The Effect of Ground Terrain on Sacramento Valley Supercells

When the environment permits, the Central Valley of California can generate supercells capable of producing tornadoes. Typically, this occurs behind cold fronts, where surface winds become backed against the Sierra Nevada, and a mid level cold pool is overhead in conjunction with partial clearing and diurnal heating. The Central Valley is unique, because as a supercell that has formed on the west side of the valley continues to move eastward, it will begin to encounter more mountainous terrain. This study focuses on the tornadogenesis in Sacramento Valley supercells and the interactions of these supercells with the mountainous terrain. In order to examine the effects of the mountainous terrain on the Sacramento Valley supercells, 26 tornadoes that occured since the implementation of WSR-88D level II were examined. Primarily, 5 parameters are taken into account verifying post frontal cold pool conditions. The first is the surface temperature of the air. Typically for Northern California supercells, the temperatures in the valley after daytime heating will be 10^oC to 20^oC. Next are the 500mb temperatures. These need to be very cold, in the -25^oC to -40^oC range. These temperatures cause steep lapse rates that are supportive of convection. The reason surface based lifted index is not used, is due to the low tropopause heights of cold pool convection. Because of this, the favorable temperature ranges are used to verify a favorable thermodynamic environment. Additionally, a characteristic of the post frontal regime supercells is offshore open cell convection over the ocean. To verify the presence of this convection, Surface Based (SB) CAPE is used. Weak inland SB CAPE values less than 500J/kg are often observed with slightly higher SB CAPE values offshore, usually higher than 500J/kg. To verify the directional shear in these environments, surface winds are analyzed to verify central valley backing, and 700 mb winds are verified to be originating from the west, to complete a veering wind profile with height. This data is gathered from the NCEP North American Regional Reanalysis plotting tool. Next, in order to analyze the effects of the mountainous terrain, an index named maximum positive continuous elevation change (abbr. MPCEC) was created to model the relief of the terrain, and correlated with the maximum increase in gate to gate shear in a mesocyclone during tornadogenesis. MPCEC is derived using google earth. Using the radar data, the path that a mesocyclone took over a period of two radar scans is input into google earth. Google earth will generate an elevation profile, and the base to peak height of the highest hill or cliff is considered the value for the MPCEC parameter. Based on the results of the study, it is reasonable to assert that a high MPCEC value could lead to a rapid intensification in the mesocyclone of a supercell. Delving deeper into why exactly this occurs, there are two possible explanations. There are two possible explanations as to why topography can increase the rate of intensification of a mesocyclone. The first explanation is the compaction of the mesocyclone against steeper terrain. In figure 12, it was apparent that the tornado had run into a hill, which may have caused the possible area for rotation to shrink, causing the vortex to stretch. This vorticity stretching would, in turn, cause an increase in intensity. Another explanation is topographical channeling. In high relief terrain, often times erosion channels can help create localized areas of topographical channeling which can orient surface winds in a much more favorable direction for locally enhanced low level shear. Enhanced low level shear can often mean a faster intensifying mesocyclone. This intensification can cause an increased likelihood of a storm producing a tornado. However, it must be noted, that due to a small sample size tornadic supercells in readable level II radar data, there are limitations in the conclusivity of this study. Furthermore, topographic enhancement is not required for tornadogenesis in the Central Valley. The majority of supercells that occured in the central valley that actually ended up producing tornadoes were under relatively flat conditions, with an MPCEC with 1-20 meters. Only a select few tornado producing supercells dealt with a high value of MPCEC, and were able to benefit from it. Based on these limitations, we cannot assert that topographic enhancement is the primary factor to tornadogenesis in the central valley, rather it is just one factor that can be taken into account when forecasting an increase in mesocyclone strength of a supercell. Altogether, the limited data does show a strong correlation with the degree of relief and gate to gate shear increase.