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C. Lyn Comer
,
Braedon Stouffer
,
David J. Stensrud
,
Yunji Zhang
, and
Matthew R. Kumjian

Abstract

Convective boundary layer (CBL) depth can be estimated from dual-polarization WSR-88D radars using the product differential reflectivity (ZDR ), because the CBL top is co-located with a local ZDR minimum produced by Bragg scatter at the interface of the CBL and the free troposphere. Quasi-vertical profiles (QVPs) of ZDR are produced for each radar volume scan and profiles from successive times are stitched together to form a time-height plot of ZDR from sunrise to sunset. QVPs of ZDR often show a low-ZDR channel that starts near the ground and rises during the morning and early afternoon, identifying the CBL top. Unfortunately, results show that this channel within the QVP can occasionally be misleading. This motivated creation of a new variable: DVar , which combines ZDR with its azimuthal variance and is particularly helpful at identifying the CBL top during the morning hours. Two methods are developed to track the CBL top from QVPs of ZDR and DVar . Although each method has strengths and weaknesses, the best results are found when the two methods are combined using inverse variance weighting. The ability to detect CBL depth from routine WSR-88D radar scans rather than from rawinsondes or lidar instruments would vastly improve our understanding of CBL depth variations in the daytime by increasing the temporal and spatial frequency of the observations.

Restricted access
Takuya Kurihana
,
Ilijana Mastilovic
,
Lijing Wang
,
Aurelien Meray
,
Satyarth Praveen
,
Zexuan Xu
,
Milad Memarzadeh
,
Alexander Lavin
, and
Haruko Wainwright

Abstract

The complexity of growing spatiotemporal resolution of climate simulations produces a variety of climate patterns under different projection scenarios. This paper proposes a new data-driven climate classification workflow via an unsupervised deep learning technique that can dimensionally reduce the vast volume of spatiotemporal numerical climate projection data into a compact representation. We aim to identify distinct zones that capture multiple climate variables as well as their future changes under different climate change scenarios. Our approach leverages convolutional autoencoders combined with k–means clustering (standard autoencoder) and online clustering based on the Sinkhorn-Knopp algorithm (clustering autoencoder) across the continental United States (CONUS) to capture unique climate patterns in a data-driven fashion from the Geophysical Fluid Dynamics Laboratory Earth System Model (GFDL-ESM2G). The developed approach compresses 70 years of GFDL-ESM2G simulation at 0.125° spatial resolution across the CONUS under multiple warming scenarios to a lower dimensional space by a factor of 660000, and then tested on 150 years of GFDL-ESM2G simulation data. The results show that five climate clusters capture physically reasonable and spatially stable climatological patterns matched to known climate classes defined by human experts. Results also show that using a clustering autoencoder can reduce the computational time for clustering by up to 9.2 times when compared to using a standard autoencoder. Our five unique climate patterns resulting from the deep learning-based clustering of the lower dimensional space thereby enable us to provide insights on hydrometeorology and its spatial heterogeneity across the continental US immediately without downloading large climate datasets.

Open access
Qinghua Ding
and
Hailan Wang

Abstract

This study aims to understand the underlying mechanism of large scale circulation control on atmospheric rivers (AR) and precipitation variability across the Contiguous United States (CONUS) in winter. The El Niño-Southern Oscillation (ENSO), known as a key driver of global circulation, has shown a modest impact on CONUS precipitation, prompting us to focus our attention on other climate drivers. Here, we find that barotropic instability over the exit region of the North Pacific subtropical jet stream plays a critical role in forming a downstream stationary Rossby wave train during winter (referred to as the West Mode). This wave pattern influences CONUS precipitation by affecting AR activity and explains approximately 50% of rainfall changes in the Western US, as well as numerous extreme wet and drought years along the West Coast, such as the wet winter in 2022/23. Over the past eight decades, the West Mode exhibited limited sensitivity to both Sea Surface Temperature (SST) and increasing anthropogenic forcing and was more influential in shaping interannual and interdecadal CONUS precipitation variability than ENSO. This result may explain why ENSO alone can only account for a limited portion of CONUS precipitation variability, thereby imposing an inherent constraint on the precision of seasonal predictions of CONUS precipitation made by climate models. Due to the significance of the West Mode in governing precipitation variability over the Western US, winter precipitation in that region may possess some resilience to the effects of global warming in the coming decades, as supported by large ensemble simulations driven by projected radiative forcing.

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Siegfried D. Schubert
,
Yehui Chang
,
Anthony M. DeAngelis
,
Young-Kwon Lim
,
Natalie P. Thomas
,
Randal D. Koster
,
Michael G. Bosilovich
,
Andrea M. Molod
,
Allison Collow
, and
Amin Dezfuli

Abstract

In late December of 2022 and the first half of January 2023, an unprecedented series of atmospheric rivers (ARs) produced near-record heavy rains and flooding over much of California. Here, we employ the NASA GEOS AGCM run in a “replay” mode, together with more idealized simulations with a stationary wave model, to identify the remote forcing regions, mechanisms, and underlying predictability of this flooding event. In particular, the study addresses the underlying causes of a persistent positive Pacific–North American (PNA)-like circulation pattern that facilitated the development of the ARs. We show that the pattern developed in late December as a result of vorticity forcing in the North Pacific jet exit region. We further provide evidence that this vorticity forcing was the result of a chain of events initiated in mid-December with the development of a Rossby wave (as a result of forcing linked to the MJO) that propagated from the northern Indian Ocean into the North Pacific. As such, both the initiation of the event and the eventual development of the PNA depended critically on internally generated Rossby wave forcings, with the North Pacific jet playing a key role. This, combined with contemporaneous SST (La Niña) forcing that produced a circulation response in the AGCM that was essentially opposite to the positive PNA, underscores the fundamental lack of predictability of the event at seasonal time scales. Forecasts produced with the GEOS-coupled model suggest that useful skill in predicting the PNA and extreme precipitation over California was in fact limited to lead times shorter than about 3 weeks.

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Chenyu Zheng
,
Shaojun Zheng
,
Ming Feng
,
Lingling Xie
,
Lei Wang
,
Tianyu Zhang
, and
Li Yan

Abstract

The East African Coastal Current (EACC) is an important western boundary current of the tropical South Indian Ocean and plays an important role in the ocean circulation and biogeochemical cycles in the Indian Ocean. This study investigates the interannual variability of the EACC and its dynamical mechanisms. The result shows that the EACC has interannual variability associated with the El Niño-Southern Oscillation (ENSO) during 1993-2017. The EACC shows a significantly positive correlation with the Niño3.4 index with a correlation coefficient of 0.65, lagging the Niño3.4 index by 18 months. During the decaying phases of El Niño (La Niña) events, the negative (positive) sea level anomaly (SLA) propagates westward as upwelling (downwelling) Rossby waves from the southeast Indian Ocean to the southwest Indian Ocean, and then strengthens (weakens) the EACC due to zonal SLA gradient off the East African coast under geostrophic equilibrium. The SLA gradually weakens in the southeast Indian Ocean during its westward propagation but strengthens in the southwest Indian Ocean promoted by local wind stress curl anomaly. This study can improve our understanding of the relationship between the western boundary current of the tropical South Indian Ocean and large-scale ENSO air-sea processes, and is important for managing marine fisheries and ecosystems on the East African coast.

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Josep Bonsoms
,
Marc Oliva
,
Juan I. López-Moreno
, and
Xavier Fettweis

Abstract

The Greenland Ice Sheet (GrIS) meltwater runoff has increased considerably since the 1990s, leading to implications for the ice sheet mass balance and ecosystem dynamics in ice-free areas. Extreme weather events will likely continue to occur in the coming decades. Therefore, a more thorough understanding of the spatiotemporal patterns of extreme melting events is of interest. This study aims to analyze the evolution of extreme melting events acrossthe GrIS and determine the climatic factors that drive them. Specifically, we have analyzed extreme melting events (90th percentile) across the GrIS from 1950 to 2022 and examined their links to the surface energy balance (SEB) and large-scale atmospheric circulation. Extreme melting days account for approximately 35-40% of the total accumulated melting per season. We found that extreme melting frequency, intensity, and contribution to the total accumulated June, July and August (summer) melting show a statistically significant upward trend at a 95% confidence level. The largest trends are detected across the northern GrIS. The trends are independent of the extreme melting percentile rank (90th, 97th, or 99th) analyzed, and are consistent with average melting trends that exhibit an increase of similar magnitude and spatial configuration. Radiation plays a dominant role in controlling the SEB during extreme melting days. The increase in extreme melting frequency and intensity is driven by the increase of anticyclonic weather types during summer and more energy available for melting. Our results help to enhance the understanding of extreme events in the Arctic.

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R. M. Samelson
,
S. M. Durski
,
D. B. Chelton
,
E. D. Skyllingstad
, and
P. L. Barbour

Abstract

The dependence of surface-current damping on the definition of surface current for the relative wind is examined in coupled ocean-atmosphere numerical simulations of the northern California Current System (nCCS) during March through October 2009. The model response is analyzed for wind stress computed from relative wind for six different choices of effective model surface velocity. Simulations without surface-current coupling are also considered. As a function of the geographically varying uppermost grid-level depth, the model uppermost grid-level velocity is found to have a wind-drift component with a log-layer structure. Mean geostrophic wind work is concentrated in the shelf and slope regions during March through May (MAM) and in the deep offshore region in June through September (JJAS). The surface-current damping effect on ocean kinetic energy depends more strongly on the parameterization of atmospheric planetary boundary layer (PBL) turbulence than on the surface-current coupling formulation: weaker PBL mixing gives stronger surface-current damping. The damping effect is stronger in the less energetic, offshore region than in the more energetic region closer to the coast. During MAM, the changes in kinetic energy and geostrophic wind-work in the shelf and slope regions are spatially correlated, while during JJAS, the changes in geostrophic wind-work are strongly modulated by SST-stress coupling. The wind-drift-corrected surface-current formulations result in large changes in the effective wind-work based on the product of stress and relative-wind surface current but in only small changes in the kinetic energy of the circulation.

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Shaohua Chen
,
Haikun Zhao
,
Philip J. Klotzbach
,
Jian Cao
,
Jia Liang
,
Weican Zhou
, and
Liguang Wu

Abstract

On interannual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave (SSW) train relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG–TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of large-scale circulation and tropical cyclones.

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Valentina Ortiz-Guzmán
,
Martin Jucker
, and
Steven C. Sherwood

Abstract

The Southern Hemisphere climate and weather are affected by several modes of variability and climate phenomena across different time and spatial scales. An additional key component of the atmosphere dynamics that greatly influences weather is quasi-stationary Rossby waves, which attract particular interest as they are often associated with synoptic-scale extreme events. In the Southern Hemisphere extratropical circulation, the most prominent quasi-stationary Rossby wave pattern is the zonal wavenumber 3 (ZW3), which has been shown to have impacts on meridional heat and momentum transport in mid- to high latitudes and on the Antarctic sea ice extent. However, little is known about its impacts outside of polar regions. In this work, we use ERA5 reanalysis data on monthly time scales to explore the influence of phase and amplitude of ZW3 on temperature and precipitation across the Southern Hemisphere midlatitudes. Our results show a significant impact in various regions for all seasons. One of the most substantial effects is observed in precipitation over southeastern Brazil during austral summer, where different phases of the ZW3 force opposite anomalies. When using the ZW3 phase and amplitude as prior information, the probability of occurrence of precipitation extremes in this region increases up to three times. Additionally, we find that this ZW3 weather signature is largely independent of the zonally symmetric Southern Annular Mode (SAM); neither does it seem to be linked to El Niño–Southern Oscillation (ENSO) or Indian Ocean dipole (IOD) signal.

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A. R. Siders
and
Dana Veron
Open access