A Study of Urban Effects on Radar First Echoes

The properties of 3 cm radar first echoes are used to study the effects of the St. Louis, Mo., metropolitan area on precipitation initiation in summer convective clouds. Based on a sample of 4553 first echoes, obtained on 82 echo-producing days of 1972–75, it is shown that the area-normalized frequency of first echo formation over the city and in the “near” downwind region is approximately a factor of 2 greater than for nearby rural regions. The maximum enhancement in first echo formation occurs over the downtown area and along the Mississippi River, which separates St. Louis from industrial suburbs to the east. The downwind extent of the region of first echo enhancement appears to be limited to about 1 h of wind travel. The enhancement occurs mainly on weekdays. Temperatures of first echo tops and bases indicate that precipitation initiation is most frequently through drop collection, though there is evidence that ice processes may contribute a small fraction of the first echoes. Urban first echoes have lower and warmer bases and greater vertical thickness than rural first echoes.