Main Atmospheric Patterns Research Articles

Climatology of 500‐hPa geopotential height anomalies associated with May tornado outbreaks in the United States

AbstractThis study supports efforts directed toward research on large‐scale atmospheric patterns and on the variability of tornado outbreaks. Specifically, we applied rotated principal component analysis to identify synoptic‐scale patterns of 500‐hPa geopotential height associated with tornado outbreaks in the United States. We created a database of historic tornado outbreaks using kernel density estimation on events composed of at least seven tornadoes of magnitude (E)F2 or higher (major tornado outbreaks) that occurred in May from 1950 to 2019 (91 events). Results of the analysis show that the first three principal components explained the majority, that is, 74% of the total variation. Based on the analysis of congruence coefficients, the Promax oblique transformation was chosen as the most representative rotation in portraying physically meaningful modes and resulted in three main atmospheric patterns. Two of these, to the best of our knowledge, have not been yet identified as the most representative of tornado outbreaks in any previous studies. Additionally, results suggest that although synoptic patterns associated with major May tornado outbreaks remain the same over time, partial variability in their geographic location exhibits some cyclical behaviour, especially on decadal and multidecadal scales. Identifying these patterns can serve as a first step in determining how they may change under anthropogenic climate change in the future.

Role of orography in the spatial distribution of precipitation over the Spanish Mediterranean zone

The effects of orography on the precipitation field in the Spanish Mediterranean zone are studied through numerical simulations from a climatological point of view. The main atmospheric patterns that produce significant precipitation in the considered area, derived in a previous study, have been used as input data. The numerical simulations have been performed using the HIRLAM model. We isolate, by means of a factor-separation technique, the separate effects of the Iberian and non-Iberian orographies, as well as the synergy between the 2 orographies. The diversity of simu- lated-precipitation spatial distributions is clearly linked to the wide variety of flows over the Iberian Peninsula. However, a clear distinction between Atlantic and western Mediterranean disturbances is obtained. When Atlantic westerlies occur over the Iberian Peninsula, rainfall in the Spanish Mediter- ranean zone is enhanced by orography, mainly by that of Iberia. Otherwise, when the Iberian Penin- sula is affected by Mediterranean disturbances, local orographic enhancements along with the redis- tribution of precipitation due to non-Iberian orography and synergistic effects are observed. The North African Atlas range, due to its primary influence, emerges as one of the factors governing the spatial distribution of rainfall across the Spanish Mediterranean zone.