Oct 17, 2003 MISR

1. Oct 17, 2003 MISR The Origin and Structure of Southern California Climate Variations Alex Hall, Sebastien Conil, Mimi Hughes, Greg Masi UCLA Atmospheric and Oceanic Sciences

2. Experimental Design MM5 model with boundary conditions from NCEP reanalysis data resolution of innermost domain is 6 km, time period is from 1995 to present. One can think of this as a reconstruction of weather conditions over this time period consistent with three constraints: (1) our best guess of the large-scale conditions, (2) the physics of the MM5 model, and (3) the prescribed topography, consistent with model resolution.

3. DIURNAL CYCLE AMPLITUDE

5. For the details on the causes of small-scale structure in diurnal cycle amplitude, see poster by Mimi Hughes

6. PRECIPITATION

7. Southern California Climatological Precipitation (cm/yr) For the details on the origins of small-scale variations in climatological precipitation, see poster by Greg Masi.

8. MODES OF VARIABILITY

9. A view of the Santa Anas from space, taken by the Multi-angle Imaging SpectroRadiometer (MISR) on February 9, 2002.

10. The winds simulated by the model during the Santa Ana event of February 9-12, 2002. Note the intense flow, reaching speeds on the order of 10 meters per second, being channeled through mountain passes. The fact that the model simulates this and other Santa Ana events with the correct timing demonstrates that the conditions for Santa Ana events are contained within the boundary condition information provided to the model.

11. Cluster Analysis To classify the regimes of wind variability in Southern California, we performed a probabalistic cluster analysis algorithm (Smyth et al. 1999) on the October to March daily-mean winds. The clustering technique provides an quantitative means of defining preferred modes of wind variability. We chose to focus on the wet season because of the interesting combination of phenomena (Santa Ana events and precipitation) during this period. We found that the wind regimes can be well-described in terms of three clusters, which together account for 82% of the days. Note that this implies that 18% of the days do not belong to any cluster, and are unclassified.

12. The “Santa Ana” regime is characterized by intense offshore flow through mountain passes. Its average duration is 1.7 days and it accounts for 13% of the total days in the analysis.

13. The “weak northwesterly” regime is characterized by relatively weak winds over land, and moderate alongshore flow over the ocean. Its average duration is 1.6 days and it accounts for 27% of the total days in the analysis.

14. The “strong northwesterly” regime is characterized by weak winds over land, and strong alongshore flow over the ocean. Its average duration is 2.4 days and it accounts for 42% of the total days in the analysis. This is therefore the dominant wind regime in Southern California.

15. These are the primary modes of pressure variability during the October-March season when the entire Siberian-Pacific-North American sector is considered. The middle pattern corresponds to the Pacific -- North American pattern (PNA). These modes are completely uncorrelated with the local Southern California modes identified by the cluster analysis. For more details on the analysis of the local modes of variability, see poster by Sebastien Conil.

16. A Local Perspective on Climate We show in our examination of (1) The amplitude of the diurnal cycle (2) The geographical distribution of precipitation (3) Local modes of wind variability in Southern California that spatial and temporal variability in local climate is either generated by in situ by local processes, or related in a non-trivial way to the large-scale forcing. __________________________ If we hope to predict climate on spatial scales relevant for humans and ecosystems, we must first understand these local processes and the complex relationships of large-scale forcing to local variability.