84th AMS Annual Meeting

Monday, 12 January 2004: 4:00 PM
Severe weather as seen via a preliminary sounding climatology and a wind adjusted convective index (WACI)
Room 602/603
Ivory J. Small, NOAA/NWS, San Diego, CA
Poster PDF (1013.8 kB)
A preliminary thermodynamic climatology for the 1998-2000 warm seasons (June through September) was constructed to investigate how convection in southern California is affected by the distribution of moisture in the vertical, the mid level lapse rate, and mid level wind speed. The parameters analyzed were the 750 and 600 mb dew point depressions, the 750 to 500 mb lapse rate, and the 500 mb wind speed. A preliminary "wind adjusted convective index" (WACI) was developed. The WACI assumes that the optimal mid level moisture content is about 75 % relative humidity (a dew point depression of approximately 4 degree C), and the contribution of moisture to the index falls off to either side. This is to lower the index values for environments that may be so cloudy that solar heating is hampered, or so dry that entrainment rapidly weakens the updrafts. Another assumption is that light to very light winds, generally less than 15 knots at 500 mb, are better for updraft strength in an environment such as southern California. This is based on the assumption that convection is mainly triggered by solar heating (especially over localized and/or elevated source regions). In these cases, stronger winds tend to shear the cells apart and/or move the cells from their source regions rather quickly. This removes the heating as well as upslope and convergent ridge top flow as factors aiding updraft strength. The intent of the index was to target days with the potential for moderate to strong updrafts that may produce severe weather and\or flooding. There is a good match with the development data. For the 2 highest WACI values, large hail was reported on both days, and of the 6 days with the highest WACI values, large hail was reported on 4 days.

For WACI values at and above 45 and for days when either severe weather or flooding was reported, the soundings were separated into 4 groups. The 4 distinct sounding types were linked to certain types of convection, such as severe weather and flash flooding, and will be discussed. The data suggests that WACI values along with sounding types can be used as a first guess for predicting the type of severe weather and/or flooding that is more likely to occur on a given day, and could affect aviation in southern California.

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