ALBERT

Description: ABSTRACT During Intensive Observation Period 13 (15–16 October 2012) of the first Special Observing Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX), Southern Italy (SI) was affected by two consecutive heavy precipitation events (HPEs). Both HPEs were associated with multi-cell V-shaped retrograde regeneration mesoscale convective systems (MCSs). The life cycle of two MCSs in connection with their dynamic and thermodynamic environments were analysed using a combination of ground-based, airborne and spaceborne observations and numerical simulations. Rain gauges revealed that heavy precipitation occurred in two phases: the first one from 1300 to 1700 UTC (35 mm h −1 ) was caused by a V-shaped system initiating over the Tyrrhenian Sea in the early morning of 15 October. Convection was triggered by the low-level convergence between the south-westerlies ahead of an upper-level trough positioned over south-eastern France and very moist southerlies from the Strait of Sicily. The convection was favoured by high convective available potential energy (1500 J kg −1 ) resulting from warm and moist conditions at low levels associated with high sea surface temperatures in the Sicily Channel. In addition, humidity at mid-level was enriched by the presence of an elevated moisture plume from tropical Africa, favouring the efficiency of the convection to produce more precipitation. The second phase of heavy precipitation (2300 UTC on 15 October to 0200 UTC on 16 October, 34 mm h −1 ) was caused by a MCS initiating over Algeria around 1300 UTC, which subsequently traveled over the Strait of Sicily toward Sicily and SI. Convection was maintained by the combination of large low-level moisture contents and a marked convergence ahead of the cold front. Unlike other MCSs forming in the same region earlier on that day, this huge V-shaped system did affect SI because the strong upper-level flow progressively veered from southwesterly to south-southwesterly.

Description: The distribution of mass density along the field lines affects the ratios of toroidal (azimuthally oscillating) Alfvén frequencies, and given the ratios of these frequencies we can get information about that distribution. Here we assume the commonlyused power law form for the field line distribution, ρ m  =  ρ m , eq ( LR E / R ) α , where ρ m , eq is the value of the mass density ρ m at the magnetic equator, L is the L shell, R E is the Earth's radius, R is the geocentric distance to a point on the field line, and α is the power law coefficient. Positive values of α indicate that ρ m increases away fromthe magnetic equator, zero value indicates that ρ m is constant along the magnetic field line, and negative α indicates that there is a local peak in ρ m at the magnetic equator. Using 12 years of observations of toroidal Alfvén frequencies by the Geostationary Operational Environmental Satellites (GOES), we study the typical dependence of inferred values of α on the magnetic local time (MLT), the phase of the solar cycle as specified bythe F10.7 extreme ultraviolet solar flux, and geomagnetic activity as specified by the auroral electrojet (AE) index. Over the mostly dayside range of the observations, we find that α decreases with respect to increasing MLT and F10.7, but increases with respect to increasing AE. We develop a formula that depends on all three parameters, α 3Dmodel  = 2.2 + 1.3 ⋅  cos (MLT ⋅ 15 ∘ ) + 0.0026 ⋅ AE ⋅  cos ((MLT − 0.8) ⋅ 15 ∘ ) + 2.1 ⋅ 10 − 5  ⋅ AE ⋅ F10.7 − 0.010 ⋅ F10.7, that models the binned values of α within a standard deviation of 0.3. While we do not yet have a complete theoretical understanding of why α should depend on these parameters in such a way, we do make some observations and speculations about the causes. At least part of the dependence is related to that of ρ m , eq ; higher α , corresponding to steeper variation with respect to MLAT, occurs when ρ m , eq is lower.

Description: Abstract An up‐to‐date map of the Antarctic Circumpolar Current (ACC) fronts is constructed from the latest version of mean dynamic topography from satellite altimetry, and reveals the narrowest ACC width in the Udintsev Fracture Zone (UFZ), with the strongest concentration of the three major ACC fronts within a limited distance as short as 170 km, about 40% narrower than that at Drake Passage. At 144°W, at the entrance of the UFZ, which lies between the Pacific‐Antarctic Ridge (PAR) and its eastwardly‐offset segment (offset PAR segment), there is a triple confluence of the Subantarctic Front (SAF), Polar Front (PF), and Southern ACC Front. Downstream of this longitude, the SAF progressively meanders northward over the relatively shallow offset PAR segment before channeling through the Eltanin Fracture Zone, thus diverging from the PF which proceeds through the UFZ. In‐situ observations from two recent cruises at 144°W confirm the satellite altimetry‐derived frontal circulation in the UFZ region, and yield a baroclinic transport relative to the bottom of 113 x 106 m3 s‐1, comparable to that through Drake Passage. The hydrographic sections show no Antarctic bottom water colder than 0.2°C. Characteristics of major water masses are described and the implications for their potential downstream modifications at Drake Passage are discussed in terms of the meridional overturning circulation across the ACC. Mesoscale eddy activity with periods shorter than 90 days is predominantly concentrated in the immediate downstream area of the offset PAR segment, suggesting a substantial poleward eddy heat flux there.

Description: Magnetospheric compression due to impact of enhanced solar wind dynamic pressure P dyn has long been considered as one of the generation mechanisms of electromagnetic ion cyclotron (EMIC) waves. With the Van Allen Probe-A observations, we identify three EMIC wave events that are triggered by P dyn enhancements under prolonged northward IMF quiet time preconditions. They are in contrast to one another in a few aspects. Event 1 occurs in the middle of continuously increasing P dyn while Van Allen Probe-A is located outside the plasmapause at post-midnight and near the equator (magnetic latitude (MLAT) ~ -3 o ). Event 2 occurs by a sharp P dyn pulse impact while Van Allen Probe-A is located inside the plasmapause in the dawn sector and rather away from the equator (MLAT ~ 12 o ). Event 3 is characterized by amplification of a pre-existing EMIC wave by a sharp P dyn pulse impact while Van Allen Probe-A is located outside the plasmapause at noon and rather away from the equator (MLAT ~ -15 o ). These three events represent various situations where EMIC waves can be triggered by P dyn increases. Several common features are also found among the three events. (i) The strongest wave is found just above the He + gyrofrequency. (ii) The waves are nearly linearly polarized with a rather oblique propagation direction (~28 o to ~39 o on average). (iii) The proton fluxes increase in immediate response to the P dyn impact, most significantly in tens of keV energy, corresponding to the proton resonant energy. (iv) The temperature anisotropy with T ⊥  〉 T || is seen in the resonant energy for all the events, although its increase by the P dyn impact is not necessarily always significant. The last two points (iii) and (iv) may imply that, in addition to the temperature anisotropy, the increase of the resonant protons must have played a critical role in triggering the EMIC waves by the enhanced P dyn impact.

Description: Surface mass change estimates from Gravity Recovery and Climate Experiment (GRACE) spherical harmonic solutions are contaminated by North-South stripe noise due largely to aliasing of high frequency variations into monthly samples. These meridional stripes are especially troubling for ice mass balance studies of the Greenland Ice Sheet (GrIS) where large ice mass variations are known to occur along North-South trending coastlines. By assuming that mass variations and noise have different patterns in both space and time over Greenland, we use Extended Empirical Orthogonal Functions (EEOF) to filter out this noise. The method is compared with a conventional approach, by examining both continent-wide estimates, and regional changes. GRACE results are compared with independent regional estimates derived from a climate model. The EEOF filter is effective at separating ice mass change signals from meridional stripe noise, with better rejection of high temporal frequency noise and less signal attenuation and spatial smoothing compared to a conventional method. We use EEOF filtered GRACE data to examine regional seasonal variations. Consistent with surface and other data, results show ice mass loss along the West, South-West and East coasts during summer and gain in these regions during winter. In addition, there is summer ice mass gain in the central region of the GrIS.

Description: The Suomi National Polar-Orbiting Partnership (S-NPP) satellite, launched in late 2011, carries the Visible Infrared Imaging Radiometer Suite (VIIRS) and several other instruments. VIIRS has similar characteristics to prior satellite sensors used for aerosol optical depth (AOD) retrieval, allowing the continuation of space-based aerosol data records. The Deep Blue algorithm has previously been applied to retrieve AOD from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) measurements over land. The SeaWiFS Deep Blue data s et also included a SeaWiFS Ocean Aerosol Retrieval (SOAR) algorithm to cover water surfaces. As part of NASA's VIIRS data processing, Deep Blue is being applied to VIIRS data over land, and SOAR has been adapted from SeaWiFS to VIIRS for use over water surfaces. This study describes SOAR as applied in version 1 of NASA's S-NPP VIIRS Deep Blue data product suite. Several advances have been made since the SeaWiFS application, as well as changes to make use of the broader spectral range of VIIRS. A preliminary validation against Maritime Aerosol Network (MAN) measurements suggests a typical uncertainty on retrieved 550nm AOD of order ±(0.03+10%), comparable to existing SeaWiFS/MODIS aerosol data products. Retrieved Ångström exponent and fine mode AOD fraction are also well-correlated with MAN data, with small biases and uncertainty similar to or better than SeaWiFS/MODIS products.

Description: ABSTRACT During Intensive Observation Period 15a (20 October 2012) of the first Special Observation Period of the Hydrological cycle in the Mediterranean Experiment, north-eastern Spain experienced heavy precipitation (130 mm in 24 h) associated with a retrograde regeneration mesoscale convective system (MCS) developing in the exit region of the Ebro River Valley (ERV). The life cycle of the MCS that brought intense hourly rainfall (34 mm) from the foothill of Iberian Plateau to the central Pyrenees, as well as the detailed structure of moist marine flow upstream, were analysed using a combination of ground-based, airborne and space-borne observations as well as model analyses. Over the Balearic Sea, the south-westerlies along the north-eastern flank of a surface low converged with south-easterlies from north Africa, creating a near surface moisture tongue in the region of the Balearic Islands, and a southeast-northwest oriented convergence line within a cloud cluster advecting from northern Africa. Airborne lidar measurements, acquired upstream of the ERV, evidenced water vapour mixing ratios in excess of 15 g kg −1 in the marine atmospheric boundary layer. In the mid-level (700 hPa), the presence of an elevated moisture plume from tropical Africa contributed for about one third to the large moisture content present over the western Mediterranean Sea. In this moist environment, the MCS was initiated over the orography of the north-eastern tip of the Iberian plateau, due to the combined influence of the approaching convergence line ahead of the surface low and the convergence resulting from weak north-westerlies channeled in the ERV and the easterlies impinging on the coastal range. After the initiation phase, the MCS further developed over the foothill of Iberian Plateau and moved into the ERV and along the southern flank of the Pyrenees, thanks to the penetration of the warm and moist maritime south-easterly flow through the narrow gap between the north-eastern part of the Iberian Plateau and the Catalan coastal range.