2011 - Cooperative Institute for Research in Environmental Sciences ...

Tom Chase
Land and Ocean Surface Impacts
on Global Hydrology
We continue to examine
the impact of surface
hydrology on climate,
particularly in the massively
irrigated regions
of Asia, and have found
evidence of substantial
impacts both in observational
and model simulation
studies. For example,
irrigation in India seems
to have reduced monsoon
rainfall and slowed the
East Asian Jet maximum,
indicating that circulations
around the globe
could be affected by
human disturbances to
the land surface in remote
areas.
We have continued examining minimum temperature
regulation by convection at high latitudes and have
updated our observational analysis of this phenomenon in
the last year. This work documents that mid-tropospheric
temperatures at high latitudes reach a minimum (about
-40°C) early in the winter and then never get much colder,
despite the lack of solar input for months. We hypothesize
that this is due to convective heating driven by sea-surface
temperature slightly below 0°C in cold air masses that
migrate over the ocean. Surface air rises moist adiabatically
to reach the observed minimum by 500 mb. We are proposing
to extend this to maximum temperatures and lower
latitudes in future work.
Finally, we also have been looking into low-level inversions
in the Western United States, which we found to
have substantially decreased in frequency—but increased
in strength—over the period of record in six Western cities
(Figures 1 and 2 show trends in inversion frequency and
strength in Denver, Colo.). This is of interest because air
quality in the West is a function of inversion frequency and
strength and because climate models have predicted that
inversions would be more frequent in a warming climate.
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Denver, Colorado
Frequency Anomaly
Linear Regression
Locally Weighted Regression
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1994 1996 1998 2000 2002 2004 2006 2008
Figure 1: Monthly anomalies in the occurrence of two or more consecutive
days in Denver, Colo., during which low-level daytime inversions were present.
Y-axis is fraction of days per month, and x-axis is calendar year. Locally
weighted regressions smooth the original data using a 0.7 span.
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Denver, Colorado
Monthly Mean Strength
Linear Regression
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1994 1996 1998 2000 2002 2004 2006 2008
Figure 2: Monthly anomalies in frequencies (fraction of days per month
during which inversions were present) of low-level inversions. Y-axis is
fraction of days per month, and x-axis is calendar year.
CIRES Annual Report 2011 29