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Sipra Biswas
,
Kallol Sarkar
, and
Tapan Kumar Das

Abstract

Being situated in the estuary of the flood-dominated Hooghly River system, the macrotidal Indian Sundarban Delta (ISD) has become one of the most complex, dynamic and rapidly changing landforms on the earth’s surface. To study horizontal areal shifting of shoreline and its impact on mangrove-cover in the region, United State Geological Survey (USGS)-satellite data of 1980, 1990, 2000, 2010 and 2021 were used. Remote sensing and GIS techniques were employed in the investigation. Simultaneous prograding and retrograding shoreline shifting was distinguished almost in all the parts, though sediment-starved eastern and macrotidally more active southern lobes experienced dominantly retreating shift, and sediment-engorged western lobe demonstrated to be more dynamic. Net areal change over north-south tracks followed the trend of decreasing accretion to increasing erosion while going from west to east, whereas that over west-east tracks followed the trend of exponentially increasing erosion while going from north to south. Overall accretion of ∼91 sq. km in the ISD accounted for augmentation of sparse vegetation of ∼13 sq. km, whereas, ∼243 sq. km erosion called for depletion of sparse & moderate vegetation of ∼18 & ∼174 sq. km respectively over the 41-year period. Various oceanographic and riparian forces and actions, episodic natural events etc. vis-a-vis several anthropogenic interventions— all together contributed to such changes. The findings may help the coastal environmentalists, professionals, planners, decision-makers and implementers in formulating and taking up of suitable strategic measures for integrated and effective coastal zone management in this estuarine wetland-forest.

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Chong-Chi Tong
,
Ming Xue
,
Chengsi Liu
,
Jingyao Luo
, and
Youngsun Jung

Abstract

To improve the representation of all relevant scales in initial conditions for large-domain convection-allowing models, a new multi-scale ensemble Kalman filter (MEnKF) algorithm is developed and implemented within the GSI data assimilation framework coupled with the FV3 limited area model. The algorithm utilizes ensemble background error covariances filtered to match the observations assimilated. This is realized in a sequential manner: 1) When assimilating coarse-resolution observations such as radiosondes, ensemble background perturbations are filtered to remove scales smaller than those the observations can represent, along with relatively large horizontal localization radii to ensure low-noise and balanced analysis increments. 2) The resulting ensemble analyses from the first step then serve as the background to assimilate denser observations such as radar data with smaller localization radii. Several passes can be taken to assimilate all observations. In this paper, vertically increasing horizontal filter scales are used when assimilating rawinsonde and surface observations together while radar data are assimilated in the second step.

The algorithm is evaluated through six convective storm cases during May 2021, with cycled assimilation of either conventional data only or with additional radar reflectivity followed by 24-h ensemble forecasts. Overall, positive impacts of the MEnKF on forecasts are obtained regardless of reflectivity data; its advantage over the single-scale EnKF is most significant in surface humidity and temperature forecasts up to at least 12 hours. More accurate hourly precipitation forecasts with MEnKF can last up to 24 hours for light rain. Furthermore, MEnKF forecasts higher ensemble probabilities for the observed hazardous events.

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Falk Feddersen
,
Olavo B. Marques
,
James H. MacMahan
, and
Robert L. Grenzeback

Abstract

Wave spectra and directional moment measurements are of scientific and engineering interest and are routinely estimated with wave buoys. Recently, both fixed-location and uncrewed aircraft system (UAS)-mounted lidar have estimated surfzone wave spectra. However, nearshore wave statistics seaward of the surfzone have not been measured with lidar due to low return number, and nearshore directional moments have not been measured at all. We use a multibeam scanning lidar mounted on a gasoline-powered UAS to estimate wave spectra, wave slope spectra, and directional moments on the inner shelf in ≈10-m water depth from an 11-min hover and compare to a collocated wave buoy. Lidar returns within circular sampling regions with varying radius R are fit to a plane and a 2D parabola, providing sea surface and slope time series. Wave spectra across the sea–swell (0.04–0.4 Hz) band are robustly estimated for R ≥ 0.8 m. Estimating slope spectra is more challenging. Large R works well in the swell band, and smaller R works well at higher frequencies, in good agreement with a wave buoy inferred slope spectrum. Directional Fourier coefficients, estimated from wave and slope spectra and cross-spectra, are compared to a wave buoy in the sea–swell band. Larger R and the 2D parabola-fit yield better comparison to the wave buoy. Mean wave angles and directional spreads, functions of the directional Fourier coefficients, are well reproduced at R = 2.4 m and the 2D parabola-fit, within the uncertainties of the wave buoy. The internal consistency of the UAS-lidar-derived results and their good comparison to the Spotter wave buoy demonstrate the effectiveness of this tool for estimating wave statistics.

Significance Statement

Previously fixed-location or hovering lidar has been used to estimate wave spectra in the surf and swash zone where lidar returns are high due to the reflectance of foam. We present a methodology to accurately estimate wave spectra and directional properties on the inner shelf where waves are not breaking using a hovering uncrewed aircraft system with a mounted lidar. The estimated wave spectra and directional statistics are compared well with a Spotter wave buoy, demonstrating the method’s robustness.

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Andrew L. Stewart
,
Yan Wang
,
Aviv Solodoch
,
Ru Chen
, and
James C. McWilliams

Abstract

Eastern Boundary Upwelling Systems (EBUSs) host equatorward wind-driven near-surface currents overlying poleward subsurface undercurrents. Various previous theories for these undercurrents have emphasized the role of poleward alongshore pressure gradient forces (APF). Energetic mesoscale variability may also serve to accelerate undercurrents via mesoscale stirring of the potential vorticity gradient imposed by the continental slope. However, it remains unclear whether this eddy rectification mechanism contributes substantially to driving poleward undercurrents in EBUS. This study isolates the influence of eddy rectification on undercurrents via a suite of idealized simulations forced either by alongshore winds, with or without an APF, or by randomly-generated mesoscale eddies. It is found that the simulations develop undercurrents with strengths comparable to those found in nature in both wind-forced and randomly forced experiments. Analysis of the momentum budget reveals that the along-isobath undercurrent flow is accelerated by isopycnal advective eddy momentum fluxes and the APF, and retarded by frictional drag. The undercurrent acceleration may manifest as eddy momentum fluxes or as topographic form stress depending on the coordinate system used to compute the momentum budget, which reconciles these findings with previous work that linked eddy acceleration of the undercurrent to topographic form stress. The leading-order momentum balance motivates a scaling for the strength of the undercurrent that explains most of the variance across the simulations. These findings indicate that eddy rectification is of comparable importance to the APF in driving poleward undercurrents in EBUSs, and motivate further work to diagnose this effect in high-resolution models and observations, and to parameterize it in coarse-resolution ocean/climate models.

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Daphne S. LaDue
,
David Roueche
,
Frank Lombardo
, and
Lara Mayeux

Abstract

When a tornado strikes a permanent or mobile/manufactured home, occupants are at risk of injury and death from blunt force trauma caused by debris-loaded winds and failure of the structure. Mechanisms for these failures have been studied for the past few decades and identified common weaknesses in the structural load path. Also under study in recent decades, much has been learned about how people receive and understand warnings and determine how, when, and if they will shelter in advance. Recent research, for example, shows most people do not shelter until close to impact, after seeing, hearing, or feeling the approaching tornado. To advance beyond these innovations, a new, multi-disciplinary approach was fielded in nine Southeast U.S. tornadoes between 2019 and 2022. For each tornado, 1) wind engineering assessments documented near-surface wind fields, 2) structural engineering assessments documented the primary wind load path for each structure, and 3) social science interviews captured the survivor’s narrative and asked several follow-up questions to assure key items of interest were addressed in each interview. When possible, the team was multi-disciplinary during the interview, enabling survivors to ask questions and better understand their experiences. Most survivors became aware of the approaching tornado with at least a few minutes lead time and most were able to reach a place of refuge. Most survivors recalled sensory experiences during the tornado and about half could describe direction or temporal sequences of damage. A case study of the Cookeville, Tennessee, Tornado of 3 March 2020 illustrates the power of the integrated data assessment.

Open access
Free access
Xiangzhou Song
,
Xuehan Xie
,
Yunwei Yan
, and
Shang-Ping Xie

Abstract

Based on data collected from 14 buoys in the Gulf Stream, this study examines how hourly air–sea turbulent heat fluxes vary on subdaily time scales under different boundary layer stability conditions. The annual mean magnitudes of the subdaily variations in latent and sensible heat fluxes at all stations are 40 and 15 W m−2, respectively. Under near-neutral conditions, hourly fluctuations in air–sea humidity and temperature differences are the major drivers of subdaily variations in latent and sensible heat fluxes, respectively. When the boundary layer is stable, on the other hand, wind anomalies play a dominant role in shaping the subdaily variations in latent and sensible heat fluxes. In the context of a convectively unstable boundary layer, wind anomalies exert a strong controlling influence on subdaily variations in latent heat fluxes, whereas subdaily variations in sensible heat fluxes are equally determined by air–sea temperature difference and wind anomalies. The relative contributions by all physical quantities that affect subdaily variations in turbulent heat fluxes are further documented. For near-neutral and unstable boundary layers, the subdaily contributions are O(2) and O(1) W m−2 for latent and sensible heat fluxes, respectively, and they are less than O(1) W m−2 for turbulent heat fluxes under stable conditions.

Significance Statement

High-resolution buoy observations of air–sea variables in the Gulf Stream provide the opportunity to investigate the physical factors that determine subdaily variations in air–sea turbulent heat fluxes. This study addresses two key points. First, the observed subdaily amplitudes of heat fluxes are related to various processes, including wind fields and air–sea thermal effect differences. Second, the global sea surface heat budget is known to not be in near-zero balance and it ranges from several to tens of watts per square meter. Therefore, consideration of the relatively strong influence of subdaily variability in air–sea turbulent heat fluxes could provide a new strategy for solving the global heat budget balance problem.

Open access
Jeffrey Anderson
,
Chris Riedel
,
Molly Wieringa
,
Fairuz Ishraque
,
Marlee Smith
, and
Helen Kershaw

Abstract

The uncertainty associated with many observed and modeled quantities of interest in Earth system prediction can be represented by mixed probability distributions that are neither discrete nor continuous. For instance, a forecast probability of precipitation can have a finite probability of zero precipitation, consistent with a discrete distribution. However, nonzero values are not discrete and are represented by a continuous distribution; the same is true for rainfall rate. Other examples include snow depth, sea ice concentration, amount of a tracer or the source rate of a tracer. Some Earth system model parameters may also have discrete or mixed distributions. Most ensemble data assimilation methods do not explicitly consider the possibility of mixed distributions. The Quantile Conserving Ensemble Filtering Framework (Anderson 2022, 2023) is extended to explicitly deal with discrete or mixed distributions. An example is given using bounded normal rank histogram probability distributions applied to observing system simulation experiments in a low-order tracer advection model. Analyses of tracer concentration and tracer source are shown to be improved when using the extended methods. A key feature of the resulting ensembles is that there can be ensemble members with duplicate values. An extension of the rank histogram diagnostic method to deal with potential duplicates shows that the ensemble distributions from the extended assimilation methods are more consistent with the truth.

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Jakob Boventer
,
Matteo Bramati
,
Vasileios Savvakis
,
Frank Beyrich
,
Markus Kayser
,
Andreas Platis
, and
Jens Bange

Abstract

One of the most widely used systems for wind speed and direction observations at meteorological sites is based on Doppler Wind LiDAR (DWL) technology. The wind vector derivation strategies of these instruments rely on the assumption of stationary and homogeneous horizontal wind, which is often not the case over heterogeneous terrain. This study focuses on the validation of two DWL systems, operated by the German Weather Service (DWD) and installed at the boundary layer field site Falkenberg (Lindenberg, Germany), with respect to measurements from a small, fixed-wing uncrewed aircraft system (UAS) of type MASC-3. A wind vector intercomparison at an altitude range from 100 to 500 m between DWL and UAS was performed, after a quality control of the aircraft’s data accuracy against a cup anemometer and wind vane mounted on a meteorological mast also operating at the location. Both DWL systems exhibit an overall root mean square difference in wind vector retrieval of less than 22% for wind speed and lower than 18° for wind direction. The enhancement or deterioration of these statistics is analyzed with respect to scanning height and atmospheric stability. The limitations of this type of validation approach are highlighted and accounted for in the analysis.

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Shihua Liu
,
Sihua Huang
,
Yanke Tan
,
Zhiping Wen
,
Xiaodan Chen
, and
Yuanyuan Guo

Abstract

Previous studies have pointed out that the tropical easterly jet (TEJ) core varies longitudinally or latitudinally. Whether there is a linkage between longitudinal and latitudinal variations of the TEJ core remains unclear. We found that, on the interannual time scale, the northward (southward) movement of the TEJ core is typically accompanied by a westward (eastward) shift, characterized by a noticeable northwest–southeast (NW–SE) displacement. This NW–SE shift is most evident in July. A locational index is defined to capture this shift by the difference of area-averaged 200-hPa zonal winds between the western Arabian Sea (AS) and the southern tip of the Indian Peninsula. Observations and numerical simulations demonstrated that the northwestward-shifted (southeastward-shifted) TEJ core is caused by the joint and individual influences from the enhanced (suppressed) convective activities over the eastern AS and suppressed (enhanced) convective activities over the northern Bay of Bengal–South China Sea (BOB–SCS). Enhanced (suppressed) convective activities over the eastern AS can induce upper-tropospheric divergence (convergence) and anticyclonic (cyclonic) circulations to the northwest of the convection, leading to anomalous easterly (westerly) over the western AS. The suppressed (enhanced) convective activities over the northern BOB–SCS can further facilitate the northwestward (southeastward) shift through inducing anomalous cyclonic (anticyclonic) circulation centering at the BOB and the associated anomalous westerly (easterly) over the southern tip of the Indian Peninsula. The NW–SE shift of the TEJ core may have an implication for the change in the area of the intense rainfall in South Asia.

Significance Statement

The purpose of this study is to explore the linkage between the zonal and meridional variations of the core of the tropical easterly jet (TEJ) and its underlying mechanisms. We found that the TEJ core features a pronounced northwest–southeast shift and this phenomenon only occurs in July. Thus, we defined a locational index to depict this unique characteristic and reveal its relationship with the anomalous convective activities over the eastern Arabian Sea and the northern Bay of Bengal–South China Sea. These results may help improve our understanding of the characteristics and mechanisms of the variations of the TEJ core.

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