Thomas Griesser

The Comprehensive Historical Upper-Air Network

To better understand variability in weather and climate, it is vital to address past atmospheric circulation. This need requires meteorological information not just from the surface but also at upper levels. Current global upper-level datasets only reach back to the 1940s or 1950s and do not cover some important periods in the first half of the twentieth century. Extending the observational record is therefore considered important in order to analyze climate variability in the past and verify global climate models used to predict future climate change. Although earlier upper-air data from platforms such as radiosondes, aircraft, pilot balloons, registering balloons, and kites are available from various sources, no systematic compilation and quality assessment of upper-level data prior to the International Geophysical Year (1957/58) has ever been performed. Here we present the Comprehensive Historical Upper-Air Network (CHUAN). It is a consistent global historical upper-air dataset that has been derived fr...

A multi-data set comparison of the vertical structure of temperature variability and change over the Arctic during the past 100 years

We compare the daily, interannual, and decadal variability and trends in the thermal structure of the Arctic troposphere using eight observation-based, vertically resolved data sets, four of which have data prior to 1948. Comparisons on the daily scale between historical reanalysis data and historical upper-air observations were performed for Svalbard for the cold winters 1911/1912 and 1988/1989, the warm winters 1944/1945 and 2005/2006, and the International Geophysical Year 1957/1958. Excellent agreement is found at mid-tropospheric levels. Near the ground and at the tropopause level, however, systematic differences are identified. On the interannual time scale, the correlations between all data sets are high, but there are systematic biases in terms of absolute values as well as discrepancies in the magnitude of the variability. The causes of these differences are discussed. While none of the data sets individually may be suitable for trend analysis, consistent features can be identified from analyzing all data sets together. To illustrate this, we examine trends and 20-year averages for those regions and seasons that exhibit large sea-ice changes and have enough data for comparison. In the summertime Pacific Arctic and the autumn eastern Canadian Arctic, the lower tropospheric temperature anomalies for the recent two decades are higher than in any previous 20-year period. In contrast, mid-tropospheric temperatures of the European Arctic in the wintertime of the 1920s and 1930s may have reached values as high as those of the late 20th and early 21st centuries.

Reconstruction of Global Monthly Upper-Level Temperature and Geopotential Height Fields Back to 1880

Abstract This work presents statistically reconstructed global monthly mean fields of temperature and geopotential height (GPH) up to 100 hPa for the period 1880–1957. For the statistical model several thousand predictors were used, comprising a large amount of historical upper-air data as well as data from the earth’s surface. In the calibration period (1958–2001), the statistical models were fit using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) as the predictand. After the weighting of the predictors, principal component (PC) analyses were performed on both the predictand and predictor dataset. Multiple linear regression models relate each principal component time series from the predictand with an optimal subset of principal component time series from the predictor. To assess the quality of the reconstructions, statistical split-sample validation (SSV) experiments were performed within the calibration period. Furthermore, the reconstructions were compared w...

The early twentieth century warm period in the European Arctic

The European Arctic experienced a pronounced warming around 1920 and a sustained warm period in the 1920s and 1930s. The causes of this climatic event are not fully known. However, understanding this event is considered important for assessing current and future climate change in the Arctic. Here we investigate the role of atmospheric circulation variability based on newly available historical upper-air data and statistical reconstructions of atmospheric circulation. The strongest warming at the ground from the 1910s to the 1920s and 1930s was found in wintertime. Historical upper-air data in this region from the 1930s show warm temperatures also in the lower troposphere. Reconstructed geopotential height fields suggest stronger than normal meridional transport of warm air into the European Arctic during the warm period compared to the preceding cold period. We propose that the 1920-1940 warm period can be subdivided into two periods with distinct circulation regimes: During the 1920s, warm, relatively clean air masses from the North Atlantic lead to a warming, while during the 1930s warm, rather polluted air masses from Western Europe played an important role. This is reflected in a sudden increase in sulphate concentrations in an ice core from Svalbard around 1930. The aerosols might have amplified the warming via changing cloud long wave emissivity, but this mechanism remains to be further studied. The circulation anomalies in the North Atlantic region during the early 20 th century warm period that are shown in this paper form an observation-based counterpart against which model studies can be compared.