ALBERT

Description: This study aims to investigate some characteristics of the moist processes of the Madden-Julian oscillation (MJO), by making use of joint HDO (or δ D) and H 2 O vapor measurements. The MJO is the main intraseasonal mode of the tropical climate, but is hard to properly simulate in global atmospheric models. The joint use of δ D-H 2 O diagnostics yields additional information compared to sole humidity measurements. We use mid-tropospheric Infrared Atmospheric Sounding Interferometer (IASI) satellite δ D and H 2 O measurements to determine the mean MJO humidity and δ D evolution. Moreover, by making use of high temporal resolution data, we determine the variability in this evolution during about eight MJO events from 2010 to 2012 (including those monitored during the CINDY/DYNAMO campaign). This data has a higher spatio-temporal coverage than previous δ D measurements, enabling the sampling of individual MJO events. IASI measurements over the Indian Ocean confirm earlier findings that the moistening before the precipitation peak of an MJO event is due to water vapor slightly enriched in HDO. There is then a HDO depletion around the precipitation peak that also corresponds to the moister environment. Most inter-event variability determined in the current study occurs 5 to 10 days after the MJO event. In 75% of the events, humidity decreases while the atmosphere remains depleted. In a quarter of the events, humidity increases simultaneously with an increase in δ D. After this, the advection of relatively dry and enriched air brings back the state to the mean. Over the maritime continent, δ D-H 2 O cycles are more variable on timescales shorter than the MJO and the inter-event variability is larger than over the Indian Ocean. The sequence of moistening and drying processes as revealed by the q- δ D cycles can be used as a benchmark to evaluate the representation of moist processes in models. This is done here by comparing observations to simulations of the isotope enabled LMDZ GCM nudged with reanalysis wind fields. These simulations also give informations to investigate possible physical origins of the observed q- δ D cycles.

Description: A one-dimensional flux form advection scheme with a new correction of time-splitting errors is implemented in the numerical weather prediction (NWP) model COSMO of the German Meteorological Service. The advection scheme is applied for calculating the transport of all prognostic humidity variables in COSMO. Moreover, the advective transport of passive tracers is investigated. With the modified COSMO configuration several realistic case studies are performed and compared with the operational COSMO version. The main focus of these investigations is on the behavior of the new advection scheme in regions with strong deformational flow fields, such as steep orographical terrain and meteorological fronts. The numerical results of these case studies indicate that the modified COSMO model yields similar results as the operational version. The observed differences between the two model versions are minimized when the advection scheme of the operational COSMO model is applied with a numerically expensive Strang-splitting approach. At the same time, the results of the new advection scheme are barely affected by the application of Strang-splitting. Since, in addition to this, the new advection scheme behaves numerically more stable than the original scheme it is an attractive alternative for calculating the transport of scalar quantities in NWP models.

Description: [1]  Recent observations suggest that the mean mesoscale oceanic zonal velocity field is dominated by alternating jet-like features often referred to as striations. Here the generating dynamics of Northeast Pacific striations are explored with a set of 120-year eddy-permitting model simulations. Simulations are conducted with decreasing complexity towards idealized configurations retaining the essential dynamics and forcing necessary for striation development. For each simulation, we diagnose the spin-up of the ocean model and the sensitivity of striation generation to topography, coastal geometry, and the wind stress, which modulates the gyre circulation and the nonlinearity of the flow field. Results indicate that Northeast Pacific striations develop predominantly at the eastern boundary and migrate westward in congruence with beta-plumes both in the nonlinear and quasi-linear regimes. Mean striations have their source in the coastline geometry, which provides quasi-steady vorticity sources energized by eastern boundary current instabilities.

Description: Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission show that a persistent layer of Mg + ions occurs around 90 km in the Martian atmosphere, but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global ablation rate of 0.06 metric tonnes of magnesium sol -1 , out of a cosmic dust input of 2.7 ± 1.6 t sol -1 . The absence of detectable Mg at 90 km requires that at least 50% of the ablating Mg atoms ionize through hyperthermal collisions with CO 2 molecules. Dissociative recombination of MgO + .(CO 2 ) n cluster ions with electrons to produce MgCO 3 directly, rather than MgO, also avoids a buildup of Mg to detectable levels. The meteoric injection rate of Mg, Fe and other metals – constrained by the IUVS measurements - enables the production rate of metal carbonate molecules (principally MgCO 3 and FeCO 3 ) to be determined. These molecules have very large electric dipole moments (11.6 and 9.2 Debye, respectively), and thus form clusters with up to 6 H 2 O molecules at temperatures below 150 K. These clusters should then coagulate efficiently, building up metal carbonate-rich ice particles which can act as nucleating particles for the formation of CO 2 -ice clouds. Observable mesospheric clouds are predicted to occur between 65 and 80 km at temperatures below 95 K, and above 85 km at temperatures about 5 K colder.

Description: We present direct number density retrievals of carbon dioxide (CO 2 ) and molecular nitrogen (N 2 ) for the upper atmosphere of Mars using limb scan observations during October and November 2014 by the Imaging Ultraviolet Spectrograph on board NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We use retrieved CO 2 densities to derive temperature variability between 170 and 220 km. Analysis of the data shows (1) low-mid latitude northern hemisphere CO 2 densities at 170 km vary by a factor of about 2.5, (2) on average, the N 2 /CO 2 increases from 0.042 ± 0.017 at 130 km to 0.12 ± 0.06 at 200 km, and (3) the mean upper atmospheric temperature is 324 ± 22 K for local times near 14:00.

Description: We identify molecular nitrogen (N 2 ) emissions in the Martian upper atmosphere using the Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We report the first observations of the N 2 Lyman-Birge-Hopfield (LBH) bands at Mars and confirm the tentative identification of the N 2 Vegard-Kaplan (VK) bands. We retrieve N 2 density profiles from the VK limb emissions and compare calculated limb radiances between 90 and 210 km against both observations and predictions from a Mars general circulation model (GCM). Contrary to earlier analyses using other satellite data, we find that N 2 abundances exceed GCM results by about a factor of 2 at 130 km but are in agreement at 150 km. The analysis and interpretation are enabled by a linear regression method used to extract components of UV spectra from IUVS limb observations.

Description: To improve the representation of surface and groundwater flows, global land surface models rely heavily on high-resolution digital elevation models (DEMs). River pixels are routinely defined as pixels with drainage areas that are greater than a critical drainage area (A cr ). This parameter is usually uniform across the globe, and the dependence of drainage density on many environmental factors is often overlooked. Using the 15” HydroSHEDS DEM as an example, we propose the calibration of a spatially variable A cr as a function of slope, lithology, and climate, to match drainage densities from reference river networks at a 1:50,000 scale in France and Australia. Two variable A cr models with varying complexities were derived from the calibration, with satisfactory performances compared to the reference river networks. Intermittency assessment is also proposed. With these simple tools, river networks with natural heterogeneities at the 1:50,000 scale can be extracted from any DEM.