Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 239

Since urban atmospheres are also polluted, and particles can have a similar
effect, it is difficult to separate particulate and moisture influences.
Whether UME or moisture deficit, urban areas act to increase turbulence,
convection, cloud formation, and rainfall. Lowry ( 1998 ) and McKendry ( 2003 )
suggest that cities induce stronger convection zones, especially associated with
the UHI, which support enhanced cloud formation and thunderstorm develop-
ment. Variations in urban topography play a secondary role. But clouds are also
an important controller of the strength of the UHI. An increased cloud cover
generally reduces the strength of the UHI. Urban areas also create an excess of
large and giant nuclei, many of which are hygroscopic, enhancing cloud con-
densation processes. On occasion, too many nuclei can be produced, creating too
many cloud droplets and preventing efficient cloud droplet growth to raindrops.
Precipitation and severe weather can be enhanced, especially associated with
frontal systems. Enhancement usually occurs at some distance downwind of the
city, associated with the urban plume (Figure 7.4a ), because time is needed for
the raindrop formation process to develop. One of the earliest, and most con-
troversial, urban rainfall enhancement descriptions was the LaPorte, Indiana,
anomaly. Here major increases in rainfall were blamed on excess particulates
and convection, blown downwind from the city of Chicago (Lowry 1998 and
others). Summertime rainfall downwind of St Louis also showed increases
blamed on urban effects (Brazel and Quatrocchi 2005 ). Not only did 15%
more rainfall occur on the east of the southeast side of St Louis, but also storms
with more than 25mm of rain were 50%more frequent. The historical precipita-
tion records show significant increases which parallel the growth of Mexico City
(Jauregui and Romales 1996 ). For example, the frequency of rain periods greater
than 20mm h 1 has increased, especially between 1200 and 1800, in the warmer
part of the day. While there is no evidence of an urban influence upwind of the
city, increased rainfall downwind of the prevailing winds is clearly evident. The
spatial distribution of rain shows a significant relation with the core of the UHI,
where additional convective heating is available. However this relation is com-
plex and non-linear, and there is considerable spatial variation within the urban
area from storm to storm. Broadly similar results have been found for several
other US cities, depending on the storm type. Urban enhancement of precipita-
tion has the greatest effect on the strength and frequency of smaller rainstorms.
Bonan's ( 2002 ) summary describes weekly variations in urban rainfall asso-
ciated with the urban cycle of pollutants. During the working week, assuming no
major change in synoptic situation, pollutants accumulate in the atmosphere,
reaching a maximum on Thursday, Friday, and Saturday. This can lead to
increased rainfall toward the end of the working week and on the weekends,
while Monday and Tuesday show little indication of rainfall enhancement over
the city or downwind.
Lowry ( 1998 ) argues that, since precipitation is discontinuous, there is con-
siderable uncertainly about urban effects. Complications include the role of local