water level difference proposed in recent studies for different systems is further confirmed and discussed in this system. The quasi-steady state balance leads to a relatively high R2 value of greater than 0.8 between the modeled water level gradient and actual observed gradient. We have also applied a regression model, derived from the momentum balance requirement, for the subtidal exchange flow as a function of wind components and their squares which yield an R2 value greater than 0.7. With a confidence in the regression model, we further implement it for twelve years from 26 February 2007 to 10 April 2019. Four extreme events during this 12-year period of time are discussed–they include the Hurricane Ike (2008), Tropical Storm Lee (2011), a warm front, and a cold front. This hindcast of the exchange flows over multiple years can provide a useful tool for coastal management and research for estuarine channels where continuous observations of velocity are not always available.

Aerosols influence cloud and precipitation development in complex ways due to myriad feedbacks at a variety of scales from individual clouds through entire storm systems. This paper describes the implementation, testing, and results of a newly modified bulk microphysical parameterization with explicit cloud droplet nucleation and ice activation by aerosols. Idealized tests and a high-resolution, convection-permitting, continental-scale, 72-h simulation with five sensitivity experiments showed that increased aerosol number concentration results in more numerous cloud droplets of overall smaller size and delays precipitation development. Furthermore, the smaller droplet sizes cause the expected increased cloud albedo effect and more subtle longwave radiation effects. Although increased aerosols generally hindered the warm-rain processes, regions of mixed-phase clouds were impacted in slightly unexpected ways with more precipitation falling north of a synoptic-scale warm front. Aerosol impacts to regions of light precipitation, less than approximately 2.5 mm h−1, were far greater than impacts to regions with higher precipitation rates. Comparisons of model forecasts with five different aerosol states versus surface precipitation measurements revealed that even a large-scale storm system with nearly a thousand observing locations did not indicate which experiment produced a more correct final forecast, indicating a need for far longer-duration simulations due to the magnitude of both model forecast error and observational uncertainty. Last, since aerosols affect cloud and precipitation phase and amount, there are resulting implications to a variety of end-user applications such as surface sensible weather and aircraft icing.

Atmospheric fronts are important for the day‐to‐day variability of weather in the midlatitudes, particularly during winter when extratropical storm‐tracks are at their maximum intensity. Fronts are often associated with heavy rain, and strongly affect the local space‐time distribution of rainfall. A recently developed objective front identification method that distinguishes between cold, warm and quasi‐stationary fronts, is applied to reanalysis data and combined with a daily global gridded data set to investigate how precipitation around the globe is associated with atmospheric fronts. A large proportion (up to 90%) of rainfall in the major storm‐track regions is associated with fronts, particularly cold and warm fronts. Precipitation over the oceanic storm‐tracks is mostly associated with cold fronts, while over the Northern Hemisphere continents precipitation is mainly associated with warm fronts. There are seasonal and regional variations in the proportion of precipitation associated with fronts. Key Points Objectively identified fronts are linked to global precipitation data The proportion of precipitation associated with fronts highest over midlatitudes Average precipitation greater when front present than when no front present

A systematic analysis of the main weather types influencing southern Australian rainfall is presented for the period 1979–2015. This incorporates two multi-method datasets of cold fronts and low pressure systems, which indicate the more robust fronts and lows as distinguished from the weaker and less impactful events that are often indicated only by a single method. The front and low pressure system datasets are then combined with a dataset of environmental conditions associated with thunderstorms, as well as datasets of warm fronts and high pressure systems. The results demonstrate that these weather types collectively account for about 86% of days and more than 98% of rainfall in Australia south of 25° S. We also show how the key rain-bearing weather systems vary throughout the year and for different regions, with the co-occurrence of simultaneous lows, fronts and thunderstorm conditions particularly important during the spring and summer months in southeast Australia.

The model of atmospheric rivers (ARs) has been around since the 1990s. A closely related model is the warm conveyor belt (WCB) developed in the 1970s. Looking further back in time, a phenomenon known as the “moist tongue” was intensively investigated in the late 1930s and early 1940s by Rossby and his collaborators using the innovation of isentropic analysis. This article aims to establish a historical perspective on the development of the moist tongue model and its relevance to the current models of WCBs and ARs. As it turns out, the moist tongue was identified as an extension of moist air into a region of lower moisture content on the selected isentropic charts. Most moist tongues are driven by large-scale cyclonic and anticyclonic eddies and are often accompanied by surface cold fronts in close proximity. Ahead of the moist tongues, areas of continuous precipitation are caused mainly by the motion of moist air up the steep isentropic slopes over warm fronts or topographical features. In the warm season, the mere presence of a moist tongue could be sufficient to give thunderstorms. A reanalysis dataset is used to reexamine the structures and evolutions of two moist tongue events in 1936. It is shown that not all but some of the moist tongues fit well with the modern conceptual models of WCB and AR. These two case studies also serve to elucidate the usefulness of reanalysis data for investigating historical high-impact weather events that were poorly understood due to the lack of observational data.

The frontal structures of extratropical cyclones developing in the northwestern Pacific storm track are relatively poorly understood compared with those in Europe and the Atlantic Ocean, for which representative conceptual models have been developed. In this paper, the structures of cyclones and their associated fronts in the northwestern Pacific (NP), as well as in the Okhotsk Sea and Sea of Japan (OJ), are examined at their developing and mature stages using Japanese 55-year Reanalysis dataset. Furthermore, the frontal structures in the NP are compared with those in the northwestern Atlantic (NA). At the time of maximum deepening rate, cyclones in the NP are accompanied by strong warm and cold fronts, whereas cyclones in the OJ are more frequently accompanied by cold fronts than by warm fronts and tend to have stronger cold fronts than warm fronts. The weaker warm fronts than cold fronts to the east and northeast of cyclones in the OJ is likely due to the cyclones developing to the north and away from the region where the horizontal gradient of environmental potential temperature is strong. A comparison between mature cyclones in the NP and NA shows that the warm fronts in the NA tend to extend northeastward, whereas those in the NP extend more southeastward. These differences in warm fronts between NP and NA are suggested to be due to the difference in the horizontal structures of the warm currents between NP and NA.