DETECTION OF PERIODIC CLIMATE ANOMALIES OVER GREENLAND 
WITH MICROWAVE RADIOMETERS 
Wolfgang Wagner‘, Per Gloersen? 
! Student, Institute for Photogrammetry and Remote Sensing, Vienna University of Technology 
? Senior Scientist, Oceans and Ice Branch, Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Centre 
Commission VII, Working Group 8 
KEY WORDS: Radiometry, Change Detection, Spectral Analysis, Greenland, Ice Sheet 
ABSTRACT 
We investigate the potential of microwave radiometers for the detection of periodic climate anomalies over Greenland. A multiple- 
window harmonic analysis technique is applied to a nine year record of Scanning Multichannel Microwave Radiometer (SMMR) 
data. We concentrated on the 37GHz vertically polarized channel to obtain a good correlation between the signal and ice crystal 
size over the skin depth. Year to year differences in microwave emission from the percolation facies of the Greenland Ice Sheet are 
explained by the absence or varying degree of surface melt during summer. Quadrennial oscillations were found to occur more 
frequently in the brightness temperature record than oscillations with periods of one or two years. 
1. INTRODUCTION 
The Greenland ice sheet is the largest ice sheet in the Northern 
Hemisphere. The present ice sheet has a surface area of 
1,750,000 km? (Steffen et al., 1993), that is twenty one times 
the size of Austria and roughly the size of Mexico. Little is yet 
known about the Greenland ice sheet despite its important role 
for climate studies and for climate itself (Thomas, 1993). 
Research has been hampered by its vast size and the harshness 
of the polar environment. Remote sensing satellites are able to 
provide measurements over large areas and will thus contribute 
to our understanding of the Greenland ice sheet. The 
usefulness of microwave radiometers, like the Scanning 
Multichannel Microwave Radiometer (SMMR), for the study 
of the polar ice sheets has already been demonstrated. The 
most prominent application of microwave radiometers is 
detection of surface melting on the ice sheets. Since moisture 
in the near-surface firn causes a marked increase in microwave 
brightness temperatures (Ty), very high values of T, indicate 
surface melting. Thereby the extent and duration of surface 
melting can be derived. Presently long term-series of such 
information are being established (Zwally and Fiegels, 1994). 
In this study we investigated the potential of microwave 
radiometers for the detection of periodic climate anomalies 
over Greenland. By studying the spatial and temporal 
variabilities of geophysical parameters, global patterns can be 
depicted which makes it possible to link weather and climate 
anomalies in one region of the globe to another (Lau and 
Busalacchi, 1993; Gloersen, 1995). 
2. METHOD 
To find periodic signals in the microwave emission from the 
Greenland ice sheet we applied a novel harmonic analysis 
technique to a nine year record of SMMR brightness 
temperatures. The harmonic analysis technique was developed 
for investigating seismic, atmospheric carbon dioxide, and 
other time series records (Park et al., 1987; Kuo et al. 1990). 
Several names can be found for it in the literature. We will 
refer to this technique as the ,multiple-window harmonic 
analysis". The multiple-window harmonic analysis aims for 
finding the harmonic components of a time series. The method 
is novel in that it uses several windows to fit a sinusoid model 
to the data and that it utilizes a F-test statistic to test the fit of 
the sinusoid model. 
We analyzed data from the Scanning Multichannel Microwave 
Radiometer (SMMR) which was operated on board of the 
Nimbus 7 satellite from October 26, 1978 to August 20, 1987. 
The SMMR is a 10-channel instrument receiving both 
horizontally (H) and vertically (V) polarized radiation at 5 
frequencies. Only the 37 GHz V and H channels were 
considered because of the greater sensitivity of the 37 GHz 
data to near-surface changes compared to the longer- 
wavelength data (Schuman et al., 1995). The spatial resolution 
of the data at 37 GHz is approximately 30 km. The SMMR 
instrument was generally operated on an every-other-day basis. 
Our data set thus comprised 1611 images for both the 37 GHz 
vertically polarized channel and the 37 GHz horizontally 
polarized channel. The SMMR data have been mapped onto a 
rectangular grid with a grid cell size of 25 km x 25 km over a 
stereographic projection. For a detailed description of the 
SMMR see Gloersen et al. (1992). 
Brightness temperature time series were established for each 
grid cell. The time series were detrended assuming that the 
trend is linear. Following the procedures described in the 
Appendix ,,Multiple-window harmonic analysis“ we calculated 
the complex amplitude, u, of the sinusoid model and the F-test 
parameter, F, from the detrended data. The sum over time is 
taken on six-day intervals instead of the original two-day 
intervals to match the revisit time of the SMMR. The complex 
amplitude gives information about the magnitude and phase of 
the sinusoid model immersed in the time series, and the F-test 
744 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996 
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