Fronts and frontogenesis in relation to vorticity

Soon after the discovery of the polar front, it was realized that fronts were subject to processes which either increased or diminished their intensity. Thus, fronts may form in fields where the distributiori of the meteorological elements is continuous; and, in other cases, fronts may dissolve and develop into a field of continuous distribution of the various elements. The processes which lead to the formation of a front or the increase in intensity of an existing front, are called jrontogenetical processes; and the processes which lead to the dissolution of fronts are calledjrontolytical processes. In theoretical treatments of fronts it has been customary to simplify the problem by assuming that a frontal surface is a mathematical discontinuity, and doubts have been raised against the validity of this simplification. Petterssen has shown that both the dynamic and the kinematic boundary conditions that hold for perfect discontinuities hold also for layers of transition of finite thickness within which the meteorological elements vary continuously. We are, therefore, justified in treating frontal surfaces and fronts as either strict discontinuities or as finite layers of transition. Frontogenesis may therefore be defined as the process that tends to create a surface of discontinuity in the atmosphere. Whether or not this process results in a strict discontinuity is immaterial

A 4D feature tracking algorithm: a multidimensional view of cyclone systems

An objective 4D algorithm developed to track extratropical relative vorticity anomaly 3D structure over time is introduced and validated. The STACKER algorithm, structured with the TRACKER single level tracking algorithm as source of the single-level raw tracks, objectively combines tracks from various levels to determine the 3D structure of the cyclone (or anticyclone) events throughout their life cycle. Stacker works progressively, beginning with two initial levels and then adding additional levels to the stack in a bottom-up and/or top-down approach. This allows an iterative stacking approach, adding one level at a time, resulting in an optimized 4D determination of relative vorticity anomaly events. A two-stage validation process is carried out with the ERA-Interim dataset for the 2015 austral winter. First the overall tracking capability during an austral winter, taking into account a set of climate indicators and their impacts on Southern Hemisphere circulation, was compared to previous climatologies, in order to verify the density and distribution of the cyclone events detected by STACKER. Results show the cyclone density distribution is in very good agreement with previous climatologies, after taking into account potential differences due to climate variability and different tracking methodologies. The second stage focuses on three different long-lived events over the Southern Hemisphere, during the winter of 2015 spanning seven different pressure levels. Both GOES satellite imagery, infrared and water vapour channels, and ERAInterim cloud cover products are used in order to validate the tracks obtained as well as the algorithm’s capability and reliability. The observed 3D cyclone structures and their time evolution are consistent with current understanding of cyclone system development. Thus, the two-stage validation confirms that the algorithm is suitable to track multilevel events, and can follow and analyse their 3-D life cycle and develop full 3D climatologies and climate variability studie

Relative effects of wind stress curl, topography, and stratification on large‐scale circulation in Lake Michigan

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95379/1/jgrc8845.pd

North Atlantic Climate Variability: Phenomena, Impacts and Mechanisms

Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena, their climatic impacts and our present state of understanding of their underlying caus

Interannual variations in precipitation: the effect of the North Atlantic and Southern oscillations as seen in a satellite precipitation data set and in models

Precipitation is a parameter that varies on many different spatial and temporal scales. Here we look at interannual variations associated with the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO), comparing the spatial and temporal changes as shown by three data sets. The Global Precipitation Climatology Project (GPCP) product is based upon satellite data, whereas both the National Centers for Environmental Prediction (NCEP) and European Centre for Medium-Range Weather Forecasts (ECMWF) climatologies are produced through reanalysis of atmospheric circulation models. All three products show a consistent response to the NAO in the North Atlantic region, with negative states of the NAO corresponding to increases in precipitation over Greenland and southern Europe, but to a decrease over northern Europe. None of the climatologies display any net change in total rainfall as a result of the NAO, but rather a redistribution of precipitation patterns. However, this redistribution of rain is important because of its potential effect on oceanic overturning circulation. Similarly, all three data sets concur that the SO has a major effect on precipitation in certain tropical regions; however, there is some disagreement amongst the data sets as to the regional sensitivity, with NCEP showing a much weaker response than GPCP and ECMWF over Indonesia. The GPCP and NCEP climatologies show that the various phases of El Niño and La Niña act to redistribute, rather than enhance, the freshwater cycle. Given that the models incorporate no actual observations of rain, and are known to be imperfect, it is surprising how well they represent these interannual phenomena