Storm Of February Research Articles

Effects of storm events on the shelf-to-basin sediment transport in the southwestern end of the Gulf of Lions (Northwestern Mediterranean)

Abstract. Shelf-to-basin sediment transport during storms was studied at the southwestern end of the Gulf of Lions from November 2003 to March 2004. Waves, near-bottom currents, temperature and sediment fluxes were measured on the inner shelf at 28-m depth, in the Cap de Creus submarine canyon head at 300-m depth and in the northwestern Mediterranean basin at 2350-m depth. This paper is a synthesis of results published separately in different papers; it includes some new data and focusses on the subject of storms. It is the first paper in which simultaneous data about the effect of storms on the shelf, the slope and in the basin are shown together. During the winter studied, there were two severe E-SE storms with significant wave heights ≥ 7 m: one in December 2003 and one in February 2004. During these storms, coastal water was exported off-shelf producing strong near-bottom currents (up to 82 cm s−1) at the canyon head that resuspended sediment and increased the downcanyon sediment fluxes by several orders of magnitude. The suspended sediment flux increase in the canyon head was much larger during the February storm than during the December storm. At the deep basin site, particle fluxes also increased drastically (1–2 orders of magnitude) immediately after the February storm but not after the December storm. The reason was that the February storm was reinforced by dense shelf water cascading and was long enough (43 h) to transfer large amounts of resuspended sediment from shallow shelf areas to the canyon head and from there to the northwestern Mediterranean basin. Thus, in the western Gulf of Lions, severe winter E-SE storms occurring during the dense shelf water cascading period can significantly increase the transfer to deep-sea (> 2000 m) environments of shelf and slope resuspended material, including anthropogenic contaminants and organic matter.

Biological and water quality responses to hydrologic disturbances in third‐order forested streams

AbstractWe compared stream ecosystem responses to two types of disturbances: flood and debris flows. A large storm in February 1996 disturbed four similarly sized sub‐watersheds of the Calapooia River, in the Cascade Mountains of Oregon, USA. All sub‐watersheds had flood and in two, 31 and 81% of the perennial channel, a debris flow affected the channel. For 8 years, we used a suite of approaches: stream temperature, nutrient regime, periphyton and macroinvertebrate assemblages, and resident trout abundance and habitat to evaluate the persistence of instream impacts. Differences in stream temperatures and nitrate‐nitrogen concentrations were evident, probably, due to the removal of vegetation and modification of riparian soils at the debris flow sites. Instream biological responses varied. After the event, fish, including trout, were rare with no fish at the debris flow sites. Within 6 years, trout densities (Oncorhynchus mykiss and Oncorhynchus clarki) were similar and young‐of‐the‐year trouts were common. In contrast, periphyton and macroinvertebrate assemblages differed. Periphyton biomass was lower and nitrogen‐fixing periphyton was more abundant at the debris flow sites. Macroinvertebrate assemblage diversity was higher at the debris flow sites due to fewer dominant taxa. Macroinvertebrate functional feeding groups also differed with fewer gatherers and more scrapers at the debris flow sites. Debris flow impacts related to loss of riparian canopy will probably persist until mature red alder stands are re‐established along stream‐reaches affected by debris flows to provide nitrogen input and shade. Copyright © 2011 John Wiley & Sons, Ltd.

Nonlinear disturbance of the dipole field by an axisymmetric plasma distribution

The results of numerical modeling of magnetic field variations with increasing pressure in the inner magnetosphere regions in the axisymmetric case with pressure isotropy are presented. The radial dependence of magnetic field disturbance for the given plasma pressure distribution is determined. The dependence of magnetic field disturbance near the Earth on the total pressure with allowance for the influence of magnetic field of the currents in the plasma on the field value and distribution is obtained. The obtained solutions are applied in an analysis of the large magnetic storm of February, 1986, during which the minimum value of the Dst variation was 307 nT. The plasma pressure values experimentally measured near the equatorial plane were used. It is shown that, with allowance for nonlinear effects, the value of observed Dst variation corresponds to the disturbance caused by the axisymmetric part of a ring current.

Modelling the sea ice in the Nares Strait

A three dimensional coupled ocean (HYCOM) and sea ice model (CICE) is applied to a regional setup of the Lincoln Sea, the Nares Strait, and the Baffin Bay. As the sea ice model is originally developed for global simulations, boundary conditions have been implemented for the regional setup. The model results are compared with satellite images and with the large scale simulation that is specified for the boundary conditions. The main focus in this paper is on the simulated variations in the modeled sea ice flux and oceanic volume flux through the Nares Strait in the period September 2005–August 2008. The total sea ice area flux in 2006 is 14 × 10 3 km 2/year and in 2007 it is 69 × 10 3 km 2/year. The reason for this difference is mainly the blocking of the ice flow in the Nares Strait in spring 2006. The corresponding volume fluxes are 20 km 3/year and 120 km 3/year. The average annual oceanic volume flux varies from 0.6 Sv to 1.3 Sv from 2006 to 2008. The freshwater flux with a reference salinity of 34.8 varies in the same period from 12 mSv to 29 mSv. The opening and closing of the North Water, which is one of the largest polynyas in the world, are investigated. A February storm event with opening and refreezing of the North Water is well reproduced. The model results show that the net heat flux in the polynya area at the surface is positive into the ocean from May until the end of July, and hence while the wind is responsible for opening the polynya, the main mechanism for maintaining the polynya in late spring and early summer is the surface heat flux.

Natural versus anthropogenic sources of channel sand and fine gravel following integrated logging in the Letts Creek catchment, NSW

Summary Integrated logging immediately before a large storm in February 1992 caused extensive erosion of a feeder road in compartment 588/2 of Yambulla State Forest that allegedly supplied large amounts of sand and fine gravel to the downstream river channel where pools were supposedly infilled. However, most sediment eroded on the feeder road was actually stored on slopes and in filter strips before reaching channels. Subsequent soil conservation works rehabilitated the worst-eroded areas and they are still functioning effectively today. There is no direct connection of sand and fine gravel between the general harvest area and downstream channels because of many intervening sediment discontinuities, such as ponds, drainage lines, filter strips and floodouts1. The catastrophic flood of February 1971 was the largest for at least the last 50 y in the Towamba catchment and caused substantial widening of lower Letts Creek and Wog Wog River. It also reorganised channel-spanning boulder steps on upper Letts Creek and a tributary in and immediately downstream of compartment 588/2, and may have caused gully erosion on the tributary of Letts Creek. Bank erosion supplied the sand and fine gravel in the river bed of these headwater channels. A cluster of large floods between 1988 and 1992 exacerbated bank erosion in the two channels draining the logged compartment and infilled pools.

Geomorphic Work during a “150-Year” Storm: Contrasting Behaviors of River Channels in a New Zealand Catchment

The geomorphic work accomplished during an extreme storm event by three primary tributaries in the western Manawatu river catchment (New Zealand) is assessed using sequential aerial photographs. Areas of floodplain and channel erosion were quantified as a measure of the work accomplished during floods triggered by a “150-year” storm in February 2004. Of the three rivers studied, the smallest (Kiwitea, 225 km2) accomplished the most geomorphic work, eroding 1.1 km2 of floodplain along a 30 km long reach. Here spatially discontinuous channel transformation was associated with large-scale bank erosion in response to a flood estimated to be more than five times bigger than the mean annual flood (annual recurrence interval [ARI] ∼100 years). Total energy expenditure in the Kiwitea flood was ∼14,900 × 103 joules. The larger Pohangina (547 km2) and Oroua (329 km2) rivers were less effective, expending ∼14,400 × 103 joules and ∼5,300 × 103 joules and eroding 0.36 and 0.6 km2 of floodplain, respectively. The contrasting amount of geomorphic work between these tributaries relates to valley floor and channel configurations, which prime discrete reaches for instability, sensitizing them to perturbation by this flood event. In the Kiwitea, greatest erosion occurred where floodwaters were confined by terrace bluffs at bends that locally enhance stream powers. The wider channels of the Pohangina and Oroua Rivers wander across broader gravelly floodplains, permitting widespread dissipation of flood flows across the wider active channel and valley floor. Hydrologic, hydraulic, and geomorphic variables thus explain the variability in geomorphic work accomplished during the event. Ultimately, diverse channel behaviors reflect varied catchment and reach sensitivity to the flood event.

Auroral precipitation dynamics during strong magnetic storms

The dynamics of the auroral precipitation boundaries in the daytime (0900–1200 MLT) and nighttime (2100–2400 MLT) sectors during two strong magnetic storms of February 8–9, 1986, and March 13–14, 1989, with a Dst value at a maximum of approximately −300 and −600 nT, respectively, are studied using the DMSP satellite data. It is shown that, during the main phase of a storm, a shift to lower latitudes of the poleward and equator ward boundaries of the daytime precipitation is observed. In the nighttime sector, the equatorward boundary of the precipitation also shifts to lower latitudes, whereas the position of the poleward boundary depends weakly on the magnetic activity level even in the periods of very strong magnetic disturbances. The increase in the polar cap area occurs mainly due to the equatorward shift of the daytime precipitation. A high correlation degree between the equatorward shift of the poleward boundary of the daytime precipitation and the position of the equatorward boundary of the precipitation at the nighttime side of the Earth is demonstrated. The analysis of the events shows that (1) the magnetic activity level in the nighttime sector of the auroral zone influences considerably the position of the daytime precipitation boundaries during magnetic storms and that (2) the ring current inputs considerably into the value of the Dst variations.

Ice Damage to Trees on the Virginia Tech Campus from Ice Storms

The purpose of this study was to analyze ice damage to 228 trees of 9 species on the Virginia Tech campus. Damage was caused by three severe ice storms in February and March 1994. There were significant differences among species in amount of damage. Four ways of expressing percent damage were compared (% individuals damaged, % basal area of damaged trees, % crown damage, and an average of the three). The average method yielded the most significant comparisons, followed by percent crown damage. The species, ranked in four groups by mean total damage, are as follows: most damage (Acer saccharum, Chamaecyparis nooktakensis, Ulmus americana, and Acer nigrum: 29.7–26.4%); less damage (Quercus alba and Platanus occidentalis: 18.4–13.7%); lesser damage (Cornus florida: 10.2%); and least damage (Quercus rubra and Quercus palustris: 6.7–0%). The differences among groups were significant at P ≤ 0.05. Ice damage also caused a significant decrease in crown growth of four species. Comparisons with other studies revealed good correspondence, in general, with two or three exceptions. Our conclusions are that three factors were chiefly responsible for the relatively severe damage to trees: 1) the severity of the ice storms; 2) the open, exposed siting of all the trees, similar to trees growing at the edge of a forest; and 3) the high percentage of large trees with internal decay and asymmetric crowns.

Sediment dynamics during wet and dry storm events on the Têt inner shelf (SW Gulf of Lions)

The importance of short-term processes, such as floods and storms, on sediment delivery and reworking on the Têt inner shelf was investigated. The Têt inner shelf is a small, event-dominated system located in the south-western part of the Gulf of Lions. The expected sedimentary scenario in this environment is that fluvial sediment should be deposited on the prodelta and later dispersed around the shelf and slope by wave and current reworking during storms. This paper investigates differences in inner shelf sediment dynamics between storm events occurring during usual river discharges and those occurring during river floods. Waves, current velocities, near-bottom water turbidity, bottom sediment grain-size and sea-floor erosion/accretion were concurrently measured at 28-m water depth on the Têt inner shelf from November 2003 to March 2004. Two major storms took place on 4 December 2003 and 21 February 2004, and two moderate storms occurred on 8 December 2003 and 14 March 2004. The two major storms displayed similar wave characteristics: maximum significant wave height ( H s) > 7 m, peak period ( T p) > 12 s and wave direction around 90°. The main environmental differences during the two major storm events correspond to the amount of sediment discharged from the Têt River. About 2 × 10 4 t of sediment was delivered by the river during the 4 December flood (wet storm) and less than 5 × 10 2 t during the 21 February storm (dry storm). Sediment dynamics were quite similar during the storm events: resuspension caused by waves and sediment advection towards the southeast due to near-bottom shelf currents were the dominant sedimentary processes. The result was a bottom sediment erosion of several centimetres at the study site during both the wet and dry events. The main differences between the wet and dry events arose after the storm. Immediately after the peak of the wet storm, sediment supplied by the Têt River (and probably from other rivers) was deposited around the river mouth. A few days later, during a moderate storm, this unconsolidated sediment was resuspended, transported offshore and deposited on the inner shelf. In contrast, a moderate storm which occurred some days after the dry February storm caused bottom erosion on the inner shelf, because no fresh sediment was available on the shallower area. The results from this study indicate that the Têt inner shelf at 28-m water depth is mainly a bypass zone for sediment that is transported S–SE towards the middle–outer shelf and slope, although ephemeral sediment deposits may be favoured by the sediment supplied from flood events. Sediment transport across the shelf, from the river to the slope, follows a complex, multi-step pattern that needs to be addressed using a multi-event approach.

Suspended sediment fluxes and transport processes in the Gulf of Lions submarine canyons. The role of storms and dense water cascading

Contemporary suspended sediment transport was studied in seven submarine canyons of the Gulf of Lions (GoL). Current meters equipped with turbidity sensors were moored 4 m above bottom at 300 m depth in the canyon axis from November 2003 to May 2004. Sediment transport events were monitored and studied in relation to forcing conditions. There was a large flood in early December, during which discharges from all of the coastal rivers increased by more than one order of magnitude. A smaller flood of the Rhone River occurred later in mid-January followed by a persistent high river discharge that lasted until mid-February. There were also several E–SE storm events during the measurement period, two of them causing large swell, one in early December (max H s: 8.4 m), coinciding with the major river flood, and one in late February (max H s: 7 m) during a period of lower river discharge. Most of the variability in down-canyon current speeds was linked to strong downwelling induced by E–SE storms and to cascading of dense shelf water induced by N and NW winds. The intensity and timing of these processes strongly varied spatially. Eastern storms generated higher waves and induced stronger downwelling in the western than in the eastern sector of the GoL. Shelf dense water cascading events were enhanced during eastern storms and they were also more intense in the western sector of the GoL. These events occurred frequently from January to May along the western canyons and from February to April along the eastern canyons. From February to April they occurred simultaneously in all the canyons. Sediment transport was mainly down-canyon and mostly concentrated during storm-induced downwelling and dense water cascading events. During the high river discharge season most of the newly-supplied and resuspended sediment remained stored on the shelf. However, during the late February storm and cascading event, the stored sediment was quickly resuspended and transported, mainly through the westernmost submarine canyon (Cap de Creus), following a flushing pattern. Later, although minor storms and shelf dense water cascading hardly increased suspended sediment concentrations within the canyons, they induced strong near-bottom current velocities, increasing down-canyon sediment fluxes and generating small but significant sediment transport events. Most of the shelf-canyon transfer took place during the late February storm and cascading event through the Cap de Creus canyon where the net suspended sediment flux was between one and two orders of magnitude higher than in the other canyons

A Case Study of Digital Elevation Model-Based Slope Stability Assessment on Managed Forestland in the Oregon Coast Range

Abstract An aerial photo-based inventory of landslides on recently harvested and reforested land after a significant landslide-producing storm in February 1996, was compared with a digital elevation model-based assessment of slope stability (shallow landsliding stability model [SHALSTAB]) for Confederated Tribes of Siletz Indians (CTSI) and surrounding forestland. The SHALSTAB predictions of landslide locations did not correlate well with the locations of observed landslides. Eighty-nine percent of the landslides on the more stable landform in the southern portion of the CTSI ownership occurred on land that SHALSTAB indicated to be at a low risk of landsliding. Seventy-two percent of the landslides on the less stable landform to the north occurred on land that SHALSTAB indicated to be at a low risk of landsliding. Conversely, only 11 and 28%, respectively, of the observed landslides occurred on lands predicted to be “chronically unstable” or at “high risk” by SHALSTAB. This level of correct prediction of landsliding was judged to be unacceptable for SHALSTAB to be used for slope stability assessment as a part of forest management planning. West. J. Appl. For. 21(4):195–202.

Airglow and other F-layer variations in the Indian sector during the geomagnetic storm of February 5–7, 2000

Abstract A CCD based all-sky imaging system was used to monitor various nightglow emissions (OI 630-nm, OI 557.7- nm, OI 777.4-nm, Na (589.3-nm) and hydroxyl (OH) airglow) at a low latitude station, Kolhapur (16.8°N, 74.2°E, dip lat 10.6°N) in India to study the characteristics of night airglow variations observed during the period of a moderate/weak geomagnetic storm with SSC (Storm Sudden Commencement) commencing at 15:42 UT (21:12 IST (Indian Standard Time = UT + 5.5 hrs)) on February 5, 2000. The images on the night of February 6 show the development of strong ionization anomaly (EIA) with bright intensity regions in OI 630-nm and the signature of rising bubbles with very low intensity. Though the signature of ionospheric plasma bubbles were not observed on the night of February 7, the OI 630-nm images showed the presence of large scale enhanced airglow moving to the southeast direction. The speed was significantly fast (≈300 m/s). There were bright intensity regions also observed in OI 557.7-nm airglow, but no intensity enhancement was seen in other mesospheric emissions (Na (589.3-nm) and hydroxyl (OH) airglow ) during this magnetic disturbance. The ionosonde observations at the nearby station, Visakhapatnam (lat. 17.67°N, long. 83.32°E) also showed enhancement in electron density parameter ((foF2)2) at the station on each night compared to the night of February 4–5 (quiet day) around the same time interval maximizing around 23:00 IST on the night of February 7. The Total Electron Content (TEC) fluctuations obtained from GPS phase delays between two L-band signals (L1 = 1575 MHz and L2 = 1227 MHz) showed no enhancement at night at four mid latitude stations in Japan. However, noontime TEC values were enhanced on February 6 and 7 with maximum intensity occurring on February 7. It is inferred that enhanced storm time effects in the ionosphere were mainly confined to the low latitude region, the effects were subdued in the mid latitude stations.

Validation of a region-wide model of landslide susceptibility in the Manawatu–Wanganui region of New Zealand

Since European settlement 160 years ago, much of the indigenous forest in New Zealand hill country has been cleared for pastoral agriculture, resulting in increased erosion and sedimentation. To prioritise soil conservation work in the Manawatu–Wanganui region, we developed a model of landslide susceptibility. It assigns high susceptibility to steep land not protected by woody vegetation and low susceptibility everywhere else, following the commonly used approach for identifying inappropriate land use. A major storm on 15–16 February 2004 that produced many landslides was used to validate the model. The model predicted hills at risk to landsliding with moderate accuracy: 58% of erosion scars in the February storm occurred on hillsides considered to be susceptible. The model concept of slope thresholds, above which the probability of landsliding is high and below which the probability is low, is not adequate because below 30° the probability of landsliding is approximately linearly related to slope. Thus, reforestation of steep slopes will need to be combined with improved vegetation management for soil conservation on moderate slopes to significantly reduce future landsliding.

Forecasting for a Remote Island: A Class Exercise

Students enrolled in a satellite meteorology course at North Carolina State University, Raleigh, recently had an unusual opportunity to apply their forecast skills to predict wind and weather conditions for a remote site in the Southern Hemisphere. For about 40 days starting in early February 2001, students used satellite and model guidance to develop forecasts to support a research team stationed on Bouvet Island (54°26′S, 3°24′E). Internet products together with current output from NCEP's Aviation (AVN) model supported the activity. Wind forecasts were of particular interest to the Bouvet team because violent winds often developed unexpectedly and posed a safety hazard. Results were encouraging in that 24-h wind speed forecasts showed reasonable reliability over a wide range of wind speeds. Forecasts for 48 h showed only marginal skill, however. Two critical events were well forecasted—the major February storm with wind speeds of over 120 kt and a brief calm period following several days of strong winds in early March. The latter forecast proved instrumental in recovering the research team.

Storm‐time distortion of the inner magnetosphere: How severe can it get?

First results are presented of an effort to model the storm‐time distortion of the magnetic field in the inner magnetosphere using space magnetometer data. Strong geomagnetic storms are relatively rare events, represented by only a small fraction of the data used in the derivation of existing empirical geomagnetic field models. Hence using those models for the mapping of the storm‐time magnetosphere is at most an extrapolation based on trends, obtained from quiet and moderately disturbed data. To overcome that limitation, a set of data was created, containing only clear‐cut events with Dst ≤ −65 nT, with the goal to derive models of the inner and near geomagnetic field (R < 15 RE), representing strongly disturbed geomagnetic configurations and their evolution during the storm cycle. The final data set included about 143,000 records with 5‐min average B‐vectors, covering 37 major storms between 1996 and 2000. Most of the data came from GOES‐8, ‐9, ‐10, Polar, and Geotail spacecraft, and two storms in February–March of 1998 were also partially covered by the data of Equator‐S. In all cases, only those storms were selected for which concurrent solar wind and IMF data were available for the entire duration of the event. Interplanetary medium data were provided by Wind, ACE, and, to a lesser extent, by IMP 8 and Geotail. The inner magnetospheric field was represented using the newly developed T01 model [Tsyganenko, 2002a, 2002b], with a duskside partial ring current with variable amplitude and scale size, an essential part of the storm‐time current system. The modeling revealed an enormous distortion and dawn‐dusk asymmetry of the inner magnetosphere during the peak of the storm main phase, caused by the combined effect of the symmetric and partial ring currents, cross‐tail current, and Birkeland currents. We found that during storms with Dst < −250 nT the tail‐like deformation of the nightside field penetrates so close to Earth that the quasi‐dipolar approximation breaks down at distances as small as 3–4 RE. This finding yields a quantitative answer to the question of why the auroras expand to unusually low latitudes during extremely strong storms. It also may provide a natural explanation for the observed impulsive injections and energizations of charged particles on the innermost L‐shells. Finally, it questions the validity of using the dipole or quasi‐dipole approximation in numerical simulations of severe storms in the inner magnetosphere.

Piezometric response in shallow bedrock at CB1: Implications for runoff generation and landsliding

Experimental observations comparing two steep unchanneled valleys in the Oregon Coast Range, one intensively instrumented (CB1) and the other monitored for runoff but which produced a debris flow (CB2), shed light on the mechanisms of shallow flow in bedrock, its interaction with the vadose zone, and its role in generating landslides. Previous work at CB1 led to the proposal that during storms pulses of rainfall transmit pressure waves through the vadose zone and down to the saturated zone to create rapid pore pressure response and runoff [Torres et al., 1998]. Here, we document the associated rapid pore pressure response in the shallow fractured bedrock that underlies these colluvium‐mantled sites and examine its influence on the generation of storm flow, seasonal variations in base flow, and slope stability in the overlying colluvial soil. Our observations document rapid piezometric response in the shallow bedrock and a substantial contribution of shallow fracture flow to both storm flow and seasonal variations in base flow. Saturated hydraulic conductivity in the colluvial soil decreases with depth below the ground surface, but the conductivity of the near‐surface bedrock displays no depth dependence and varies over five orders of magnitude. Analysis of runoff intensity and duration in a series of storms that did and did not trigger debris flows in the surrounding area shows that the landslide inducing storms had the greatest intensity over durations similar to those predicted by a simple model of piezometric response. During a monitored storm in February 1992, the channel head at the base of the neighboring CB2 site failed as a debris flow. Automated piezometric measurements document that the CB2 debris flow initiated several hours after peak discharge, coincident with localized development of upward spikes of pressure head from near‐surface bedrock into the overlying colluvial soil in CB1. Artesian flow observed exfiltrating from bedrock fractures on the failure surfaces at CB2 further implicates bedrock fracture flow in both runoff generation by subsurface storm flow and suggests a connection to landslide initiation. Our observations show that the timing of shallow landslide initiation can be delayed relative to both peak rainfall and peak runoff and support the argument that the influence of fracture flow on shallow landsliding helps explain the wide variability in the occurrence of slope instability in topographically analogous locations.

Statistical Prediction of Seasonal East Coast Winter Storm Frequency

A statistical method for predicting seasonal winter storm activity is developed using discriminant analysis. A large pool of plausible predictors is initially considered, which includes sea surface temperature anomalies from the Niño-3.4 region, the tropical Atlantic, and adjacent to the southeastern U.S. coast; land temperature over the southeastern United States; and values of the Southern, North Atlantic, and quasi-biennial oscillation indices. These values are screened using a chi-squared procedure to isolate the most relevant variables and averaging periods. A reduced pool of predictors is then used to develop a series of discriminant functions relating the variables to East Coast winter storm activity categories (e.g., above or below normal). The selection and addition of variables to the discriminant functions is based upon the cross-validated skill score. In all cases, the discriminant forecasting procedure exhibits a high degree of skill relative to a random reference forecast that is constrained to follow the unconditional climatological distribution of storms. For December through February (DJF) storm frequency, only 4 out of 46 seasons are misclassified among three overlapping activity categories (i.e., ≤5, 5–8, and ≥8 storms). For discrete boundaries (≤5, 6–7, and ≥8 storms), about a third of the seasons are misclassified with average seasons being the most problematic. In both cases, two-category errors occur only twice. Gulf sea surface temperatures during the previous storm season is the strongest discriminator of DJF storm activity, with warmer than normal Gulf temperatures associated with active seasons. It is shown that this variable is related to both El Niño and the North Atlantic Oscillation (NAO) phase, as heightened storm activity tends to occur during the positive phase of the NAO and El Niño conditions. Over the period from October–April, forecast skill is weaker, but still high. Using a two-category forecast with overlapping boundaries (≤12 and ≥11 storms), 84% of the active seasons and 71% of the nonactive seasons are predicted. For this longer season, sea surface temperature off the southeast U.S. coast during the preceding summer is the best discriminator. Again warmer than normal temperatures are associated with active storm seasons.

Imaging observations of midlatitude ionospheric disturbances during the geomagnetic storm of February 12, 2000

Regular all‐sky imaging observations of the F region OI 630 nm nightglow emission are carried out at Rikubetsu (43.5°N) and Shigaraki (34.8°N), Japan since 1998. In this paper, we present observations of the OI 630 nm emission images from these two sites with a cluster of other observations during the geomagnetic storm of February 12, 2000. The airglow observations were obtained between 1500 UT and 2000 UT (0000–0500 LT) on this night. Images from both Rikubetsu and Shigaraki show the presence of mesoscale‐enhanced airglow bands moving slowly (∼20–30 m s−1) to the southwest direction. The mesoscale band structures were also detected by an extensive network of GPS signal observations over Japan. The airglow observations at Shigaraki also show intense OI 630 nm emission in the south with several small‐scale intensity depletion structures. We suggest that these small‐scale intensity depletion structures (scale size 30–50 km) are caused by possible nonlinear interaction between the mesoscale TIDs and enhanced regions of the equatorial ionospheric anomaly. The ionosonde observations from Wakkanai (45.4°N) and Kokubunji (35.7°N) show spread F signatures between 1130 and 2100 UT and 1215 and 1715 UT, respectively, whereas no spread F was observed at Yamagawa (31.2°N) and Okinawa (26.3°N). The phase fluctuations of GPS signal were observed at latitudes higher than 30°N for 1400–1900 UT. These observations suggest enhanced storm time ionospheric disturbances were confined in the midlatitudes.

The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range

Decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes. Forests, however, have variations in vegetation species and age which can dominate the local stability of landslide-initiation sites. To assess the influence of this variability on root cohesion we examined scarps of landslides triggered during large storms in February and November of 1996 in the Oregon Coast Range and hand-dug soil pits on stable ground. At 41 sites we estimated the cohesive reinforcement to soil due to roots by determining the tensile strength, species, depth, orientation, relative health, and the density of roots [Formula: see text]1 mm in diameter within a measured soil area. We found that median lateral root cohesion ranges from 6.8–23.2 kPa in industrial forests with significant understory and deciduous vegetation to 25.6–94.3 kPa in natural forests dominated by coniferous vegetation. Lateral root cohesion in clearcuts is uniformly [Formula: see text]10 kPa. Some 100-year-old industrial forests have species compositions, lateral root cohesion, and root diameters that more closely resemble 10-year-old clearcuts than natural forests. As such, the influence of root cohesion variability on landslide susceptibility cannot be determined solely from broad age classifications or extrapolated from the presence of one species of vegetation. Furthermore, the anthropogenic disturbance legacy modifies root cohesion for at least a century and should be considered when comparing contemporary landslide rates from industrial forests with geologic background rates.Key words: root strength, cohesion, landslide, debris flow, land use, anthropogenic disturbance.

Quantifying contributions to storm runoff through end‐member mixing analysis and hydrologic measurements at the Panola Mountain Research Watershed (Georgia, USA)

AbstractThe geographic sources and hydrologic flow paths of stormflow in small catchments are not well understood because of limitations in sampling methods and insufficient resolution of potential end members. To address these limitations, an extensive hydrologic dataset was collected at a 10 ha catchment at Panola Mountain Research Watershed near Atlanta, GA, to quantify the contribution of three geographic sources of stormflow. Samples of stream water, runoff from an outcrop, and hillslope subsurface stormflow were collected during two rainstorms in the winter of 1996, and an end‐member mixing analysis model that included five solutes was developed. Runoff from the outcrop, which occupies about one‐third of the catchment area, contributed 50–55% of the peak streamflow during the 2 February rainstorm, and 80–85% of the peak streamflow during the 6–7 March rainstorm; it also contributed about 50% to total streamflow during the dry winter conditions that preceded the 6–7 March storm. Riparian groundwater runoff was the largest component of stream runoff (80–100%) early during rising streamflow and throughout stream recession, and contributed about 50% to total stream runoff during the 2 February storm, which was preceded by wet winter conditions. Hillslope runoff contributed 25–30% to peak stream runoff and 15–18% to total stream runoff during both storms. The temporal response of the three runoff components showed general agreement with hydrologic measurements from the catchment during each storm. Estimates of recharge from the outcrop to the riparian aquifer that were independent of model calculations indicated that storage in the riparian aquifer could account for the volume of rain that fell on the outcrop but did not contribute to stream runoff. The results of this study generally indicate that improvements in the ability of mixing models to describe the hydrologic response accurately in forested catchments may depend on better identification, and detailed spatial and temporal characterization of the mobile waters from the principal hydrologic source areas that contribute to stream runoff. Copyright © 2001 John Wiley & Sons, Ltd.