On January 1, 1997, a significant icing outbreak occurred over northern and central California. This outbreak was noteworthy in that two thirds of the moderate or greater icing pilot reports (PIREPS) occurred at altitudes above 20,000 feet. Also, the severity of the icing caused unspecified damage to a Boeing 737 aircraft between 24,000 and 28,000 feet ober Fresno CA.
This study attempts to diagnose the cause of the icing outbreak using zero and three hour forecasts from the Rapid Update Cycle (RUC) model. This study also recommends which meteorological parameters need to be examined in order to forecast this unusual icing event.
There were five PIREPS of severe icing over northern and central California on January 1, 1997. Two of the five were over Fresno, one was over Sacramento, one was over Santa Rosa and one was over Santa Barbara. All of the severe icing PIREPS were at or above 18,000 feet.
RUC model soundings and profiles of relative humidity, temperature and equivalent potential vorticity are presented for some of the significant icing reports. Profiles of cloud liquid water, droplet size and parameterized vertical motion from a new Aviation Weather Center (AWC) algorithm and zero hour forecasts of Convective Available Potential Energy (CAPE) are also included. All of this data is used in an attempt to understand the cause of this icing outbreak.
Preliminary results indicate that convection was the cause of this icing event. RUC model forecasts of CAPE in the lowest layer of the atmosphere revealed small values of CAPE which are supportive of slow convection which is one way to produce supercooled drops necessary for icing of this severity.
It is also hypothesized that small values of CAPE, a very deep layer of moisture and neutral lapse rates or a mechanism to increase the lapse rate in the layer of interest are all necessary components of a significant high level icing outbreak.
The 8th Conference on Aviation, Range, and Aerospace Meteorology