BUOYANCY AND SHEAR

The Sounding Hodograph Analysis Research Program (SHARP) (Hart and Korotky 1991) was used to construct a proximity sounding and hodograph by insertion of the 2300 UTC observation from Moffett Field into the 0000 UTC 5 May 1998 KOAK radiosonde observation. CAPE was calculated on the basis of a lifted surface parcel.


 Fig. 5. Proxmity sounding for KNUQ at 2300 4 May 1998. Wind speeds in knots.
(Click on image for clearer version)

The sounding shown in above represents a great departure from the typical warm season thermal and moisture structure in the San Francisco Bay Region. The usually-seen marine inversion is absent, due to the influence of the offshore cold upper-level low. The relatively moist nature of the surface air mass involved in the low-level marine intrusion (shown by the northwesterly wind at the surface) is evidenced by atypically high surface dewpoints of nearly 16oC. The superposition of relatively cold middle- and upper- tropospheric temperatures over an uncharacteristically warm and moist marine intrusion actually created a stratification which, for California, was highly unstable. Surface-based CAPE of 1732 J kg-1 and a 500 mb Lifted Index of -6 underscored the potential for strong-to-severe convection on this day.

The hodograph given below shows that winds were generally backing with height with the sea breeze-related surface wind contributing to very large NEGATIVE shear in the 0-0.5 and 0-1 km layers. The motion of the tornadic storm as obtained from radar is plotted along with the storm relative flow vectors.

 Fig. 6. Hodograph for KNUQ at 2300 UTC 4 May 1998. Wind speed in knots.
(Click on image for clearer version)

Table 1. Shear and SREH values extracted from OAK hodograph modified for surface conditions at Moffett Field (NUQ) at 23 UTC 4 May 1998

 Layer
 Negative Shear (10-3s-1)

 Storm Relative Helicity (m2/s2)

0-1 km
0-2 km
0-3 km
0-4 km
0-5 km
0-6 km

14.7
6.9
5.1
3.5
3.6
3.1

 -70
-90
-111
-111
-126
-128


Values of negative shear and storm relative helicity (SREH) for the KNUQ hodograph given in the hodograph are listed in Table 1. The 0-6 km negative shear of ~3.0 X 10 -3 s-1 is marginally sufficient for supercells (Weisman and Klemp 1982). However, the very strong negative shear in the lowest layers is consistent with findings summarized in Monteverdi et al. (2000) and Lipari and Monteverdi (2000), in particular, that tornadic events in California are associated with very strong shear in the lowest layers.

The Moffett Field hodograph also shows some cyclonic curvature. Modeling studies (Rotunno and Klemp 1982) for storms developing in association with cyclonically-curved hodographs suggest that initially mirror image supercells should form if sufficient deep-layer shear exists, but that the left-moving member would be favored. Such a left-moving supercell storm would be characterized by a clockwise-rotating updraft. However, the relatively minimal curvature evident in the KNUQ hodograph also suggests that the supercell would not be tornadic, or, at, least, that the traditional "supercell cascade" to tornado genesis would not be expected for this case. This is verified by the relatively low SREH values in the "inflow" layer (i.e., 0-3 km layer) of -111 m2 s-2.