Presented on September 20, 2023, by Bin Wang, Professor Emeritus Department of Atmospheric Sciences School of Ocean and Earth Sciences University of Hawaii at Manoa
Abstract
Five multi-year La Niña (ML) events have occurred since 1998, including two rare ‘triple’ La Nina. The clustered ML occurrence is phenomenal given that only ten ML events occurred since 1920o. Climate models have trouble reproducing the observed trends in ML and the Pacific mean climate. While an ML can be caused by the preceding extreme El Niño and attendant massive upper-ocean heat content discharge, three recent multiyear La Niña episodes (2007–08, 2010–11, and 2020–22) did not follow this paradigm. Why so many MLs emerged recently and whether they will become common sparked worldwide discussion, yet remains a puzzle.
We examined 20 La Niña events from 19201-2022 and found ML distinguishes from single-year La Niña by a conspicuous onset rate, which foretells its accumulative intensity and climate impacts. The eight MLs occurred after 1970, primarily following either a super El Niño (SE) or a central Pacific El Niño (CPE), forming two types of ML: SE2ML and CPE2ML. The latter is an emerging form in recent decades. We hypothesize that both types arise from the western Pacific (WP) warming and increased SST gradients in the west-to-central Pacific. The ML’s rapid onset and persistence are triggered by the WP warming-enhanced zonal advective feedback for CPE2ML and thermocline feedback for SE2ML. The WP warming also favors initiating SE and CPE events in the west Pacific, increasing the odds of ML occurrence. The results from the CESM2 large-ensemble climate simulations principally support the observed ML features and the ML-WP warming linkages. More MLs will exacerbate adverse socioeconomic impacts if the WP continues to warm relative to the CP.
Yi-Leng (Dave) Chen, Professor Department of Atmospheric Sciences School of Ocean and Earth Sciences University of Hawaii at Manoa
Abstract
During the early summer rainy season over Taiwan, three types of low-level jets are observed, including a synoptic low-level jet (SLLJ) situated in the 850–700 hPa layer in the frontal zone, a marine boundary layer jet (MBLJ) embedded within the southwesterly monsoon flow over the northern South China Sea at approximately the 925 hPa level, and an orographically induced jet at approximately the 1 km level off the northwestern Taiwan coast (e.g., barrier jet (BJ)). The characteristics and physical processes of the formation of these three types of low-level jets are reviewed, and their roles in the development of heavy rainfall are discussed.
Yuqing Wang Department of Atmospheric Sciences and International Pacific Research Center School of Ocean and Earth Sciences University of Hawaii at Manoa
Abstract
In this presentation, the relationship between the observed rapid eyewall contraction (in terms of the radius of maximum wind–RMW) and the rapid intensification of hurricanes over the North Atlantic and Eastern North Pacific is statistically analyzed based on the best-track data. Results show that the RMW (eyewall) contracts rapidly well preceding the rapid intensification in hurricanes. To further understand the involved dynamics, we performed ensemble axisymmetric numerical simulations, which reproduced the observed feature. Further analysis indicates that because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon could not be understood based on the AAM-based dynamics. Instead, results from the budgets of tangential wind following the RMW and the rate of change in the RMW provide dynamical insights into the simulated relationship between the intensification rate and RMW contraction rate. During the rapid RMW contraction stage, due to the weak hurricane intensity and large RMW, the negative radial gradient of radial vorticity flux and small curvature of tangential wind distribution near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus relatively small intensification rate. As the RMW contracts rapidly together with the increase in hurricane intensity, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to substantial increase in radial absolute vorticity flux near the RMW and thus the rapid hurricane intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of tangential wind distribution near the RMW as the hurricane intensifies rapidly together with the decrease in the RMW. These findings help better understand hurricane structure and intensity changes.
José Martínez-Claros Postdoctoral scholar Scripps Institution of Oceanography La Jolla, CA
Abstract
Atmospheric rivers (ARs) are large-scale moisture transport systems in the atmosphere, commonly associated with an extratropical cyclone. Two key variables are used to characterize ARs and estimate their impact, that is Integrated Vapor Transport (IVT) and Integrated Water Vapor (IWV). This vertically-integrated approach to atmospheric rivers has limitations related to moisture sourced in the tropics, loosely defined as tropical moisture exports (TMEs). Part of the challenge is understanding how tropical convection fits in this vertically-integrated moisture model, and ultimately how it affects the precipitation caused by this large-scale disturbance. Even though the physical mechanisms behind TMEs are not well understood, many researchers have hypothesized that tropical moisture fluxes can exacerbate the hydrologic impact caused by atmospheric rivers, particularly at landfall. In recent years, the concept of moisture quasi-equilibrium (MQE) has elucidated the average behavior of moisture in the tropics, by setting an anti-correlation between vertically integrated moisture (saturation fraction) and moist convective instability, which depends on vertical temperature anomalies. This moisture feedback mechanism, which has proven to perform significantly well in cloud-resolving models and observations of tropical cyclogenesis, is not unique to the tropics. I propose an alternate approach to characterize tropical moisture exports in atmospheric rivers from the perspective of MQE, and its governing role over moisture convergence and precipitation. I will first describe the ideas that led to the development of the MQE concept, as well as the ideas that led to its application as a tool to understand atmospheric rivers. I will then elaborate on my current research, which explores the role played by TMEs in NE Pacific atmospheric rivers that impact the US West Coast. Results in this study show that the TMEs that evolve in moist convectively stable environments have higher column-integrated moisture (saturation fraction) and a higher precipitation rate, in agreement with MQE. A correlation between saturation fraction and precipitation peaks is also observed in the water vapor budget in atmospheric rivers with TMEs. I finally will discuss how the MQE signal can be used as a tool to predict precipitation extremes at landfall, directly attributed to TMEs, in cases such as the Valentines’ Day AR of 2019.
Bio
José was born in Honduras. He became interested in meteorology and atmospheric science in 1998, after Hurricane Mitch made landfall in Honduras and destroyed the majority of the country, leaving an aftermath of 10,000 reported deaths. Following his dream of understanding the atmosphere, particularly extreme weather events, he enrolled in the meteorology BS program at the Universidad de Costa Rica, where he and his mentor Dr. Walter Fernandez published a paper on atmospheric lightning distributions in Costa Rica. He then went on to obtain a MS in lightning physics under the advisory of Dr. Richard Sonnenfeld, and a PhD in atmospheric physics under the advisory of Dr. David Raymond, at the New Mexico Institute of Mining and Technology (NMT). While at NMT, José published a paper and wrote a dissertation on the role of tropical dynamics in early summer atmospheric rivers. In 2021, he was brought to Scripps Institution of Oceanography by Dr. Forest Cannon of the Center for Western Weather and Water Extremes (CW3E) to continue investigating the elusive role of the tropics in winter season NE Pacific atmospheric rivers that affect the U.S. West Coast.
Niklas Schneider Director, International Pacific Research Center Professor, Department of Oceanography University of Hawai’i at Mānoa
Abstract:
The Gulf Stream Convergence Zone is a dipole of long-term averaged surface-wind divergence and convergence over the cold and warm flanks of the Gulf Stream, respectively. Using satellite observations of equivalent neutral winds and sea surface temperatures, we show that this dipole results from the aggregation of the atmospheric boundary layer responses to the Gulf Stream sea surface temperature front and winds of transients in the North Atlantic storm track. The boundary layer is based on observed impulse response functions for surface wind divergences, and captures modulations of boundary layer vertical mixing and atmospheric pressure over, and downwind of, ocean mesoscale sea surface temperatures. When forced with observed mean sea surface temperatures and synoptic-scale winds, the empirical boundary layer recovers the long term dipole of surface divergence, and the preponderance of surface wind divergence and convergence when winds are in the direction of, or opposed to, gradients of sea surface temperatures, respectively. These sea surface temperature impacts can also be seen in strong surface wind convergences associated with mid-latitude cyclones and fronts.
Prof. Yuqing Wang Department of Atmospheric Sciences International Pacific Research Center School of Ocean and Earth Science and Technology University of Hawaii at Manoa
Abstract:
In this presentation, some applications of the newly developed time-dependent theory of tropical cyclone (TC) intensification will be briefly introduced and discussed. One of the applications is to help determine the key environmental factors and their relative importance/contributions to the observed TC intensity change. This has been done by introducing an environmental ventilation parameter into the time-dependent theory. The environmental ventilation parameter is a multiplication of individual ventilation parameters induced by all individual factors and is quantified by machine learning algorithm. Six environmental factors are evaluated, including the large-scale environmental vertical wind shear (VWS), the climatological ocean heat content (COHC), the divergence in the upper troposphere (D200), the mid-level relative humidity (RHMD), the gradient of maximum potential intensity (dMPI) along the TC track, and the TC translational speed (SPD). The results show that the environmental VWS contributes the most to the slowing down of TC intensification or the weakening of TCs. Other environmental factors contribute about equally but secondary compared with the environmental VWS. The second application of the time-dependent theory is to real TC intensity forecasting. This is done by considering the environmental ventilation parameter as an unknown latent variable and is determined by a combination of the Bayesian hierarchical model (BHM) and neural network (NN) and ensemble mean based on selected environmental factors. With the determined environmental ventilation parameter, the time-integration of the time-dependent equation of TC intensification rate can provide TC intensity forecast at required lead times. Our preliminary results for TCs over the North Atlantic show that the model has a good skill in predicting TC intensity change up to 10 days, indicating that the scheme has a potential to be used for real-time TC intensity forecasting after implementation and further testing in an operational real-time setting in the near future.
Dr Nadya Moisseeva Post-Doctoral Researcher Atmospheric Sciences Department University of Hawaii at Manoa
Abstract:
Volcanic activity and the associated vog (volcanic smog) emissions into the atmosphere often result in adverse air quality conditions and present a hazard to human health and the environment. Since 2010 the Vog Measurement and Prediction Program at UH Manoa has been dedicated to improving our understanding of vog dispersion and providing the public and emergency responders with accurate and timely forecasts of volcanic air quality in Hawaii. In this talk, I will present the newest version of the VMAP air quality modelling framework for volcanic gas emissions called VogCast. Building on a decade of experience of VMAP researchers, VogCast is designed to simplify ensemble air quality prediction by linking together multiple state-of-art models of meteorology, plume-rise, emissions, and dispersion. This talk will focus on the application of VogCast to the most recent eruption of Maunaloa in December 2022. We will discuss the unique challenges associated with modeling this event and how it differed from typical eruptive activity at Kilauea. Using remotely sensed data, I will demonstrate the strengths and limitations of the VogCast framework, and highlight the key steps to improving forecast accuracy. Lastly, I’ll briefly touch on VMAP’s current modelling and observational efforts aimed at improving our understanding of vog dispersion in Hawaii.
Bio:
Nadya Moisseeva is a postdoctoral researcher at the Vog Measurement and Prediction Program at UH Manoa. She joined VMAP in 2021 after finishing her doctoral work at the University of British Columbia under Prof. Roland Stull. Her PhD focused on high-resolution numerical and analytical modelling of wildfire smoke plumes, atmospheric dynamics, air quality and turbulence. Currently, her main research interest is in developing multi-scale (turbulence-resolving to mesoscale) air quality simulations in complex terrain to help characterize vog dispersion in Hawaii.
Meiji HONDA Professor, Faculty of Science Niigata University, Japan
Abstract:
We propose a new scheme based on geopotential height fields to detect cutoff lows starting in the preexisting trough stage. The intensity and scale derived from the proposed scheme will allow for a better understanding of the cutoff low life cycle. These cutoff lows often accompany mesoscale disturbances, causing adverse weather-related events, such as intense torrential rainfall and/or tornadoes. The proposed scheme quantifies the geometric features of a depression from its horizontal height profile. The height slope of a line intersecting the depression bottom and the nearest tangential point (optimal slope) locally indicates the intensity and scale of an isolated depression. The strength of the proposed scheme is that, by removing a local background height slope from a geopotential height field, the cutoff low and its preexisting trough are seamlessly detected as an identical depression. The distribution maps for the detected cutoff lows and preexisting troughs are illustrated along with their intensities, sizes, and local background flows estimated from snapshot height fields. We conducted climatological comparisons of cutoff lows to determine the utility of the proposed scheme.
Prof. Yali Luo Nanjing University of Information Science and Technology Chinese Academy of Meteorological Sciences The State Key Laboratory of Severe Weather Beijing, China
Abstract:
Extreme precipitation is an issue of worldwide concern, but its microphysics remain elusive. Using multisource data including 5-yr dual-polarization radar observations, convective and microphysical characteristics of extreme precipitation features (EPFs) over a densely populated monsoon coastal region in South China are investigated including the dependence on rainfall extremity and subseasonal variations.
The EPFs are sorted into three groups according to the extreme rainfall intensity: 84 – 126 mm hr -1 (ER1), 126 – 186 mm hr -1 (ER2), ≥186 mm hr -1 (ER3). The more extreme rainfall shows a notable increase and decrease in the fractions of “intense” convection (7.6%, 20.6%, 31.6%) and “weak” convection (41.3%, 22.9%, 18.9%), respectively, while that of the “moderate” convection remain about 50%. The higher rainfall extremity is accompanied by statistically significant increases in ice and liquid water contents and a slight decrease in the fraction of coalescence in liquid-phase processes. While the raindrop size distributions (RSDs) of ER1 to ER3 similarly feature a mean size larger than “maritime-like” droplets and a concentration much higher than “continental-like” raindrops, the mean size and concentration of raindrops tend to increase with the increasing rainfall extremity.
During the pre-monsoon period, precipitation systems are the largest in area but their EPFs are the least frequent and have the lowest raindrop concentration, likely due to the colder, drier environment with large vertical wind shear (VWS). Onset of the summer monsoon increases the frequency and convective intensity of EPFs, leading to an increase in raindrop size, consistent with the substantial increases of CAPE and moisture during the active- monsoon period. EPFs share similar convective intensity and RSD between the post-monsoon and active-monsoon periods, although the post-monsoon EPFs are slightly less frequent and have a smaller horizontal scale related to the reduced 0–6-km VWS. EPFs associated with tropical cyclones have the weakest convective intensity but the most active warm-rain processes with the RSD being closer to the maritime regime.
Bio:
Yali Luo received the M.S. degree from the Chinese Academy of Meteorological Sciences (CAMS), Beijing, China, in 1996, and the Ph.D. degree from The University of Utah, Salt Lake City, UT, USA, in 2003.
After spending 3.5 years as a Research Scientist at the NASA Langley Research Center, Hampton, VA, USA, she took a scientist position at the State Key Laboratory of Severe Weather, CAMS, in July 2007. She was a Chief Scientist with CAMS from September 2019 to December 2022, where she was also the Chief Scientist of the Southern China Monsoon Rainfall Experiment (SCMREX; 2013－2021) that was a Research and Development Project (RDP) of the World Weather Organization’s World Weather Research Programme. She took a professor position at Nanjing University of Information Science and Technology in early 2023.
She works on the understanding of physical mechanisms governing the evolution and long-term changes of weather/climate extremes especially heavy precipitation, using integrated observational datasets combined with numerical modeling. She has about 100 refereed publications in journals, such as the Bulletin of the American Meteorological Society and the Science Bulletin. She is one of the lead authors of the IPCC’s Special Report on Extremes (SREX). Her research interests also include evaluation and improvement of physics parameterization schemes in atmospheric models, i.e., cloud- precipitation microphysics and convection schemes.
