Pollution Reduces Winter Precipitation In winter, moist air flows off the ocean and
rises over the hills downwind of a coastal city,
dropping its rain and snow mainly as it ascends the
hills. As pollution from the city is pushed into the
clouds by the hills downwind of the city, it
interferes with droplet formation in the clouds and
makes them smaller, as observed by NASA's
satellites. The smaller cloud droplets convert more
slowly into precipitation. Instead of precipitating,
much of the water in the clouds evaporates, reducing
the net rainfall downwind of the urban area by up to
15% to 25% on a seasonal basis. First is the
unpolluted, then the polluted case. Credit: NASA
The SOAR research
aircraft approaching and penetrating a
mountain wave cloud over the California Sierra
Nevada on 27th February 2005. The accompanying
CIP image shows 100 to 200mm hydrometeors
recorded during the cloud penetration at
–17oC)
SOAR is engaged in a study to document and model the
effects of urban and industrial air pollution in
California on clouds, precipitation, and stream flows
in mountainous terrain downwind of the pollution
sources. This effort involves hydrological analyses,
satellite-based cloud analyses and numerical modeling
in order to obtain insights into the
recently-documented (Givati and Rosenfeld, 2004a)
detrimental impacts of air pollution on precipitation
in several locations in the world, most recently in
California.
The focus of the overall investigation of the effect
of pollution on Sierra Nevada winter precipitation is
on the nature and source of the pollutants that are
apparently decreasing the orographic component of the
precipitation over the portions of the Sierra Nevada
that are climatologically downwind of known pollution
sources such as the San Francisco/Oakland/San Jose
Metropolis and Southern California including Los
Angeles and San Diego. A program, called the
Suppression of Precipitation (SUPRECIP) Experiment,
was conducted to provide the needed documentation. The
number, sizes and concentrations of ingested aerosols
and the resulting internal cloud microphysical
structure were documented in February and March of
2005.
An
important component of SUPRECIP was the use the SOAR
cloud physics aircraft, to reach two objectives:
1.
Measure atmospheric aerosols in pristine and polluted
clouds and the impact of the aerosols on cloud-base
microstructure, on the evolution with height of the
cloud drop-size distribution and on the development of
precipitation under warm and mixed-phase processes.
2.
Validate the multi-spectral satellite inferences of
cloud structure and the effect of pollutants on cloud
processes especially the suppression of precipitation.
Instrumentation during SUPRECIP
The SOAR research
aircraft, leased for up to 70 hours of flight time,
equipped with cloud-physics instrumentation and
aerosol instruments. The cloud physics instruments
used were the DMT CIP and DMT CDP. In addition, the
DMT CCN counter, the DMT modified PCASP and the Texas
A&M University DMA/TDMA were used during this
campaign. The DMT modified FSSP was used for
comparisons with the DMT CDP.
Satellite inferences of
cloud microstructure were made in terms of their
effective radius. The satellite inferences were made
for all of the cloud pixels within a series of boxes
along the flight track. Each box was defined such that
it encompassed some of the individual aircraft cloud
passes. This made it possible to compare the effective
diameters for the cloud passes at the height and
temperature of the pass with the satellite inferences
of the effective diameters at the 50th percentile for
the composite cloud for all clouds in the box.
Considering the differences in scale (i.e., individual
cloud passes vs. the composite cloud within a box that
contains the cloud passes) and time, the agreement is
remarkably good (linear correlation = 0.73), giving
increased credibility to the satellite inferences of
suppressed precipitation-forming processes associated
with pollution.
