Temporal and spatial evolution of the large scale meteorological patterns (LSMPs) for California Central Valley hot spells

Extreme hot spell events have major impacts on human activities in the California Central Valley (CCV). Previous studies (Grotjahn and Faure 2008; Grotjahn 2011, 2013, 2014) showed that Large Scale Meteorological Patterns (LSMPs) amplify heat over the regional scale CCV. This study diagnoses the temporal and spatial evolution as well as the thermodynamics and dynamics of LSMPs during extreme CCV hot spells. Hot spells begin when at least 6 of 15 CCV stations are in the highest 5% of their normalized daily maximum temperature anomaly (using each station's long term daily mean, LTDM, and LTDM standard deviation). Hot spells must persist at least 3 consecutive days and be separated from other events by 6 days or longer. 22 such events occur during summer (JJAS) 1981-2010. Air parcel trajectories arrive at the critical region for high temperature anomaly from different origins. During the last 4 days leading up to a hot spell onset, air parcels commonly move downward several hundred hPa. Prior to this sinking, two thirds of the parcels (cluster 1) come from the far west (west of 160W) while the rest (cluster 2) from various directions near the CCV. The LSMP evolves as follows: a strong ridge off NE Asia and a stronger trough to the east both upstream of North America, amplify a west coast ridge. In both clusters wave activity flux (WAF) propagates across the North Pacific. In mid to lower troposphere, sinking motion over western North America is coupled to offshore flow. In cluster 1 the upper troposphere WAF off Asia is from the middle and higher latitudes; in the lower troposphere warm anomalies form and amplify along the California coastline. Cluster 2 WAF off Asia is from middle and subtropical latitudes, revealing Rossby wave propagation from the tropics not seen in cluster 1. Also in contrast, warm anomalies in cluster 2 are first prominent off the US/Canada west coast then amplify and expand southward towards California's coast. Objective clustering techniques classify CCV hot spell events based on the evolution of temperature in key parts of the LSMPs. Additional dynamical and adiabatic and diabatic thermal analyses are ongoing and will be shown for each cluster.