Surface Wind Strength Research Articles

Surface wind strength and sea surface temperature connections along the south peruvian coast during the last 150 years

Surface wind strength and sea surface temperature connections along the south peruvian coast during the last 150 years

Simulations of Severe Convective Systems Using 1- versus 3-km Grid Spacing

Abstract Herein, 14 severe quasi-linear convective systems (QLCS) covering a wide range of geographical locations and environmental conditions are simulated for both 1- and 3-km horizontal grid resolutions, to further clarify their comparative capabilities in representing convective system features associated with severe weather production. Emphasis is placed on validating the simulated reflectivity structures, cold pool strength, mesoscale vortex characteristics, and surface wind strength. As to the overall reflectivity characteristics, the basic leading-line trailing stratiform structure was often better defined at 1 versus 3 km, but both resolutions were capable of producing bow echo and line echo wave pattern type features. Cold pool characteristics for both the 1- and 3-km simulations were also well replicated for the differing environments, with the 1-km cold pools slightly colder and often a bit larger. Both resolutions captured the larger mesoscale vortices, such as line-end or bookend vortices, but smaller, leading-line mesoscale updraft vortices, that often promote QLCS tornadogenesis, were largely absent in the 3-km simulations. Finally, while maximum surface winds were only marginally well predicted for both resolutions, the simulations were able to reasonably differentiate the relative contributions of the cold pool versus mesoscale vortices. The present results suggest that while many QLCS characteristics can be reasonably represented at a grid scale of 3 km, some of the more detailed structures, such as overall reflectivity characteristics and the smaller leading-line mesoscale vortices would likely benefit from the finer 1-km grid spacing. Significance Statement High-resolution model forecasts using 3-km grid spacing have proven to offer significant forecast guidance enhancements for severe convective weather. However, it is unclear whether additional enhancements can be obtained by decreasing grid spacings further to 1 km. Herein, we compare forecasts of severe quasi-linear convective systems (QLCS) simulated using 1- versus 3-km grids to document the potential value added of such increases in grid resolutions. It is shown that some significant improvements can be obtained in the representation of many QLCS features, especially as regards reflectivity structure and in the development of small, leading-line mesoscale vortices that can contribute to both severe surface wind and tornado production.

Pollen evidence of variations in Holocene climate and Southern Hemisphere Westerly Wind strength on sub-Antarctic South Georgia

The Southern Hemisphere Westerlies (SHW) play a major role in the global climate system. The winds drive ocean circulation and affect the Southern Oceans’ ability to take up atmospheric CO2. Recently, the SHW core belt has strengthened and shifted south, but there is an insufficient understanding of its long-term behaviour. Palaeoclimatic records are key for capturing long-term variability through the SHW’s effect on surface temperature and moisture availability. However, terrestrial records are sparse in the Southern Hemisphere. We use a palynological record from Lake Diamond on sub-Antarctic South Georgia to provide reconstructions of vegetation and climate for the last ~10,000 years. Influx of long-distance transported pollen is used as a measure of surface wind strength. Changes in relative pollen abundance of native taxa occupying either upland (cold) or lowland (warm) environments indicate local climatic variability. On South Georgia, we find long-distance transported pollen from South American taxa, mainly Nothofagus and Ephedra. They show a general increase in abundance throughout the Holocene, with peak influxes between 5700–5400, 2800–1500 and 1000–500 cal yr BP. These intervals coincide with colder periods inferred from the palynological record, suggesting that SHW variation and temperature on South Georgia are highly connected. Agreement with palaeoecological records from eastern Patagonia show that climatic changes have been regionally consistent. The record from Lake Diamond further illustrates the importance of remote islands in contributing to a deeper understanding of atmospheric circulation and climatic variability in the sub-Antarctic.

An Assessment of Representation of Oceanic Mesoscale Eddy‐Atmosphere Interaction in the Current Generation of General Circulation Models and Reanalyses

AbstractOceanic mesoscale eddies interact strongly with the atmosphere, inducing heat flux that acts to dissipate the eddy potential energy. So far it remains unknown how well this oceanic mesoscale eddy‐atmosphere (OMEA) interaction is represented in the current generation of general circulation models. Here we evaluate the intensity of OMEA interaction in numerical models widely used by the community. It is found that the intensity of OMEA interaction differs significantly among models in its overall magnitude and spatial distribution. In eddy‐rich regions such as Kuroshio Extension and Antarctic Circumpolar Current, the intermodel difference can reach 40%. Surface wind strength and marine atmospheric boundary layer adjustment to mesoscale heat flux anomaly are two important factors accounting for the intermodel difference. Models with stronger surface wind tend to have higher OMEA interaction. Moreover, neglecting the marine atmospheric boundary layer adjustment, ocean‐alone model simulations overestimate OMEA interaction especially at middle and high latitudes by 20%–50%.

Winter storm intensity, hazards, and property losses in the New York tristate area.

Winter storms pose numerous hazards to the Northeast United States, including rain, snow, strong wind, and flooding. These hazards can cause millions of dollars in damages from one storm alone. This study investigates meteorological intensity and impacts of winter storms from 2001 to 2014 on coastal counties in Connecticut, New Jersey, and New York and underscores the consequences of winter storms. The study selected 70 winter storms on the basis of station observations of surface wind strength, heavy precipitation, high storm tide, and snow extremes. Storm rankings differed between measures, suggesting that intensity is not easily defined with a single metric. Several storms fell into two or more categories (multiple-category storms). Following storm selection, property damages were examined to determine which types lead to high losses. The analysis of hazards (or events) and associated damages using the Storm Events Database of the National Centers for Environmental Information indicates that multiple-category storms were responsible for a greater portion of the damage. Flooding was responsible for the highest losses, but no discernible connection exists between the number of storms that afflict a county and the damage it faces. These results imply that losses may rely more on the incidence of specific hazards, infrastructure types, and property values, which vary throughout the region.

Aeolian deposits at the southeastern margin of the Tengger Desert (China): Implications for surface wind strength in the Asian dust source area over the past 20,000 years

Independent dust provenance data play an important role in understanding changes in the atmospheric dust loading over geological time. Major elemental compositions and grain-size characteristics of the aeolian deposits (Xiangshan (XS) section) at the southeastern margin of the Tengger Desert in northwestern China demonstrate that the deposits are an intermediate product of the transportation of aeolian materials from the dust source area (deserts) to the dust depositional regions (Chinese Loess Plateau) that were likely formed by dust storm processes. The > 40 μm and 63–178 μm fractions and the ratio of SiO 2/TiO 2 of the XS section were used as proxies to reconstruct the surface wind regime over the last 20 kyr. The results show that the surface winds were much stronger and fluctuate acutely from 19.1 to 11.4 kyr. The winds gradually attenuated after the beginning of the Holocene and reached a minimum between 4.0 and 3.5 kyr BP. A sharp increase in surface winds occurred after 2.5 kyr BP. The surface winds in the Asian dust source area are largely consistent with the changes in winter monsoon strength documented by the Chinese loess deposits. Also, the stronger surface winds from 19.1 to 11.4 kyr coincide with the high mineral dust concentration determined from the Greenland ice core, except for the period of Bölling–Allerød (B/A), when the atmospheric dust may have been washed out by enhanced global hydrological cycles although dust was still emitted from source areas. This suggests that the stronger surface winds in dust source areas can enhance the atmospheric dust loadings during the late glacial. The weakening surface wind regime during the Holocene may have greatly contributed to the decrease in dust concentration of the remote dust records. The paleo-environmental changes in the study area since 42.5 kyr BP are broadly in agreement with other adjacent records. However, the possible warm and humid spell of 42.5–19.1 kyr BP still requires verification.

Atmospheric controls on the annual cycle of North African dust

Dust emitted from desert regions and transported in the atmosphere has been recognized for its potential to alter the Earth's climate and environments. Satellite data show that the largest source regions of dust (i.e., hot spots) are located in dry, nonvegetated areas of West Africa and central Chad. Dust emissions from these sources follow a distinct seasonal cycle. Whereas our understanding of processes controlling the dust cycle of the Chad dust source has been improved through recent studies, our understanding of the West African sources is limited because of the remoteness of the sources and lack of surface observations. Using a satellite‐derived dust index and reanalysis atmospheric fields, we show that the annual dust cycle at the West African dust hot spots is not related to changes in mean surface wind strength but is linked to small‐scale high‐wind events. We find that the annual dust cycle correlates well with changes in near‐surface convergence associated with the annual north‐south movement of the Inter‐Tropical Convergence Zone (ITCZ). Dust emissions in West Africa are highest in June coinciding with the crossing of the convergence zone on its northward bound over the dust hot spots. The increase in convergence leads to enhanced surface gustiness suggesting that dry convection associated with an increase in the occurrence of small‐scale high‐wind events and vertical velocity are the main processes controlling the annual dust cycle at the West African dust sources.

High‐wind‐speed evaluation in the Southern Ocean

Space‐based scatterometer instruments provide crucial surface wind measurements with high resolution over global oceans. Midlatitude regions in the Southern Ocean are unique places to evaluate scatterometer winds at high‐wind bands because these regions host the strongest wind fields at the ocean surface. The objective of this study is to evaluate high wind speeds observed by Quick Scatterometer (QuikSCAT) wind measurements and produced by simulation models and compare them with weather station data in the Southern Ocean. The occurrence and intensity of high‐wind events in scatterometer measurements are compared with that of reanalysis winds, and the spatial and seasonal variability of high‐wind characteristics is examined. The results show that the speeds of scatterometer winds are similar to model simulations in the monthly mean field but consistently stronger than both European Centre for Medium‐Range Weather Forecasts and National Centers for Environmental Prediction/National Center for Atmospheric Research winds in high‐wind bands. When scatterometer winds are compared with the weather station observations at Macquarie Island, the present study finds no systematic bias at high‐wind bands across all months. However, both weather station and QuikSCAT winds are higher than the model simulations in high‐wind bands most of the time. This suggests that model simulations may underestimate surface wind strength in high‐wind bands. Such underestimation would lead to up to an 80% reduction in energy flux between the atmosphere and ocean. Even though high winds occur only sporadically and the reanalysis underestimation in high wind speed is not in itself of great magnitude, they have a significant impact on global climate.

Dust deposition and provenance at the Last Glacial Maximum and present day

Ice and ocean core data suggest an increase in atmospheric dust loading at the Last Glacial Maximum, 21,000 years ago, relative to the present day. We present results from a new dust model; in contrast to previous modelling studies, we correctly simulate the observed provenance data in both Antarctica and Greenland, and obtain reasonable agreement with the dust deposition data. The observed increases are attributable to decreases in vegetation, increased surface wind strength, and increased particle lifetime (due to the drier LGM climate). The Patagonian provenance of Antarctic LGM dust is linked to a decrease in soil moisture and an extension in area of the source region, and a high transport efficiency between the two regions. The results show a high degree of interannual variability, which must be taken into account when interpreting previous modelling results.

Magnitude of climate variability during middle Pliocene warmth: a palaeoclimate modelling study

The nature and magnitude of climatic variability during the period of middle Pliocene warmth (ca 3.29–2.97 Ma) is poorly understood. We present a suite of palaeoclimate modelling experiments incorporating an advanced atmospheric general circulation model (GCM), coupled to a Q-flux ocean model for 3.29, 3.12 and 2.97 Ma BP. Astronomical solutions for the periods in question were derived from the Berger and Loutre BL2 astronomical solution. Boundary conditions, excluding sea surface temperatures (SSTs) which were predicted by the slab-ocean model, were provided from the USGS PRISM2 2°×2° digital data set. The model results indicate that little annual variation (∼0.5°C) in SSTs, relative to a ‘control’ experiment, occurred during the middle Pliocene in response to the altered orbital configurations. Annual surface air temperatures also displayed little variation. Seasonally, surface air temperatures displayed a trend of cooler temperatures during December, January and February, and warmer temperatures during June, July and August. This pattern is consistent with altered seasonality resulting from the prescribed orbital configurations. Precipitation changes follow the seasonal trend observed for surface air temperature. Compared to present-day, surface wind strength and wind stress over the North Atlantic, North Pacific and Southern Ocean remained greater in each of the Pliocene experiments. This suggests that wind-driven gyral circulation may have been consistently greater during the middle Pliocene. The trend of climatic variability predicted by the GCM for the middle Pliocene accords with geological data. However, it is unclear if the model correctly simulates the magnitude of the variation. This uncertainty is derived from, (a) the relative insensitivity of the GCM to perturbation in the imposed boundary conditions, (b) a lack of detailed time series data concerning changes to terrestrial ice cover and greenhouse gas concentrations for the middle Pliocene and (c) difficulties in representing the effects of ‘climatic history’ in snap-shot GCM experiments.

Using ocean margin density to constrain ocean circulation and surface wind strength in the past

The density structure along the ocean margins carries an integrated imprint of ocean circulation. The difference in pressure between the eastern and western margins of the ocean reflects the net meridional geostrophic transport across the ocean basin. If one assumes that the wind driven circulation is closed in the upper ocean, the wind driven component of the net geostrophic transport is equal in magnitude and opposite in direction to the Ekman transport. The remainder of the net meridional geostrophic transport is the upper branch of the thermohaline meridional overturning circulation. The meridional geostrophic transports calculated using the ocean margin density data collected during World Ocean Circulation Experiment (WOCE) reflect both the wind driven and thermohaline overturning circulations. Density along the ocean margins for times in the past can be estimated from the oxygen isotopic composition of benthic foraminifera. These ocean margin density estimates can then be used to help constrain the shear in the meridional overturning circulation, the strength of zonal surface winds over the oceans, and the resulting wind driven circulation. These data will also provide an important complement to existing paleoceanographic reconstructions of surface conditions and deep water tracer fields which are currently used to constrain models of past ocean circulation.

Sedimentologic characteristics of a fluvial to aeolian succession in Weichselian Talsand in the Emsland (F.R.G.)

A sedimentologic study of a fluvial to aeolian succession in Weichselian Talsand in the Emsland (F.R.G.) is presented here. From bottom to top two fluviatile units, an aeolian bed, an aeolian-lacustrine unit and a second windborne deposit succeed each other in the investigated exposure. The aeolian facies represent two distinct styles of deposition by wind. The lower aeolian unit with horizontal alternating bedding presumably results from both a periodic change in mean surface wind strength and the spring thawing of seasonally frozen ground. The emerged part of the North Sea basin was the principle source of sand and silt for this sediment type. The upper aeolian unit is characterised by a low dune or sand sheet topography, inclined bedding and the predominance of plane-bed lamination. It was laid down on depositional surfaces normally too dry for the formation of adhesion stratification. Surface wind speed was generally high and its distribution over the year had become more uniform than before. Furthermore, sand supply was increasingly from local sources close to the depositional site. The deposition of the aeolian-lacustrine unit is due to lateral migration of the low dunes mentioned above. During this process sand was blown into a pool of still water and then subjected to gravitational slumping. The change from fluvial to aeolian depositional regime, observable in the Talsand, is tentatively attributed to the combined effect of intensified wind activity and lowering of the groundwater table as a result of permafrost degradation.

On the Dynamics of the General Atmospheric Circulation

The annual flow of angular momentum across latitudes 30-35� effected by north and south currents lying side by side (cyclonic and anticyclonic " eddies ") appears insufficient to balance the angular momentum generated at the surface in lower latitudes. The discrepancy suggests that there must be also a mean poleward drift of air at upper tropospheric and lower stratospheric levels across the latitudes of the mean surface high pressure belt, with a counterdrift which occurs at least in part in near-surface levels. A variety of auxiliary evidence supports the existence of these flows, which are indicated as intermittent or fluctuating components with mean speed over a long period of the order of a few miles per hour and so, at upper levels, only on the margin of direct observation. The origin of the poleward component is associated with the strong anticyclonic shear to equatorward of the jet stream axis. Since these drifts are found in latitudes where the vertical wind shear is greatest, - they constitute an important agency in the transport of angular momentum from its source region in the tropics to the sink region of higher latitudes. This significant contribution is masked in studies of interchange which invoke the assumption of geostrophic wind. The " index " representing the total strength of the tropical easterly surface winds, which fluctuates considerably, relates closely to the rate of generation of angular momentum in the tropics. On the basis of the drifts referred to above, a descriptive theory of the index cycle is developed, which links the fluctuations of index with the fluctuations in strength and in latitude of the jet stream. Quantitative checks are applied to this theory wherever possible and no discrepancy appears, while the normal sequence of synoptic events agrees broadly with that ascribed to the mutual interactions of surface wind strength, jet-stream, and drift.