Differences between the QBO in the first and in the second half of the ERA-40 reanalysis

The representation of the quasi-biennial oscillation (QBO) is investigated in the ERA-40 reanalysis. In the lower stratosphere, where there are a reasonable number of observations, the representation of the QBO is equally good throughout the record. However, strong differences between the first and the second half of the zonal wind data set are found in the upper stratosphere, with a typical offset of –10 m/s in the equatorial zonal wind in the earlier part versus the later part of the ERA-40 record. At the same time, the strength of the QBO is similar. The 7-year running means of zonal wind, wind shear and temperature reveal interesting structures with major changes occuring in the beginning and the middle of the 1980s. Possible explanations are discussed. The identified change of the reanalysed wind profiles over time in ERA-40 demands for a careful use of equatorial upper stratospheric winds from the reanalysis e.g.~for model validation purposes

Differences between the QBO in the first and in the second half of the ERA-40 reanalysis

International audienceThe representation of the quasi-biennial oscillation (QBO) is investigated in the ERA-40 reanalysis. In the lower stratosphere, where there is a good number of observations, the representation of the QBO is equally well throughout the record. However, strong differences between the first and the second half of the zonal wind data set are found in the upper stratosphere, with a typical offset of ?10 m/s in the equatorial zonal wind in the earlier part versus the later part of the ERA-40 data set. The strength of the QBO is also affected. Possible explanations are discussed. The identified change of the assimilated wind profiles over time in ERA-40 requires a careful use of equatorial upper stratospheric winds from the reanalysis for validation purposes

Modelling snow accumulation on Greenland in Eemian, glacial inception, and modern climates in a GCM

Changing climate conditions on Greenland influence the snow accumulation rate and surface mass balance (SMB) on the ice sheet and, ultimately, its shape. This can in turn affect local climate via orography and albedo variations and, potentially, remote areas via changes in ocean circulation triggered by melt water or calving from the ice sheet. Examining these interactions in the IPSL global model requires improving the representation of snow at the ice sheet surface. In this paper, we present a new snow scheme implemented in LMDZ, the atmospheric component of the IPSL coupled model. We analyse surface climate and SMB on the Greenland ice sheet under insolation and oceanic boundary conditions for modern, but also for two different past climates, the last glacial inception (115 kyr BP) and the Eemian (126 kyr BP). While being limited by the low resolution of the general circulation model (GCM), present-day SMB is on the same order of magnitude as recent regional model findings. It is affected by a moist bias of the GCM in Western Greenland and a dry bias in the north-east. Under Eemian conditions, the SMB decreases largely, and melting affects areas in which the ice sheet surface is today at high altitude, including recent ice core drilling sites as NEEM. In contrast, glacial inception conditions lead to a higher mass balance overall due to the reduced melting in the colder summer climate. Compared to the widely applied positive degree-day (PDD) parameterization of SMB, our direct modelling results suggest a weaker sensitivity of SMB to changing climatic forcing. For the Eemian climate, our model simulations using interannually varying monthly mean forcings for the ocean surface temperature and sea ice cover lead to significantly higher SMB in southern Greenland compared to simulations forced with climatological monthly means

Sensitivity of a Greenland ice sheet model to atmospheric forcing fields

International audiencePredicting the climate for the future and how it will impact ice sheet evolution requires coupling ice sheet models with climate models. However, before we attempt to develop a realistic coupled setup, we propose, in this study, to first analyse the impact of a model simulated climate on an ice sheet. We undertake this exercise for a set of regional and global climate models. Modelled near surface air temperature and precipitation are provided as upper boundary conditions to the GRISLI (GRenoble Ice Shelf and Land Ice model) hybrid ice sheet model (ISM) in its Greenland configuration. After 20 kyrs of simulation, the resulting ice sheets highlight the differences between the climate models. While modelled ice sheet sizes are generally comparable to the observed one, there are considerable deviations among the ice sheets on regional scales. These deviations can be explained by biases in temperature and precipitation near the coast. This is especially true in the case of global models. But the deviations between the climate models are also due to the differences in the atmospheric general circulation. To account for these differences in the context of coupling ice sheet models with climate models, we conclude that appropriate downscaling methods will be needed. In some cases, systematic corrections of the climatic variables at the interface may be required to obtain realistic results for the Greenland ice sheet (GIS)

Three-Dimensional, Tomographic Super-Resolution Fluorescence Imaging of Serially Sectioned Thick Samples

Three-dimensional fluorescence imaging of thick tissue samples with near-molecular resolution remains a fundamental challenge in the life sciences. To tackle this, we developed tomoSTORM, an approach combining single-molecule localization-based super-resolution microscopy with array tomography of structurally intact brain tissue. Consecutive sections organized in a ribbon were serially imaged with a lateral resolution of 28 nm and an axial resolution of 40 nm in tissue volumes of up to 50 µm×50 µm×2.5 µm. Using targeted expression of membrane bound (m)GFP and immunohistochemistry at the calyx of Held, a model synapse for central glutamatergic neurotransmission, we delineated the course of the membrane and fine-structure of mitochondria. This method allows multiplexed super-resolution imaging in large tissue volumes with a resolution three orders of magnitude better than confocal microscopy

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

Over 3 weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation accumulations of up to 80 mm within 1 h and several flash floods. This study examines the conditions and processes that made this particular thunderstorm episode exceptional, with a particular focus on the interaction of processes across scales. During the episode, a blocking situation persisted over northern Europe. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to the low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary, and prevented a significant air mass exchange. In addition, the weak geopotential height gradients led to predominantly weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Due to a weak propagation speed in combination with high rain rates, several thunderstorms were able to accumulate enormous amounts of precipitation that affected local-scale areas and triggered several torrential flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs provided the mesoscale setting for the development of a large number of thunderstorms. During the 22 d study period, more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptionally persistent low stability over 3 weeks combined with a weak wind speed in the mid-troposphere has not been observed during the past 30 years