ALBERT

Description: This paper evaluates the usage of reflected GPS signals for Earth observations to study changes of sea level and sea-ice in remote sensing. In a coastal setup, ∼670 m above Disko Bay (Greenland), signals with different carriers L1 and L2 were recorded. A method is presented that analyses the interferometric phase between the reflected and the direct signals and derives the height of the reflecting surface. The analysis includes a ray tracing and an estimation of signal coherence. It is shown that coherent reflections are related to sea-ice coverage. Absolute heights are derived with a time interval of ∼30 min. The altimetric results show semidiurnal tides that are validated using the AODTM-5 tide model. The residual height has a mean of 9.7 cm for L1 and 22.9 cm for L2. The dispersion is not significant but a significant tropospheric bias is detected with an error of up to 20 cm.

Description: In the afternoon of 15 July 2007, a thunderstorm was initiated within a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographically-induced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary-layer structure, lids and the water-vapour field in the pre-convective environment of the event are discussed. For this purpose, we investigated measurements of water-vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated-water-vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large-scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent-heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area in which CI took place. Copyright © 2011 Royal Meteorological Society

Description: In this work, we investigate the relationship between the structure and evolution (from initiation to decay) of precipitation systems, and the associated water vapour distributions during the COPS (Convective Orographically induced Precipitation Study). This international field campaign took place over an area from the Vosges to the Black Forest Mountains, across the Rhine Valley, in summer 2007. In particular, we consider water vapour retrieval through GPS integrated water vapour 2D maps and 3D tomography, and compare these to precipitation systems observed with the ground-based C-band POLDIRAD weather radar. We have demonstrated the predominant role of water vapour as a precursor to convective initiation for local convective cell generation. Water vapour accumulation on the crest of the orography is associated with ridge convection, while water vapour passing over the mountain top and creating valley outflows generates lee-side convection, often triggered by a small hill positioned within or close to the valley exit, or by a local convergence with the water vapour field over the plain. We have also noted that frontal systems seem to develop preferentially where the largest amount of water vapour is available. Likewise, in the case of frontal systems, well-formed synoptic-scale storms are associated with high water vapour signatures, while weaker systems with embedded convection appear to trail high water vapour areas where the convective element is associated with local water vapour depletion. This latter aspect could be the signature of convective cloud formation, when water vapour is transferred into liquid water, before the onset of precipitation. Copyright © 2010 Royal Meteorological Society