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1. The Synoptically-Influenced Extreme Precipitation Systems over Asian-Australian Monsoon Region observed by TRMM Precipitation Radar

   Jian, H.-W.; Chen, W.-T.; Chen, P.-J.; Wu, C.-M.; Rasmussen, K. L. (January 2021, Journal of the Meteorological Society of Japan)

   null (Ed.)

   This study investigates the synoptic-scale flows associated with extreme rainfall systems over the Asian–Australian monsoon region (90 – 160°E and 12°S – 27°N). On the basis of the statistics of the 17-year Precipitation Radar observations from Tropical Rainfall Measurement Mission, a total of 916 extreme systems, with both the horizontal size and maximum rainfall intensity exceeding the 99.9th percentiles of the tropical rainfall systems, are identified over this region. The synoptic wind pattern and rainfall distribution surrounding each system are classified into four major types: vortex, coastal, coastal with vortex, and none of above, with each accounting for 44, 29, 7, and 20 %, respectively. The vortex type occurs mainly over the off-equatorial areas in boreal summer. The coast-related types show significant seasonal variations in their occurrence, with high frequency in the Bay of Bengal in boreal summer and on the west side of Borneo and Sumatra in boreal winter. The none-of-the-above type occurs mostly over the open ocean and in boreal winter; these events are mainly associated with the cold surge events. The environment analysis shows that coast-related extremes in the warm season are found within the areas where high total water vapor and low-level vertical wind shear occur frequently. Despite the different synoptic environments, these extremes show a similar internal structure, with broad stratiform and wide convective core (WCC) rain. Furthermore, the maximum rain rate is located mostly over the convective area, near the convective–stratiform boundary in the system. Our results highlight the critical role of the strength and direction of synoptic flows in the generation of extreme rainfall systems near coastal areas. With the enhancement of the lowlevel vertical wind shear and moisture by the synoptic flow, the coastal convection triggered diurnally has a higher chance to organize into mesoscale convective systems and hence a higher probability to produce extreme rainfall. 

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2. Characterization of Convection and Rainfall Off the Pacific Coast of Colombia Using Airborne Radar and Dropsonde Data

   https://doi.org/10.1029/2024GL114186  

   Raymond, D_J; Stone, Ž. (April 2025, Geophysical Research Letters)

   Abstract The Organization of Tropical East Pacific Convection project used dropsondes deployed from high altitude and a downward‐pointing W‐band Doppler radar to document the characteristics of mesoscale convective systems (MCSs) located over the Pacific coastal waters of Colombia. MCSs dominated by ice crystal aggregates above the freezing level rather than graupel, as shown by the radar, are generally thought to indicate decaying stratiform rain systems with only light rain. However, dropsonde grids showed a broader range of MCS types in this category, some with shallow convection producing intense rainfall. The radar had difficulty in distinguishing between different types of aggregate‐dominated MCSs. 

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3. Backward Adaptive Brightness Temperature Threshold Technique (BAB3T): A Methodology to Determine Extreme Convective Initiation Regions Using Satellite Infrared Imagery

   https://doi.org/10.3390/rs12020337  

   Cancelada, Maite; Salio, Paola; Vila, Daniel; Nesbitt, Stephen W.; Vidal, Luciano (January 2020, Remote Sensing)

   Thunderstorms in southeastern South America (SESA) stand out in satellite observations as being among the strongest on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, the prevalence for extremely tall, wide convective cores and broad stratiform regions. Accurately quantifying when and where strong convection is initiated presents great interest in operational forecasting and convective system process studies due to the relationship between convective storms and severe weather phenomena. This paper generates a novel methodology to determine convective initiation (CI) signatures associated with extreme convective systems, including extreme events. Based on the well-established area-overlapping technique, an adaptive brightness temperature threshold for identification and backward tracking with infrared data is introduced in order to better identify areas of deep convection associated with and embedded within larger cloud clusters. This is particularly important over SESA because ground-based weather radar observations are currently limited to particular areas. Extreme rain precipitation features (ERPFs) from Tropical Rainfall Measurement Mission are examined to quantify the full satellite-observed life cycle of extreme convective events, although this technique allows examination of other intense convection proxies such as the identification of overshooting tops. CI annual and diurnal cycles are analyzed and distinctive behaviors are observed for different regions over SESA. It is found that near principal mountain barriers, a bimodal diurnal CI distribution is observed denoting the existence of multiple CI triggers, while convective initiation over flat terrain has a maximum frequency in the afternoon. 

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4. Understanding extreme rainfall in the Sahel of Southwestern Mali

   https://doi.org/10.1007/s00382-024-07241-y  

   Vizy, Edward K; Cook, Kerry H (June 2024, Climate Dynamics)

   West African Sahel extreme rainfall events cause flooding and property damage, and some areas are more prone to their occurrence. One favorable region is southwestern Mali. NASA IMERG precipitation and ERA5 reanalysis data are used to examine the most extreme boreal summer rainfall events from 2000 – 2019 over southwestern Mali to understand why they form, and to explain why this region has frequent activity. Events are sorted into 4 types based on the timing of the peak rainfall (before or after 00Z) and the associated mid-tropospheric circulation pattern (coastal low or ridge). The coastal low types are associated not with an increase of the low-level inflow of moisture into southwestern Mali, but a weakening of the mid-level westward transport of moisture out of the region. The timing and longevity of the event depends on whether there is a second low to the east in the southern storm track. The coastal ridge types are associated with a build-up of warm, dry air over the western Sahara that leads to a stronger temperature inversion cap over southwestern Mali, allowing instability to build beneath the cap. How fast the cap dissipates and whether there is synoptic activity to the east in the southern or northern storm track determines when convective activity occurs. Thus, southwestern Mali is exposed to coastal lows and ridges in addition to the Saharan heat low and the summer southern storm track for African easterly wave disturbances. The confluence of these factors makes southwestern Mali a region conducive for convective rainfall activity. 

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5. Impacts of Coastal Terrain on Warm-Sector Heavy-Rain-Producing MCSs in Southern China

   https://doi.org/10.1175/MWR-D-21-0190.1  

   Zhang, Murong; Rasmussen, Kristen L.; Meng, Zhiyong; Huang, Yipeng (March 2022, Monthly Weather Review)

   Abstract Warm-sector heavy rainfall in southern China refers to the heavy rainfall that occurs within a weakly forced synoptic environment under the influence of monsoonal airflows. It is usually located near the southern coast and is characterized by poor predictability and a close relationship with coastal terrain. This study investigates the impacts of coastal terrain on the initiation, organization, and heavy rainfall potential of MCSs in warm-sector heavy rainfall over southern China using quasi-idealized WRF simulations and terrain-modification experiments. Typical warm-sector heavy rainfall events were selected to produce composite environments that forced the simulations. MCSs in these events all initiated in the early morning and developed into quasi-linear convective systems along the coast with a prominent back-building process. When the small coastal terrain is removed, the maximum 12-h rainfall accumulation decreases by ∼46%. The convection initiation is advanced ∼2 h with the help of orographic lifting associated with flow interaction with the coastal hills in the control experiment. Moreover, the coastal terrain weakens near-surface winds and thus decreases the deep-layer vertical wind shear component perpendicular to the coast and increases the component parallel to the coast; the coastal terrain also concentrates the moisture and instability over the coastal region by weakening the boundary layer jet. These modifications lead to faster upscale growth of convection and eventually a well-organized MCS. The coastal terrain is beneficial for back-building convection and thus persistent rainfall by providing orographic lifting for new cells on the western end of the MCS, and by facilitating a stronger and more stagnant cold pool, which stimulates new cells near its rear edge. 

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