ALBERT

Description: Snowmelt is an important source of runoff in high mountain catchment. Snowmelt modeling for alpine regions remains challenging with scarce gauges. This study simulates the snowmelt in the Karuxung River catchment in the south Tibetan Plateau using an altitude zone based temperature-index model, calibrates the snow cover area and runoff simulation during 2003-2005, and validates the model performance via snow cover area and runoff simulation in 2006. In the snowmelt and runoff modeling, temperature and precipitation are the two most important inputs. Relevant parameters, such as critical snow fall temperature, temperature lapse rate and precipitation gradient, determine the form and amount of precipitation and distribution of temperature and precipitation in hydrological modeling of the sparsely gauged catchment. Sensitivity analyses show that accurate estimation of these parameters would greatly help in improving the snowmelt simulation accuracy, better describing the snow-hydrological behaviors, and dealing with the data scarcity at higher elevations. Specifically, correlation between the critical snow fall temperature and relative humidity and seasonal patterns of both the temperature lapse rate and the precipitation gradient should be considered in the modeling studies when precipitation form is not logged and meteorological observations are only available at low elevation. More accurate simulation of runoff involving snowmelt, glacier melt and rainfall runoff will improve our understanding of hydrological processes and help assess runoff impacts from a changing climate in high mountain catchments. This article is protected by copyright. All rights reserved.

Description: In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale (MMS) mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 d i , where d i is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island; bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma.

Description: Dipolarization fronts (DFs) are believed to play important roles in transferring plasmas, magnetic fluxes and energies in the magnetotail. Using the Cluster observations in 2003, electromagnetic energy conversion at the DFs is investigated by case and statistical studies. The case study indicates strongest energy conversion at the DF. The statistical study shows the similar features that the energy of the fields can be significantly transferred to the plasmas (load, J·E 〉0) at the DFs. These results are consistent with some recent simulations. Examining the electromagnetic fluctuations at the DFs, we suggest that the wave activities around the lower hybrid frequency may play an important role in the energy dissipation.

Description: Abstract An experimental investigation of droplet generation by a plunging breaking wave is presented. In this work, simultaneous measurements of the wave crest profile evolution and of droplets ranging in radius down to 50 μm for a mechanically generated plunging breaker during many repeated breaking events in freshwater are performed. We find three distinct time zones of droplet production, first when the jet impacts the free surface upstream of the wave crest, second when the large air bubbles entrapped by the plunging jet impact reach the free surface and burst, and third when smaller bubbles burst upon reaching the free surface later in the breaking process. These subprocesses account for 22%, 44%, and 34%, respectively, of the average of 653 droplets produced per breaking event. The probability distributions of the ranges of large and small droplet radii are well represented by power law functions that intersect at a radius of 418 μm.

Description: Whistler waves are believed to play an important role during magnetic reconnection. Here we report the near-simultaneous occurrence of two types of the whistler-mode waves in the magnetotail Hall reconnection region. The first type is observed in the magnetic pileup region of downstream and propagates away to downstream along the field lines, and is possibly generated by the electron temperature anisotropy at the magnetic equator. The second type, propagating towards the X-line, is found around the separatrix region and probably is generated by the electron beam-driven whistler instability or Čerenkov emission from electron phase-space holes. These observations of two different types of whistler waves are consistent with recent kinetic simulations, and suggest that the observed whistler waves are a consequence of magnetic reconnection.

Description: Afforestation of shelterbelts is a sustainable practice to protect highways from shifting sand dunes in desert areas. The Taklimakan Desert Highway Shelterbelt (TDHS) in China is known as “the Great Green Wall of Taklimakan Desert” and borders a 436-km distance along the highway. This study assessed the early survival, above-ground growth, and root growth of three salt- and drought-tolerant plant species ( Calligonum aborescens Litv. , Tamarix ramosissima Ledeb. , and Haloxylon ammodendron (C. A. Mey.) Bunge) in TDHS; those were drip-irrigated with local high saline groundwater. The results demonstrated that more than 80% of Haloxylon ammodendron seedlings could survive regardless of irrigation water salinities ranging from 2.82 to 29.70 g/L. In contrast, survival rates of Calligonum aborescens seedlings were greater than 65% when using irrigation water salinities ≤13.99 g/L and less than 50% when irrigated with water having salinities of 20.99–29.70 g/L, respectively. However, plant survival rates of Tamarix ramosissima were much lower than 50% when irrigated with water having salinities 〉4.82 g/L. Furthermore, under the same salinity, the height, crown width, and maximum width of basal stems were the greatest for Calligonum aborescens plants and the lowest for Tamarix ramosissima plants. Root length varied among the species depending on tree age and the applied water characteristics. We conclude that afforestation is feasible with saline water in this extreme arid shifting desert, particularly Calligonum aborescens plants should be grown at the outer margin of the shelterbelt due to its faster growth to more quickly stabilize the shifting sand. Our study may provide a good resolution for afforestation and marginal saline water utility in most arid and semiarid regions.

Description: The magnetotail current sheet is active and often flaps back and forth. Knowledge about the flapping motion of current sheet is essential to explore the related magnetotail dynamic processes, e.g. plasma instabilities. Due to the inability of single-point measurements to separate the spatial-temporal variation of magnetic field, the moving velocity of flapping current sheets cannot be revealed generally until the multi-point measurements are available, e.g. the Cluster mission. Therefore, currently the flapping behaviors are hard to be resolved only relying on single-point magnetic field analysis. In this study, with minimum variance analysis, we develop a technique based on single-point magnetic field measurement to qualitatively diagnose the flapping properties including the flapping type and the travelling direction of kink-like flapping. The comparison with Cluster multipoint analysis via several cases studies demonstrates that this technique is applicable, it should, however, be used with caution especially when the local sheet surface is either quasi-horizontal, or quasi-vertical. This technique will be useful for the planetary magnetotail exploration where no multipoint observations are available.

Description: Magnetic reconnection, as one important energy dissipation process in plasmas, has been extensively studied in the past several decades. Magnetic reconnection occurring in the downstream of a primary X-line is referred to as secondary reconnection. In this paper, we used kinetic simulations to investigate the secondary reconnection in detail. We found that secondary reconnection is reversed by the compression caused by the outflowing jet originating from the primary reconnection site, which results in the erosion of the magnetic island between the two X lines within ~3 ω ci −1 . We show the observational signatures expected in electromagnetic fields and plasma measurements in the Earth Magnetotail, associated with this mechanism. These simulation results could be applied to interpret the signatures associated with the evolution of earthward magnetic islands in the Earth's magnetotail.

Description: Global vegetation models predict rapid poleward migration of tundra and boreal forest vegetation in response to climate warming. Local plot and air-photo studies have documented recent changes in high-latitude vegetation composition and structure, consistent with warming trends. To bridge these two scales of inference, we analyzed a 24-year (1986-2010) Landsat time series in a latitudinal transect across the boreal forest-tundra biome boundary in northern Quebec province, Canada. This region has experienced rapid warming during both winter and summer months during the last forty years. Using a per-pixel (30 m) trend analysis, 30% of the observable (cloud-free) land area experienced a significant (p 〈 0.05) positive trend in the Normalized Difference Vegetation Index (NDVI). However, greening trends were not evenly split among cover types. Low shrub and graminoid tundra contributed preferentially to the greening trend, while forested areas were less likely to show significant trends in NDVI. These trends reflect increasing leaf area, rather than an increase in growing season length, because Landsat data were restricted to peak-summer conditions. The average NDVI trend (0.007/yr) corresponds to a leaf-area index (LAI) increase of ~0.6 based on the regional relationship between LAI and NDVI from the Moderate Resolution Spectroradiometer (MODIS). Across the entire transect, the area-averaged LAI increase was ~0.2 during 1986-2010. A higher area-averaged LAI change (~0.3) within the shrub-tundra portion of the transect represents a 20-60% relative increase in LAI during the last two decades. Our Landsat-based analysis subdivides the overall high-latitude greening trend into changes in peak-summer greenness by cover type. Different responses within and among shrub, graminoid, and tree-dominated cover types in this study indicate important fine-scale heterogeneity in vegetation growth. Although our findings are consistent with community shifts in low-biomass vegetation types over multi-decadal time scales, the response in tundra and forest ecosystems to recent warming was not uniform.

Description: Using a high-speed optical imaging system specifically designed for observing the lightning attachment process, we have documented the attachment process for six strokes in three natural lightning flashes. All strokes initiate at a height above ground, and propagate bi-directionally from that height, similar to the return strokes of artificially initiated (triggered) lightning previously reported by Wang at al. [2013, 2014]. Though the data are quite limited, these natural return strokes suggest a correlation between larger peak current and greater initiation height. Initiation heights determined here span 12–60 m with a typical uncertainty of less than 10 m. The initial upward return stroke luminosity speeds range from (0.8±0.2) to (2.0±0.4)×10 8 m/s. Two first return strokes downward luminosity speeds are assessed as (1.6±0.3) ×10 7 m/s and (1.4±0.3)×10 8 m/s. One of the first return strokes appeared to be initiated with a stepping pulse discharge of its leader as an inseparable part of the return stroke.