High-latitude Storm Research Articles

Application of a Two-Step Space-Time EOF Statistical Postprocessing Algorithm to Mitigate Sub-Seasonal 200hPa Geopotential Height Forecast Error

Abstract The time extended Empirical Orthogonal Function (EEOF) patterns of error developed in a companion paper are applied in this study for model data postprocessing. Projecting forecast anomalies from the model seasonal cycle from GEFSv12 outputs onto these EEOF patterns reconstructs the error principal components (PCs) in a manner that can be applied in realtime. When these reconstructed error anomalies are compared with original GEFSv12 200 hPa Geopotential Height (Z200) reforecast error anomalies, the projected and validation error signals tend to align temporally. When distributed globally, projection and validation error anomalies have statistically significant positive Pearson Correlation coefficients with each other at most grid points outside of the tropics. Categorical Heidke Skill Score (HSS) analysis reaffirms that by lead day 16 of a forecast, the algorithm skillfully predicts systematic errors for locations outside of the tropics. Hemispheric winter demonstrates the highest magnitude HSS values in the high latitudes, illustrating the underlying seasonality of Z200 behavior and predictability. On average, wintertime HSS values hover around 20%, with high latitude locations exceeding mean values of 40%. Comparatively summertime values generally exceed 10% improvement outside of the tropics. High latitude storm track regions, such as the North Atlantic, North Pacific, and South Pacific contain the most Z200 variance in the globe and the highest HSS values. This algorithm will be applied to realtime forecasts and additional variables in future work, with the end goal of implementation into the NOAA-CPC subseasonal forecasts.

Caracterización y evolución del sistema playa-duna de la costa mediterránea de Andalucía (España): influencia de procesos naturales y actuaciones antrópicas

En las últimas décadas, los impactos relacionados con la erosión costera en las costas del mundo han aumentado significativamente debido al actual desarrollo costero y a la ocupación turística, así como a los eventos naturales de erosión/inundación acelerados por el cambio climático. Las costas oceánicas son entornos muy dinámicos y cambiantes, ya que muestran una gran variabilidad temporal y espacial en respuesta a la acción de diferentes y complejos procesos costeros. Esta variabilidad a escala temporal interanual está relacionada con las variaciones climáticas estacionales del oleaje, debido a distribuciones temporales y espaciales de tormentas de alta latitud y tormentas/huracanes tropicales, o son resultado de eventos con un gran período de retorno, como el impacto de tormentas y tsunamis muy energéticos, elevación del nivel del mar y variaciones en el suministro de sedimentos de los ríos. Para prevenir y reducir tales impactos, los gestores deben conocer la sensibilidad de los sectores costeros naturales, que está relacionada con la energía de las olas, las características/evolución de las playas y la tendencia del nivel del mar, así como la vulnerabilidad potencial y el valor económico de los sectores urbanizados.Este trabajo muestra la evolución costera y los impactos de las estructuras costeras y la caracterización y evolución de los sistemas dunares a lo largo de la costa mediterránea de Andalucía (España). Para ello, se definieron 47 unidades a lo largo de la costa de estudio, y se cuantificó la evolución de las tasas de evolución (erosión/acreción/estabilidad) para el período 1956-2016, mediante el uso de la extensión DSAS del software ArcGIS. Como resultado, 9 unidades registraron acreción, 19 erosión y 19 estabilidad y, en cuanto al balance de superficie de playa, 17 unidades presentaron balance positivo y 28 negativo con un balance neto de -29.738,4 m2/año. El análisis de la evolución costera evidenció el impacto de las estructuras sólidas: la acreción se observó principalmente aguas arriba de los puertos y espigones y en correspondencia con los rompeolas; la erosión se detectó aguas abajo de los puertos y espigones y en correspondencia con los malecones, revestimientos y deltas de los ríos más largos; la estabilidad se observó en las playas pequeñas y en las áreas costeras estabilizadas localmente por estructuras de protección y obras de sustento. Estos resultados se utilizaron para determinar la distribución de los sectores costeros alineados con la corriente y la deriva y la dirección principal del transporte de sedimentos.En cuanto a la caracterización y evolución de los sistemas dunares, se cartografiaron diferentes tipos de sistemas dunares, así como la posición y fragmentación de la punta de las dunas, y la ocupación y evolución humana desde 1977 hasta 2001 y desde 2001 hasta 2016. En total, se delimitaron 53 sistemas dunares. a lo largo de la costa mediterránea de Andalucía, diferenciando tres tipos: dunas embrionarias y móviles, dunas fijadas por herbáceas y dunas estabilizadas. Se observa un descenso generalizado de la superficie dunar en el periodo 1977-2001 (-7,5 x 106 m2), ligado al aumento de la ocupación antrópica (+2,3 x 106 m2), y fragmentación de las dunas, especialmente en las provincias de Málaga y Almería. Durante el período 2001-2016 se observaron cambios menores en el nivel de fragmentación y en la superficie de las dunas. Solo se observó un aumento de la superficie de dunas en playas estables o en acreción (4 de 53 sistemas de dunas), tanto en áreas naturales como antrópicas (generalmente aguas arriba de los puertos).

On the decreases in North Atlantic significant wave heights from climate projections

Consistent projected decreases in significant wave heights ( $${\mathrm{H}}_{\mathrm{S}}$$ ) over the North Atlantic sub-basin under climate change scenarios have been endorsed by recent scientific literature. Here, the underlying causes of these projected decreases are investigated, by statistically characterizing the relationship between atmospheric circulation patterns, and wind generated waves. We apply a non-hierarchical partitioning method to the historical reference ERA5 mean sea level pressure ( $$\mathrm{MSLP}$$ ) over the North Atlantic, defining Weather Types (WTs) at annual and seasonal scales. The $${\mathrm{H}}_{\mathrm{S}}$$ fields related to those WTs are then characterized. Projected changes in the WTs frequencies of occurrence and related $${\mathrm{H}}_{\mathrm{S}}$$ , towards the end of twenty-first century, are analyzed for the RCP4.5 and RCP8.5 scenarios using a 10-member multi-model ensemble. Results show projected increases in the occurrence of WTs dominated by high-latitude storm tracks (above 50° N) and atmospheric blocking patterns, and projected decreases in the occurrence of WTs dominated by lower-latitude storm tracks and NAO− patterns, over the North Atlantic, consistent with generalized projected decreases in $${\mathrm{H}}_{\mathrm{S}}$$ .

Holocene Boulder Beach Eroded from Chromite and Dunite Sea Cliffs at Støypet on Leka Island (Northern Norway)

This project examines the role of high-latitude storms degrading a Holocene coast formed by igneous rocks composed of low-grade chromite ore and dunite that originated within the Earth’s crust near the upper mantle. Such rocks are dense and rarely exposed at the surface by tectonic events in the reconfiguration of old ocean basins. An unconsolidated boulder beach occupies Støypet valley on Leka Island in northern Norway, formerly an open channel 10,000 years ago when glacial ice was in retreat and rebound of the land surface was about to commence. Sea cliffs exposing a stratiform ore body dissected by fractures was subject to wave erosion that shed large cobbles and small boulders into the channel. Competing mathematical equations are applied to estimate the height of storm waves impacting the channel floor and cliffs, and the results are compared with observations on wave heights generated by recent storms striking the Norwegian coast with the intensity of an orkan (Norwegian for hurricane). Lateral size variations in beach clasts suggest that Holocene storms struck Leka Island from the southwest with wave heights between 5 and 7.5 m based on the largest beach boulders. This result compares favorably with recent high-latitude storm tracks in the Norwegian Sea and their recorded wave heights. The density of low-grade chromite ore (3.32 g/cm3) sampled from the beach deposit exceeds that of rocks like limestone or other igneous rocks such as rhyolite, andesite, and basalt taken into consideration regarding coastal boulder deposits associated with classic hurricanes in more tropical settings.

Inertial Waves in Axisymmetric Tropical Cyclones

AbstractThe heat engine model of tropical cyclones describes a thermally direct overturning circulation. Outflowing air slowly subsides as radiative cooling to space balances adiabatic warming, a process that does not consume any work. However, we show here that the lateral spread of the outflow is limited by the environmental deformation radius, which at high latitudes can be rather small. In such cases, the outflowing air is radially constrained, which limits how far downward it can subside via radiative cooling alone. Some literature has invoked the possibility of “mechanical subsidence” or “forced descent” in the storm outflow region in the presence of high inertial stability, which would be a thermally indirect circulation. Mechanical subsidence in the subsiding branch of a tropical cyclone has not before been observed or characterized. A series of axisymmetric tropical cyclone simulations at different latitudes and domain sizes is conducted to study the impact of environmental inertial stability on storm dynamics. In higher-latitude storms in large axisymmetric domains, the outflow acts as a wavemaker to excite an inertial wave at the environmental inertial (Coriolis) frequency. This inertial wave periodically ventilates the core of a high-latitude storm with its own low-entropy exhaust air. The wave response is in contrast to the presumed forced descent model, and we hypothesize that this is because inertial stability provides less resistance than buoyant stability, even in highly inertially stable environments.

Distant swells and their impacts on atolls and tropical coastlines. The example of submersions produced by lagoon water filling and flushing currents in French Polynesia during 1996 and 2011 mega swells

Abstract Distant swells produced by high latitude storms are poorly studied despite their significant impact on almost all major ocean basins. This article studies the consequences of extreme southern swells on the islands and atolls of French Polynesia by focusing on lagoon water filling. This phenomenon is a product of great water influx in the lagoon coming from waves breaking on the reef. This analysis is based on archives in Tahiti, and on surveys and interviews conducted in the Makemo Atoll (Tuamotu Archipelago) to identify the impacts of giant swells during August 2011 and July 1996. These swells have strongly affected all the archipelagos of the Western Pacific and have been strongly felt on the entire Pacific coastline of the American continent. In French Polynesia they caused a rise in the lagoons that reached 2.75 m (above the lowest astronomical tide), causing a major submersion event that strongly impacted the life of the inhabitants by destroying many resources on which they depended. Comparing the swells of 1996 and 2011, this article shows that the filling of the lagoon is largely determined by the duration of the intense wave event and not only by the maximum intensity reached by the swell. Finally, this article raises the question of how this phenomenon is taken into account in town / atoll planning, and the vulnerability of the coastlines of the intertropical zone to distant swells in the context of sea level rise and changes of atmospheric circulation.

Shallow water simulations of Saturn's giant storms at different latitudes

Shallow water simulations are used to present a unified study of three major storms on Saturn (nicknamed as Great White Spots, GWS) at different latitudes, polar (1960), equatorial (1990), and mid-latitude (2010) (Sánchez-Lavega, 2004; Sánchez-Lavega et al., 2011). In our model, the three GWS are initiated by introducing a Gaussian function pulse at the latitude of the observed phenomena with controlled horizontal size and amplitude. This function represents the convective source that has been observed to trigger the storm. A growing disturbance forms when the pulse reacts to ambient winds, expanding zonally along the latitude band of the considered domain. We then compare the modeled potential vorticity with the cloud field, adjusting the model parameters to visually get the closest aspect between simulations and observations. Simulations of the 2010 GWS (planetographic latitude ∼+40º, zonal velocity of the source ∼−30m s−1) indicate that the Coriolis forces and the wind profile structure shape the disturbance generating, as observed, a long region to the east of the convective source with a high speed peripheral anticyclonic circulation, and a long-lived anticyclonic compact vortex accompanied by strong zonal advection on the southern part of the storm forming a turbulent region. Simulations of the equatorial 1990 GWS (planetographic latitude +12º–+5º, zonal velocity of the source 365–400m s−1) show a different behavior because of the intense eastward jet, meridional shear at the equatorial region, and low latitude dynamics. A round shaped source forms as observed, with the rapid growth of a Kelvin-Helmholtz instability on the north side of the source due to advection and to the strong meridional wind shear, whereas at the storm latitude the disturbance grows and propagates eastward. The storm nucleus is the manifestation of a Rossby wave, while the eastward propagating planetary-scale disturbance is a gravity-Rossby wave trapped around the equator. The simulated 1960 GWS disturbance (planetographic latitude +56º, zonal velocity 4m s−1) formed a chain of periodic oval spots that mimic the few available observations of the phenomenon. For the mid and high latitude storms, simulations predict a strong injection of negative relative vorticity due to divergence of the upwelling storm material, which may produce large anticyclones on the anticyclonic side of the zonal profile, and a quick turbulent expansion on the background cyclonic regions. In general, simulations indicate that negative relative vorticity injected by storms determines the natural reaction to zonal winds at latitudes where Coriolis forces are dominant.

Dissipation of the energy imparted by mid-latitude storms in the Southern Ocean

Abstract. The aim of this study is to clarify the role of the Southern Ocean storms on interior mixing and meridional overturning circulation. A periodic and idealized numerical model has been designed to represent the key physical processes of a zonal portion of the Southern Ocean located between 70 and 40° S. It incorporates physical ingredients deemed essential for Southern Ocean functioning: rough topography, seasonally varying air–sea fluxes, and high-latitude storms with analytical form. The forcing strategy ensures that the time mean wind stress is the same between the different simulations, so the effect of the storms on the mean wind stress and resulting impacts on the Southern Ocean dynamics are not considered in this study. Level and distribution of mixing attributable to high-frequency winds are quantified and compared to those generated by eddy–topography interactions and dissipation of the balanced flow. Results suggest that (1) the synoptic atmospheric variability alone can generate the levels of mid-depth dissipation frequently observed in the Southern Ocean (10−10–10−9 W kg−1) and (2) the storms strengthen the overturning, primarily through enhanced mixing in the upper 300 m, whereas deeper mixing has a minor effect. The sensitivity of the results to horizontal resolution (20, 5, 2 and 1 km), vertical resolution and numerical choices is evaluated. Challenging issues concerning how numerical models are able to represent interior mixing forced by high-frequency winds are exposed and discussed, particularly in the context of the overturning circulation. Overall, submesoscale-permitting ocean modeling exhibits important delicacies owing to a lack of convergence of key components of its energetics even when reaching Δx = 1 km.

Geomorphological impacts of high-latitude storm waves on low-latitude reef islands — Observations of the December 2008 event on Nukutoa, Takuu, Papua New Guinea

Low-latitude reefs and reef islands usually experience relatively benign climatic and hydrodynamic conditions due to their location near to the equator, outside of the major storm belts, and they typically exhibit geomorphological traits that reflect the prevailing low-energy conditions. For example, algal ridges are poorly developed, reef flat boulder zones are modest or lacking, rubble banks are rare, and reef islands tend to be low and dominated by sand. Nukutoa is a low-lying triangular-shaped reef island of ~6ha located on the eastern rim of Takuu atoll (4°45′S, 157°2′E), Papua New Guinea, approximately 300km northeast of Bougainville. The approximately 450 residents of Takuu all live on Nukutoa. In December 2008 Takuu was struck by several days of very high water levels and waves, which washed completely over approximately 50% of Nukutoa. GPS shoreline mapping and topographic surveys of the island were undertaken in the days immediately prior to the event, and were repeated immediately after. Homes and village infrastructure were damaged during this event, which eroded around 60% of the shoreline, and deposited a sand sheet averaging around 50mm thick over approximately 13% of the island. This event was generated by two distant storms — one located >6000km away near 50°N, and affected a wide area of the Western Pacific. Oral histories record at least five similar events since the 1940s. In this paper we document the geomorphic impacts of the December 2008 event and discuss the possible significance of similar events in the past, and in the future.

Impacts of subpixel cloud heterogeneity on infrared thermodynamic phase assessment

[1] A combination of spatially collocated Atmospheric Infrared Sounder (AIRS) radiances and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud products are used to quantify the impact of cloud heterogeneity on AIRS-based assessments of cloud thermodynamic phase. While radiative transfer simulations have demonstrated that selected AIRS channels have greater sensitivity to cloud thermodynamic phase in comparison to the relevant MODIS bands, the relative trade-offs of spectral and spatial resolution differences that are inherent between AIRS and MODIS have not been quantified. Global distributions of AIRS field-of-view scale frequencies of clear sky (13–14%), heterogeneous cloud (26–28%), and homogeneous cloud (59–60%) are quantified for a four week time period using cloud fraction, and further categorization of cloud uniformity is assessed with the variance of cloud top temperature. Homogeneous clouds with window brightness temperatures (Tb) between 250 and 265 K are shown to have larger cloud thermodynamic phase signatures than heterogeneous clouds. Clouds in this limited Tb range occur 30–50% of the time in the mid- and high latitude storm track regions, are generally difficult to identify as being water or ice phase, and show strong responses in forced CO2 climate change modeling experiments. Two-dimensional histograms of Tb differences sensitive to cloud phase (1231–960 cm−1) and column water vapor (1231–1227 cm−1) show distinct differences between many homogeneous and heterogeneous cloud scenes. The results suggest the potential for a quantitative approach using a combination of hyperspectral sounders with high-spatial-resolution imagers, and their derived geophysical products, to assess cloud thermodynamic phase estimates within increasingly complex subpixel-scale cloud variability.

Storm Studies in the Arctic (STAR)

The Storm Studies in the Arctic (STAR) network (2007–2010) conducted a major meteorological field project from 10 October–30 November 2007 and in February 2008, focused on southern Baffin Island, Nunavut, Canada—a region that experiences intense autumn and winter storms. The STAR research program is concerned with the documentation, better understanding, and improved prediction of meteorological and related hazards in the Arctic, including their modification by local topography and land–sea ice–ocean transitions, and their effect on local communities. To optimize the applicability of STAR network science, we are also communicating with the user community (northern communities and government sectors). STAR has obtained a variety of surface-based and unique research aircraft field measurements, high-resolution modeling products, and remote sensing measurements (including Cloudsat) as part of its science strategy and has the first arctic Cloudsat validation dataset. In total, 14 research flights were flown between 5 and 30 November 2007, with eight coinciding with Cloudsat passes. The aircraft was outfitted with many instruments that measure cloud microphysical parameters and three unique Doppler-polarized airborne radars operating in Ka, X and W bands. The project area, instrumentation platforms, real-time forecasts, storm cases, and results thus far are discussed in this article. A number of synoptic and mesoscale features were sampled—such as fronts, upslope/terrain-enhanced precipitation, convective precipitation, and boundary layer clouds/precipitation—as well as targeted Cloudsat missions. One significant and unique event included a research flight into an intense high-latitude storm leftover from Hurricane Noel—an intense tropical and extratropical disturbance that caused many fatalities in the tropics and extensive damage on the eastern North American seaboard. These synoptic and mesoscale features and high-latitude storms will be studied in detail over the next several years. It is anticipated that scientific progress in better understanding the nature of these arctic storms and their hazards will lead to improved conceptual models and improved prediction of such events.

A New Perspective on Southern Hemisphere Storm Tracks

Abstract A detailed view of Southern Hemisphere storm tracks is obtained based on the application of filtered variance and modern feature-tracking techniques to a wide range of 45-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. It has been checked that the conclusions drawn in this study are valid even if data from only the satellite era are used. The emphasis of the paper is on the winter season, but results for the four seasons are also discussed. Both upper- and lower-tropospheric fields are used. The tracking analysis focuses on systems that last longer than 2 days and are mobile (move more than 1000 km). Many of the results support previous ideas about the storm tracks, but some new insights are also obtained. In the summer there is a rather circular, strong, deep high-latitude storm track. In winter the high-latitude storm track is more asymmetric with a spiral from the Atlantic and Indian Oceans in toward Antarctica and a subtropical jet–related lower-latitude storm track over the Pacific, again tending to spiral poleward. At all times of the year, maximum storm activity in the higher-latitude storm track is in the Atlantic and Indian Ocean regions. In the winter upper troposphere, the relative importance of, and interplay between, the subtropical and subpolar storm tracks is discussed. The genesis, lysis, and growth rate of lower-tropospheric winter cyclones together lead to a vivid picture of their behavior that is summarized as a set of overlapping plates, each composed of cyclone life cycles. Systems in each plate appear to feed the genesis in the next plate through downstream development in the upper-troposphere spiral storm track. In the lee of the Andes in South America, there is cyclogenesis associated with the subtropical jet and also, poleward of this, cyclogenesis largely associated with system decay on the upslope and regeneration on the downslope. The genesis and lysis of cyclones and anticyclones have a definite spatial relationship with each other and with the Andes. At 500 hPa, their relative longitudinal positions are consistent with vortex-stretching ideas for simple flow over a large-scale mountain. Cyclonic systems near Antarctica have generally spiraled in from lower latitudes. However, cyclogenesis associated with mobile cyclones occurs around the Antarctic coast with an interesting genesis maximum over the sea ice near 150°E. The South Pacific storm track emerges clearly from the tracking as a coherent deep feature spiraling from Australia to southern South America. A feature of the summer season is the genesis of eastward-moving cyclonic systems near the tropic of Capricorn off Brazil, in the central Pacific and, to a lesser extent, off Madagascar, followed by movement along the southwest flanks of the subtropical anticyclones and contribution to the “convergence zone” cloud bands seen in these regions.

Relationship between Vertical Wind Shear and Typhoon Intensity Change, and Development of Three-Predictor Intensity Prediction Model.

The relationship between vertical wind shear and tropical cyclone intensity change at each time interval of 12, 24, 36, 48, 60, and 72 h in the western North Pacific is investigated using the NCEP/NCAR reanalysis data from 1983 to 1996. As expected, regression coefficients at all time intervals are positive (storm intensity is represented by the minimum surface pressure), indicating that the vertical shear weakens storm intensity. When the total sample is stratified by latitude band, it is found that the intensity of low-latitude storms is more sensitive to vertical shear than that of high-latitude storms. This is consistent with theoretical results and observations for Atlantic storms. A minimal predictor model of predicting tropical cyclone intensity change in the western North Pacific up to 72 h is presented. The model has only three predictors (potential intensification, intensity change during previous 12 hours, and vertical shear), but the explained total variance is shown to be reasonably good in comparison to other statistical models with larger numbers of predictors. The average intensity prediction errors from the three-predictor model are reduced when the multiple linear regression method is replaced by the back-propagation neural network.