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Yibo Zhang
,
Chunzheng Kong
,
Zizhou Liu
,
Bingtian Li
, and
Xianqing Lv

Abstract

Satellite remote sensing can monitor sea level changes at temporal and spatial scales, plays an important role in the study of tides, and is widely used in numerical tidal models. However, these tidal models are usually computationally expensive. The equidistant nodes orthogonal polynomial fitting (ENOPF) method may overcome that drawback. This study evaluates the accuracy of the ENOPF method in fitting the major tidal constituents in the region near the Ryukyu Islands, where the water depth on either side of the islands varies significantly. The results show that the ENOPF method can accurately fit the major tidal constituents in the presence of complex topography. Furthermore, this approach can also be used to generate reasonable cotidal charts and provide valuable tidal information for hydrodynamic model simulations in the East China Sea. For the high-resolution hydrodynamic model of the East China Sea in particular, reasonable open boundary conditions can be provided by the ENOPF method.

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Yujun He
,
Bin Wang
,
Juanjuan Liu
,
Yong Wang
,
Lijuan Li
,
Li Liu
,
Shiming Xu
,
Wenyu Huang
, and
Hui Lu

Abstract

Accurately predicting the decadal variations in Sahel rainfall has important implications for the lives and economy in the Sahel. Previous studies found that the decadal variations in sea surface temperature (SST) in the Atlantic, Mediterranean Sea, Indian Ocean and Pacific contribute to those in Sahel rainfall. This study evaluates the decadal prediction skills of Sahel rainfall from all the available hindcasts contributing to phases 5&6 of the Coupled Model Intercomparison Project (CMIP5&6), in comparison with the related uninitialized simulations. A majority of the prediction systems show high skills on Sahel rainfall. The high skills may be partly attributed to external forcings, which are reflected in good performance of the respective uninitialized simulations. The decadal prediction skills of the key SST drivers and their relationships with the Sahel rainfall are also assessed. Both the hindcasts and the uninitialized simulations generally present high skills for the Atlantic Multidecadal Variability (AMV) and Mediterranean Sea SST indices and low skills for the Indian Ocean basin mode (IOBM) and Interdecadal Pacific Variability (IPV) indices. The relationship between the Sahel rainfall and the AMV or Mediterranean Sea SST index is reasonably captured by most prediction systems and their uninitialized simulations, while that between the Sahel rainfall and the IOBM or IPV index is captured by only a few systems and their uninitialized simulations. The high skills of the AMV and Mediterranean Sea SST indices as well as the reasonable representations of their relationships with the Sahel rainfall by both the hindcasts and uninitialized simulations probably play an important role in predicting the Sahel rainfall successfully.

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Sarah A. Tessendorf
,
Kyoko Ikeda
,
Roy M. Rasmussen
,
Jeffrey French
,
Robert M. Rauber
,
Alexei Korolev
,
Lulin Xue
,
Derek R. Blestrud
,
Nicholas Dawson
,
Melinda Meadows
,
Melvin L. Kunkel
, and
Shaun Parkinson

Abstract

During the Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) field campaign, cloud-top generating cells were frequently observed in the very high-resolution W-band airborne cloud radar data. This study examines multiple flight segments from three SNOWIE cases that exhibited cloud-top generating cells structures, focusing on the in-situ measurements inside and outside these cells to characterize the microphysics of these cells. The observed generating cells in these three cases occurred in cloud tops of −15 to −30 °C, with and without overlying cloud layers, but always with shallow layers of atmospheric instability observed at cloud top. The results also indicate that liquid water content, vertical velocity, and drizzle and ice crystal concentrations are greater inside the generating cells compared to the adjacent portions of the cloud. The generating cells were predominantly < 500 m in horizontal width and frequently exhibited drizzle drops coexisting with ice. The particle imagery indicates that ice particle habits included plates, columns, and rimed and irregular crystals, likely formed via primary ice nucleation mechanisms. Understanding the sources of natural ice formation is important to understanding precipitation formation in winter orographic clouds, and is especially relevant for clouds that may be targeted for glaciogenic cloud seeding as well as to improve model representation of these clouds.

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Clara Orbe
,
David Rind
,
Darryn Waugh
,
Jeffrey Jonas
,
Xiyue Zhang
,
Gabriel Chiodo
,
Larissa Nazarenko
, and
Gavin A. Schmidt

Abstract

Stratospheric ozone, and its response to anthropogenic forcings, provide an important pathway for the coupling between atmospheric composition and climate. In addition to stratospheric ozone’s radiative impacts, recent studies have shown that changes in the ozone layer due to 4xCO2 have a considerable impact on the Northern Hemisphere (NH) tropospheric circulation, inducing an equatorward shift of the North Atlantic jet during boreal winter. Using simulations produced with the NASA Goddard Institute for Space Studies (GISS) high-top climate model (E2.2) we show that this equatorward shift of the Atlantic jet can induce a more rapid weakening of the Atlantic Meridional Overturning Circulation (AMOC). The weaker AMOC, in turn, results in an eastward acceleration and poleward shift of the Atlantic and Pacific jets, respectively, on longer timescales. As such, coupled feedbacks from both stratospheric ozone and the AMOC result in a two-timescale response of the NH midlatitude jet to abrupt 4xCO2 forcing: a “fast” response (5–20 years) during which it shifts equatorward and a “total” response (~100–150 years) during which the jet accelerates and shifts poleward. The latter is driven by a weakening of the AMOC that develops in response to weaker surface zonal winds, that result in reduced heat fluxes out of the subpolar gyre and reduced North Atlantic Deep Water formation. Our results suggest that stratospheric ozone changes in the lower stratosphere can have a surprisingly powerful effect on the AMOC, independent of other aspects of climate change.

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Zhuoyong Xiao
,
Xinping Zhang
,
Xiong Xiao
,
Xin Chang
, and
Xinguang He

Abstract

Convective/advective precipitation partitions refer to the divisions of precipitation that are either convective or advective in nature, relative to the total precipitation amount. These distinct partitions can have a significant influence on stable isotope composition of precipitation. This study analyzed and compared the effect of precipitation partitions on δ 18O in precipitation (δ 18Op) by using daily precipitation stable isotope data from Changsha station and monthly precipitation stable isotope data from the Global Network of Isotopes in Precipitation (GNIP), under different time scales, time intervals (i.e., annual, warm season, and cold season), and precipitation intensities. The results showed that the correlation between convective precipitation fraction (CPF) and total precipitation amount was influenced by the intensity of convection in different time intervals. On both the daily and monthly scales, the CPF decreased as the precipitation amount increased in the warm season, while increased with increasing precipitation amount in the cold season. Regardless of the season, daily δ 18Op at Changsha station consistently increased with an increase in daily CPF. On a daily scale, the effect of convective activity on δ 18Op was stronger than that of the “precipitation amount effect” in the cold season, as compared to the situation in the warm season. As a result, the regression line slope between δ 18Op and CPF increased with increasing precipitation intensity in the warm season, meaning that as the CPF increased, the δ 18Op increased at a faster rate under higher precipitation intensity. Similarly, the slope increased with increasing precipitation intensity in the cold season. This suggests that precipitation intensity and convection intensity can affect the relationship between δ 18Op and CPF. Our findings shed light on how different precipitation partitions affect stable isotope composition of precipitation, thus enhancing our understanding of the variability of precipitation stable isotopes in the monsoon regions of China.

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Hirohiko Masunaga
and
Hanii Takahashi

Abstract

Convective lifecycle is often conceptualized to progress from congestus to deep convection and develop further to stratiform anvil clouds, accompanied by a systematic change in the vertical structure of vertical motion. This archetype scenario has been developed largely from the Eulerian viewpoint, and has yet to be explored whether or not the same lifecycle emerges itself in a moving system tracked in the Lagrangian manner. To address this question, Lagrangian tracking is applied to tropical convective systems in combination with a thermodynamic budget analysis forced by satellite-retrieved precipitation and radiation. A new method is devised to characterize the vertical motion profiles in terms of the column import or export of moisture and moist static energy (MSE). The Bottom-heavy, Mid-heavy, and Top-heavy regimes are identified for every one-square-degree grid pixel accompanying tracked precipitation systems, making use of the diagnosed column export/import of moisture and MSE. Major findings are as follows. The Lagrangian evolution of convective systems is dominated by a state of dynamic equilibrium among different convective regimes rather than a monotonic progress from one regime to the next. The transition from the Bottom-heavy to Mid-heavy regimes is fed with intensifying precipitation presumably owing to a negative gross moist stability (GMS) of the Bottom-heavy regime, whereas the transition from the Mid-heavy to Top-heavy regimes dissipates the system. The Bottom-heavy to Mid-heavy transition takes a relaxation time of about 5 h in the equilibrating processes, whereas the relaxation time is estimated as roughly 20 h concerning the Mid-heavy to Top-heavy transition.

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Carrie Lewis-Merritt
,
Justin P. Stachnik
,
Margaret A. Hollis
,
Elinor R. Martin
, and
Rachel R. McCrary

Abstract

Tropical easterly waves (TEWs) play a critical role in regulating convection and precipitation across the global tropics. TEWs act as seed disturbances for tropical cyclogenesis, serve as an essential component in monsoon precipitation, and produce large amounts of rainfall and diabatic heating that can strongly affect the large-scale circulation. To help improve our knowledge of a more elusive type of tropical wave, we use satellite and reanalysis estimates of the diabatic heating associated with TEWs that are identified by a tracking algorithm based on low-level curvature vorticity. This study uses the Tropical Rainfall Measuring Mission (TRMM) version 6 convective–stratiform heating (CSH) and spectral latent heating (SLH) orbital products to create a global climatology (1998–2015) of TEW diabatic heating. TEW-specific composites for the satellite-observed vertical structure of diabatic heating are compared to similar terms from MERRA-2 across a variety of tropical regions. There are striking differences between the reanalysis and satellite heating with MERRA-2 having much stronger background heating, especially at low levels. Both the satellite-observed and reanalysis heating profiles show stronger midlevel heating associated with TEWs relative to the unconditional background. Similar patterns of mid- and bottom-heaviness emerge in two-dimensional composites of TEW latent heating as stronger heating rates and percent contributions to the background are generally higher at 500 hPa than at 850 hPa. Although TEWs only represent a few percent of the background heating across the global tropics, they comprise 30%–50% of the heating in the prominent TEW tracks over the northeastern Atlantic and Pacific Oceans.

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Ricardo C. Muñoz
and
Laurence Armi

Abstract

Raco is a local wind occurring in central Chile where the Maipo River Canyon exits into the Santiago valley. The intensification of the easterly down-canyon flow starts any time during some cold season nights, accompanied by increases in temperature and drops in humidity. The hypothesis of the raco being a gap wind controlled by the narrowest section in the 12-km canyon exit corridor is tested with data from two events in July 2018 and July 2019. The data are analyzed in the framework of hydraulic theory and a subcritical-to-supercritical transition is documented to occur at the narrows of the gap where the Froude number is close to unity, confirmed by radiosondes launched in the narrows in 2019. For the raco flow, the sum of potential and kinetic energy is conserved upstream of the narrows, while the acceleration occurring farther downstream loses a large fraction of energy to frictional dissipation. The raco events occur under the influence of regional subsidence, but a differential nocturnal warming of the in-canyon airmass is responsible for a pressure gradient driving the raco. In the 2019 case, a ceilometer mounted on an instrumented pickup truck documented the structure and movement of the interface between the raco air and the cold-air pool (CAP) existing over the valley to the west. Together with a radiosonde launched near the CAP-raco surface front, the observations reveal the intense shear-driven mixing taking place at the interface and the factors supporting the establishment of a stationary front.

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Rui Jin
,
Hui Yu
,
Zhiwei Wu
,
Johnny C. L. Chan
,
Ming Ying
, and
Peng Zhang

Abstract

This study examines the East Asia and western North Pacific (WNP) monsoon circulation patterns for strong and weak WNP tropical cyclone (TC) numbers in summer. It suggested that years with more intense TCs are coupled with the tropical monsoon circulations, including the northward cross-equatorial airflow and the extending tropical monsoon trough toward the central-eastern WNP. However, a higher frequency of weak TCs can be largely attributed to the mutual interactions among the tropical monsoon trough west of 140°E, the westward South Asia high, and the high pressure anomaly in Northeast Asia (NEA). Then the potential influence of the NEA extratropical system is focused on. The resultant local negative potential vorticity (PV) anomaly is carried southeastward by the prevailing flow. It stimulates a descending flow around 30°N, which favors the westward retreat of the South Asian high and the decreased zonal vertical wind shear around 20°N. The associated lower-level outflow converges in the tropical WNP and reinforces the ascending motion around 10°–20°N. Meanwhile, the warm air column in NEA also contributes to anomalous easterlies in a band around 30°N, intensifying the lower-level cyclonic vorticity in the northwestern WNP. Consequently, the ascending motion, cyclonic vorticity, and the weakened zonal vertical wind shear in northwestern WNP promote the WTC formation. A set of physically based empirical models is developed using various physically based predictors to reconstruct the number of intense and weak TCs. Cross-validated hindcasts suggest that the NEA extratropical circulation can serve as an additional source of predictability for the weak TC variability.

Significance Statement

Tropical cyclones (TCs) are a highly destructive type of natural disaster that have garnered widespread attention. By comparison with intense TCs (ITCs), weak TCs (WTCs) are much more numerous and often form closer to the coastal regions of East Asia, whose mechanism has not been fully understood. In this study, we suggest that more ITCs are controlled by tropical monsoon circulations, while the WTC variability is closely coupled with both tropical and extratropical monsoon systems. In addition to the tropical monsoon trough west of 140°E and the westward South Asian high, the Northeast Asian circulation can regulate the WTC number by changing the lower-level vorticity, vertical motion, and vertical wind shear in the WTC genesis-prone region, which can be applied to improve the seasonal prediction skill of WTCs.

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Shengzhe Chen
,
Jiping Liu
,
Mirong Song
,
Jun Inoue
, and
Yifan Ding

Abstract

The Arctic has experienced rapid changes in recent decades. For the first time, we intercompare five snow mass budget processes over Arctic sea ice simulated by 22 models from the Coupled Model Intercomparison Project phase 6 (CMIP6) using new diagnostics that have not been available for previous CMIPs. Our analysis suggests that snowfall accumulation (melt) is the dominant process contributing to nearly 100% (70.4% ± 10.1%) of the annual snow growth (loss). Snow mass change through sea ice dynamics, snow–ice conversion, and sublimation contribute 10.9% ± 4.9%, 9.7% ± 5.9%, and 9.0% ± 7.7% to the total snow mass loss. The seasonal cycle of various snow processes simulated by most of the CMIP6 models generally follows similar variations. There is reduced snowfall accumulation, melt, and sea ice dynamics during 1993–2014. However, substantial temporal and spatial discrepancies are noteworthy between the CMIP6 models. There is a large spread of snowfall accumulation and snowmelt in summer and fall, snow–ice conversion from autumn to spring, sublimation in late spring and summer, and snow mass change due to sea ice dynamics from winter to midspring. About half the models show decreasing trends of snowfall accumulation during 1993–2014, with no trends in others. Divergent trends in snow–ice conversion and sublimation occur in the Greenland and Barents Seas. The discrepancies are attributed equally to internal variability and model structural differences. Future projections that remove the identified outlier models suggest a significant reduction in snowfall accumulation, snowmelt, and snow mass change due to sea ice dynamics in the Arctic Ocean from 2015 to 2099. Snow–ice conversion and sublimation are also projected to be reduced but with less confidence.

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