Land Cover Changes on Climate - Urbanization
We evaluate all urban areas over the globe in terms of
their daytime and nighttime skin temperatures ("urban heat island effect"),
surface albedo, surface emissivity, atmosphere stability, aerosol and cloud variations.
Unlike other previous studies, which either focused on small regions or used the WMO 2m surface air temperature measurements from only thousands of stations, we analyze the urban climate on local to global scales
using EOS satellite global and fine resolution measurements.
Examining all global cities helps disclose the range and extremity of
human impact on the climate system.
Our work suggested that human activities decrease urban surface albedo and emissivity, increase cloud and aerosol optical depths, enhance overlying atmospheric instability and eventually result in urban heat island effect. In addition, evident weekly cycle is noticed on clouds and aerosol properties on dense urban areas, such as New York, due to varying human activities on weekdays and weekends. This weekly cycle is consistent with the weekend-more-wet urban rainfall pattern previously reported by various researchers.
Click : New York Urban Heat Island 
Figure 1(a-b) show the monthly-mean daytime and nighttime skin temperatures for January, along the latitude of New York (around 40.45°N). The arrows point to New York city. At 5km resolution,
New York is hotter than the adjacent pixels along 75°- 77°W.
Variation Trend of Skin Temperature
I have worked on the Earth Observation System
EOS)
since 1993.
Using the most advanced climate models (CCM2/BATS, CCM3/BATS) and satellite observations
(MODIS, AVHRR, GOES, ISCCP),
we concentrate on the understanding of
land surface temperature related physical processes and the influences
of vegetation, soil moisture and clouds on skin temperature.
This work has been
awarded a NASA fellowship. Most of the results of this work have been
published and presented in several international conferences.
A 20-year LST data set is developed by our group, which overcomes the major
limitations of surface observations, namely sparse and irregular
distributions. Figure below compares the anomalies of global mean satellite
LST, WMO 2m Ta, and NCEP reanalysis of LST. An obvious positive trend
during the study period is observed from all these three surface
temperature data sets. Satellite AVHRR LSTD has increased at
0.43ºC/decade, while the rates for Ta and NCEP LST are 0.28ºC/decade and
0.34ºC/decade, respectively. The consistency of satellite LST and
NCEP/NCAR reanalysis suggests that a warming does occur during 1981-1998.
In addition, the exact rate of increase depends on how global and annual
averages are calculated.
Click: Interannual Trend 
For more information:
Jin, M. and R. E. Dickinson 2002: New observational evidence for global
warming from satellite, Geophys. Res. Lett., 29(10), 10.1029/2001GL01383
PDF file of the paper
Land Surface Modeling
We improve land surface model's emissivity parameter using MODIS global observations.
We develop urban scheme for GCM to better describe the urban physical processes.
Atmosphere-Land Surface Interactions
Aerosol and Cloud Interactions
Large aerosol optical depth over dense urban areas is observed as a result
of human heating, industry, and traffic (King et al. 2002). Such
disturbances on clouds and aerosol are responsible for the urban-induced
rainfall anomalies (Marshall et al 2002). Further quantitative
understanding of physical processes over global urban areas is needed.
Our project aims to use MODIS, in situ AERONET aerosol observations, and
TRMM rainfall observations in Year 2001 and 2002 to identify daily and
seasonal variations of aerosol and clouds properties, to understand the
aerosol-clouds interactions, and to identify cloud-aerosol relationships
with skin temperature and rainfall. In specific, we will address
scientific questions such as
a. What are the typical daily and seasonal variations of aerosol and
clouds properties over urban areas? What is the typical spatial scale of
urban-related aerosol?
b. What are the observed aerosol-cloud relationships? For example, will an
increase in aerosol concentration result in changes in cloud optical
depth?
c. What are the aerosol-rainfall-cloud relationships? On one hand, urban
aerosol increases CCN and therefore reduces the cloud drop size and may
consequently reduce rainfall. However, this mechanism may only work for
warm convection system. For strong meso-scale climate systems, the heavy
rainfall runs out of CCN and may need aerosol as new CCN for further
rainfall.
d. What is the urban cloud-surface relationship? Are clouds properties
(particle size, top temperature, liquid water path, etc) sensitive to
surface temperature, building density, vegetation coverage? Answering
these questions helps to understand how human activities change the
natural climate systems and how the nature responds.
-- Last updated May 22, 2003
