cc: tkarl@ncdc.noaa.gov
date: Wed, 7 Apr 1999 09:34:55 -0400
from: kgallo@ncdc.noaa.gov
subject: urban analysis mentioned at IPCC mtg.
to: christy@atmos.uah.edu, jhansen@giss.nasa.gov, p.jones@uea.ac.uk





John, Jim, and Phil:

Below are John Christy
s comments to Jim Hansen about the analysis I
mentioned briefly at the IPCC meetings held here at NCDC followed by my
"clarification" and additional comments. Ive included Phil in on this
email as he was also interested in the analysis and provided some helpful
comments.

 (6)  The analysis in the paper concerning the urban effect is based on
population.  Kevin Gallo showed evidence at the March IPCC meeting (later
that week) that when stations were categorized more objectively (by AVHRR
land use) an urbanization effect was found.  I'm copying this message to
Kevin so he can correct anything I might say.  Kevin carefully stratified
the stations (urban, suburban and rural) and checked differences (in
pairs?).  The trend for 1950-1990 for the rural was +0.16 K/decade while
that of the urban stations was +0.23 K/decade (or a difference of about 0.3
over the 40 years).  The suburban trend was in between, so that is evidence
that Kevin's technique is consistent.  The best method would be to
categorize sites by CHANGE in land-use over the period, but that's a tough
problem.

 I think you are probably correct by stating that "land-use changes" (a
better phrase than urbanization) causes "no more than 0.1 C warming" for
the global averages.


I guess I should clarify Johns comments. In the analysis mentioned at the
IPCC meetings, stations included in the GHCN data set were identified as
urban, suburban, and rural based on Defense Meteorological Satellite
Program-Operational Linescan System (nighttime lights) data in addition to
ancillary map and population data. The analysis was partially  initiated
because the paper by Peterson, Gallo,  et al., (1999, J. Geophys. Resr.) on
global rural temperatures was being misinterpreted by the press as to the
urban station contribution to the overall temperature signal (claims of no
impact). Urban stations were not specifically examined in this paper.

Thus, the study I initiated was an attempt to directly compare urban and
rural signals at the grid cell level. Stations were analyzed on a 5 X 5
degree grid cell basis such that each grid cell included in the analysis
must have contained urban and rural stations. The trends were initially
analyzed with the first difference method (Peterson et al., 1998, J.
Geophys. Resr.) that maximizes the number of stations included in the
analysis. We did find significant differences between the urban and rural
trends within the grid cells. Subsequent analysis with the traditional
anomaly time series (ATS) method of trend analysis revealed no significant
difference. Preliminary analysis of the first difference and ATS  results,
by others here at NCDC, indicates that the ATS may be more appropriate for
the scale (5 by 5 degree) of this analysis.

We have since begun to identify urban and rural station pairs that will be
analyzed, hopefully in time to have a publication that can be cited in the
next IPCC report.  At this time, we plan to cite the completed and nearly
published (J. Climate, May 1999) paper that examined grid cell trends in
diurnal temperature range, min, and max temperature trends in the USA over
the 1950-1996 interval. This study used the anomaly time series method of
analysis. While differences were observed between the satellite-defined
rural (-.41 degree C/100 years), suburban (-.81), and urban (-.86), trends
in DTR and min temperature, they were not statistically significant.
Conclusions included that; the general (urban, suburban, or rural) land
use/land cover associated with surface observation stations appears to be
one of the factors that can influence the trends observed in temperatures
and thus should be considered in the analysis and interpretation of
temperature trends.

I certainly support Johns suggestion that stations should optimally be
categorized by CHANGE in land-use over time. Data sets are under
preparation for specific urban areas that will soon permit such analysis on
a limited basis.

One last comment. Should any of you wish to have stations included in your
future analysis classified as urban, suburban, or rural, based on the
satellite methodology, please feel free to contact me.

Regards,

Kevin







----------------------------------------------

Kevin Gallo, Ph.D.

NOAA/National Climatic Data Center

151 Patton Ave.

Asheville, NC  28801

ph: +1-828-271-4878

fax: +1-828-271-4328

email: Kevin.P.Gallo@noaa.gov


        National Climatic Data Center




