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Bin Zheng
,
Ailan Lin
, and
Yanyan Huang

Abstract

In this study, persistent rainfall (PR) over South China (SC) is divided into two types. One type occurs multiple times in succession (defined as multiple PR, MPR); another type represents isolated PR (IPR), for which no new PR occurs for 10 days after the previous PR. The spatio-temporal structures of the 10–30-day intraseasonal oscillations (ISOs) associated with the two types of PR are compared and analyzed. The results reveal that the low-level moisture and air temperature perturbations always have a leading phase relative to the anomalous precipitation. In addition, the positive low-level moisture tendency appears in the MPR ending phase, whereas that in the IPR is close to zero. This difference results in convective development after the MPR ending phase, though not after the IPR. The moisture budget shows that the difference in moisture tendency between MPR and IPR is mainly due to meridional advection, including advections by the mean meridional flow across the perturbation moisture gradient and by the perturbation meridional flow across the mean moisture gradient. For the former, the difference is attributed to the perturbation moisture gradients, while the mean moisture gradients are responsible for the difference of the latter. Furthermore, an essential cause of the difference is the influence of higher-latitude disturbances that affect the IPR more significantly than the MPR. Two associated mechanisms are proposed. One is the perturbation stacking effect, and the other is the effect of angular momentum conservation. By contrast, the low-level temperature anomalies are not the key factor causing the difference between MPR and IPR.

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Shyama Mohanty
,
Madhusmita Swain
,
Raghu Nadimpalli
,
K. K. Osuri
,
U. C. Mohanty
,
Pratiman Patel
, and
Dev Niyogi

Abstract

The city of Mumbai, India, frequently receives extreme rainfall (>204.5 mm day−1) during the summer monsoonal period (June–September), causing flash floods and other hazards. An assessment of the meteorological conditions that lead to these rain events is carried out for 15 previous cases from 1980 to 2020. The moisture source for such rain events over Mumbai is generally an offshore trough, a midtropospheric cyclone, or a Bay of Bengal depression. The analysis shows that almost all of the extreme rain events are associated with at least two of these conditions co-occurring. The presence of a narrow zone of high sea surface temperature approximately along the latitude of Mumbai over the Arabian Sea can favor mesoscale convergence and is observed at least 3 days before the event. Anomalous wind remotely supplying copious moisture from the Bay of Bengal adds to the intensity of the rain event. The presence of midtropospheric circulation and offshore trough, along with the orographic lifting of the moisture, give a unique meteorological setup to bring about highly localized catastrophic extreme rainfall events over Mumbai. The approach adopted in this study can be utilized for other such locales to develop location-specific guidance that can aid the local forecasting and emergency response communities. Further, it also provides promise for using data-driven/machine learning–based pattern analysis for developing warning triggers.

Significance Statement

We have identified the meteorological conditions that lead to extreme heavy rains over Mumbai, India. They are that 1) at least two of these rain-bearing systems, offshore trough, midtropospheric circulation, and Bay of Bengal depression moving north-northwestward are concurrently present, 2) an anomalous high SST gradient is present along the same latitude as Mumbai, and 3) the Western Ghats orography favors the rainfall extreme to be highly localized over Mumbai.

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Erin A. Jones
and
Xuguang Wang

Abstract

The current global operational four-dimensional ensemble-variational (4DEnVar) data assimilation (DA) system at NCEP adopts a background ensemble at a reduced resolution, which restricts the range of spatial scales that the ensemble background error covariance can resolve. A prior study developed a multi-resolution ensemble 4DEnVar method and determined that this approach can provide a comparable forecast to an approach using solely high-resolution members, while substantially reducing the computational cost. This study further develops the multi-resolution ensemble 4DEnVar approach to allow for a flexible number of low- and high-resolution ensemble members as well as varying localization length scales between the high- and low-resolution ensembles.

Three 4DEnVar experiments with the same computational costs are compared. The first has an 80-member high-resolution background ensemble with single-scale optimally-tuned localization (SR-High). The second and third utilize the multi-resolution background ensembles. One has 130 low-resolution and 40 high-resolution members (MR170) while the other has 180 low-resolution and 24 high-resolution members (MR204). Both multi-resolution ensemble experiments utilize differing localization radii with ensemble resolution. Despite having the same costs, both MR170 and MR204 improves global forecasts and decreases tropical cyclone track errors for up to five days in lead time compared to SR-High. Improvements are most apparent in larger-scale features, such as jet streams and the environmental steering flow of tropical cyclones. Additionally, MR170 outperforms MR204 in terms of global and tropical cyclone track forecasts, demonstrating the value of both increasing sampling at large scales and retaining substantial information at small scales.

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Yangruixue Chen
,
Bo Liu
,
Yali Luo
,
Cristian Martinez-Villalobos
,
Guoyu Ren
,
Yongjie Huang
,
Sihan Zhang
,
Yong Sun
, and
Zhongshi Zhang

Abstract

A Lagrangian model – the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) is used to quantify changes in moisture sources and paths for precipitation over North China’s Henan province associated with tropical cyclone (TC) over the western North Pacific (WNP) during July-August of 1979-2021. During TC-active periods, an anomaly cyclone over the WNP enhances southeasterly and reduces southwesterly moisture transport to Henan. Accordingly, compared to TC-inactive periods, moisture contributions from the Pacific Ocean (PO), Eastern China (EC) and Local area (Local) are significantly enhanced by 48.32% (16.73% versus. 11.28%), 20.42% (9.44% versus. 7.84%), and 2.89% (4.91% versus. 4.77%), respectively, while moisture contributions from the Indian Ocean (IO), Southwestern China (SWC), Eurasia (EA) and South China Sea (SCS) are significantly reduced by −31.90% (8.61% versus. 12.64%), −16.27% (4.60% versus. 5.50%), −8.81% (19.10% versus. 20.95%), and −6.92% (12.18% versus. 13.09%). Furthermore, the moisture transport for a catastrophic extreme rainfall event during July 17–22 (“21⋅7” event) influenced by typhoon Infa is investigated. Compared to the mean state during TC-active periods, the moisture contribution from the PO was substantially increased by 126.32% (37.87% versus. 16.73%), while that from IO significantly decreased by −98.26% (0.15% versus. 8.61%) during the “21⋅7” event. Analyses with a bootstrap resampling method show that moisture contributions from the PO fall outside the +6σ range, for both the TC-active and TC-inactive probability distributions. Thus, the “21⋅7” event is rare and extreme in terms of the moisture contribution from the PO, with the occurrence probability being less than one in a million times.

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Julien Savre
and
George Craig

Abstract

In this work, cloud ensemble statistics are extracted from idealized Radiative Convective Equilibrium simulations performed at horizontal grid spacings Δ ranging from 2 km to 125 m. At the coarsest resolution, convection remains randomly distributed in space such that the equilibrium statistical mechanics theory proposed by Craig and Cohen (2006) (CC06, assumes Poisson distributed clouds and exponential mass flux distributions) remains valid. Using classical organization metrics, clustering is already observed at Δ = 1 km, but substantial deviations between the simulated cloud ensemble statistics and CC06 are only observed for grid spacings Δ < 500 m. At these resolutions, the cloud mass flux distributions exhibit heavy tails and cloud counts become overdispersed (higher variance than a Poisson distribution). These changes in ensemble statistics are accompanied by a shift in sub-cloud organization patterns as well as with the fact that individual cloudy updrafts start to be resolved. Consequently, a horizontal grid spacing no larger than 250 m is recommended, not only to properly resolve the dynamics of individual convective clouds, but also to capture the mesoscale organization of the cloud ensemble. Finally, it is shown that the CC06 theory and our high resolution results including mesoscale organization may be reconciled if one considers: 1) areas smaller than approximately 2 km in size, corresponding roughly to the narrow bands along which clouds develop almost randomly; and 2) individual cloud cores instead of cloud objects, core mass fluxes being shown to generally follow exponential distributions.

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Roland Walz
,
Hella Garny
, and
Thomas Birner

Abstract

The stratospheric polar vortex is dynamically coupled to the tropospheric circulation. Therefore, a better mechanistic understanding of this coupled system is important to interpret past and future circulation changes correctly. Previously, idealized simulations with a dry dynamical-core general circulation model and imposed tropical upper-tropospheric warming (TUTW) have shown that a critical warming level exists at which the polar vortex transitions from a weak and variable to a strong and stable regime. Here, we investigate the dynamical mechanism responsible for this regime transition and its influence on the troposphere by performing similar idealized experiments with (REF) and without a polar vortex (NPV). According to the critical-layer control mechanism, the strengthened upper flank of the subtropical jet in response to TUTW leads to an accelerated wave-driven residual circulation in both experiments. For the REF experiment, the stronger residual circulation is associated with changes in the lower-stratospheric thermal structure that are consistent with an equatorward shift of the polar vortex. At a certain threshold of TUTW in the REF experiment, the tropospheric jet and the stratospheric polar vortex form a confined waveguide for planetary-scale waves that presumably favors downward wave coupling events. Consistently, the polar vortex strengthens in combination with an enhanced poleward shift of the tropospheric jet compared to the NPV experiment. Overall, these idealized experiments suggest that a polar vortex strengthening can be caused by greenhouse gas–induced warmings via modifications of the waveguide. This mechanism might also be relevant to understand the polar vortex changes in more complex models.

Open access
Yeon-Woo Choi
and
Elfatih A. B. Eltahir

Abstract

For millennia, Mesopotamia was blessed by enough water supplied by the Tigris and Euphrates Rivers. However, the dwindling freshwater resource is no longer enough. In the future, climate change coupled with a growing population could considerably exacerbate the current water deficit. Based on simulations by carefully selected global and regional climate models, we conclude that these river basins may possibly face further water shortages (mainly due to a reduction in spring-season precipitation) in the next few decades (2021–50) under a scenario of high emissions of greenhouse gases. However, there is no consensus among models regarding these near-term (2021–50) projections of change in precipitation, and society is likely to face the challenge of how to prepare for this uncertain future. The story is different for the late decades of this century: we project, with significantly more confidence, a robust decrease in wet-season (winter to spring) precipitation over the headwaters of these river basins, worsening future water deficits and implying a century-long drying trend over Mesopotamia. Possible physical mechanisms are proposed and discussed. As global warming progresses, higher sea level pressure, centered on the Mediterranean Sea, will likely make upstream storms less frequent and weaker, leading to drying over Mesopotamia. Further, projections show a poleward migration of the fewer Mediterranean storm tracks, decreasing the frequency of storms that penetrate into Mesopotamia. Implementing a global net-zero carbon emissions policy by midcentury could mitigate the severity of the projected droughts in this region.

Open access