Evaluation of the Temperature Trend and Climate Forcing in the Pre- and Post Periods of Satellite Data Assimilation - Satellite-based Applications on Climate Change - page 51

4.2.2 ERA-40

The monthly ERA-40 reanalysis (Uppala et al. 2005 ) is employed for the same
periods as the NCEP/NCAR reanalysis datasets. The ERA-40 reanalysis data uses
the Integrated Forecasting System (IFS) developed jointly by ECMWF and M´t´o-
France. Derived temperatures from the satellite data (Kalnay et al. 1996 ) were
assimilated in the NCEP/NCAR reanalysis, while the satellite-measured radiances
were assimilated directly in the ERA-40 reanalysis. The reanalysis has 23 pressure
levels that range from 1 hPa to the surface (1,000 hPa).

4.2.3 MSU (Microwave Sounding Unit)

The MSU monthly temperature dataset between the end of 1978 and 2006 was
created using the brightness temperature measurements derived from channels
2 and 4 from the TIROS-N, NOAA-10, 11, 12, and 14 satellites (Zou et al. 2009 ).
The data were averaged over 2.5

2.5 latitude-longitude grids. To reduce the
biases in the intersatellite MSU instruments, NESDIS scientists (Zou et al. 2006 ,
2009 ) developed an intercalibration method based on the simultaneous nadir
overpass (SNO) matchups. Due to orbital geometry, the SNO matchups are con-
fined to the polar region where the brightness temperature range is slightly smaller
than the global range. Nevertheless, the resulting calibration coefficients are applied
globally to the entire life cycle of an MSU satellite.
Such intercalibration reduces intersatellite biases by an order of magnitude
compared to prelaunch calibration and, thus, results in a well-merged time series
for the MSU channels 2 and 4, which respectively represent the deep layer temper-
ature of the middle troposphere (~600 hPa) and lower stratosphere (~87 mb).

4.2.4 Climate Forcings: Solar, ENSO, QBO, and Stratospheric
Aerosols

The solar variability proxies used in this, and in most studies, are the solar radio
irradiance (the 10.7-cm radio flux). The solar radio irradiance spans the time period
from1947 to 2009 and can be found at http://www.ngdc.noaa.gov/stp/SOLAR . Onthe
basis of previous studies that have tested different proxies (Keckhut et al. 1995 ), the
10.7-cm solar flux, which closely tracks the temporal behavior of the UV changes on
daily, monthly, and 11-year time scales, is taken in our analysis to represent solar
variability. The NINO3.4 is the ENSO index averaged sea-surface temperature in the
equatorial Pacific (5 N-5 S, 170 W-120 W) provided by NOAA's Climate Predic-
tion Center (CPC). QBO is the equatorial zonal wind at 30 hpa from Freie Universit¨t
Berlin provided by Labitzke, and aerosol impacts are represented by the global
stratospheric aerosol optical depth (AOD) from NASA GISS climate model datasets
(Hansen et al. 2005 ) which is indicative of volcanic influences.