Andrea N. Grant

Recent Arctic warming vertical structure contested.

Arising from: R. G. Graversen, T. Mauritsen, M. Tjernström, E. Källén & G. Svensson 451, 53–56 (2008)10.1038/nature06502; Graversen et al. replyThe vertical structure of the recent Arctic warming contains information about the processes governing Arctic climate trends. Graversen et al. argue, on the basis of ERA-40 reanalysis data, that a distinct maximum in 1979–2001 warm-season (April–October) Arctic temperature trends appears around 3 km above ground. Here we show that this is due to the heterogeneous nature of the data source, which incorporates information from satellites and radiosondes. Radiosonde data alone suggest the warming was strongest near ground.

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...

A New Look at Radiosonde Data prior to 1958

Historical radiosonde data are known to suffer from inhomogeneities. The first radiosonde intercomparison was made at Payerne, Switzerland, in 1954, and a major international effort to standardize the network, including launch times, was made for the International Geophysical Year (IGY) in 1957–58. Data from before this period, in some cases extending back as far as 1934, have been viewed with even more suspicion than recent data. These early data are scattered among numerous archives with a variety of station identifier schemes and quality-control procedures, and some of the data have only recently been digitized from paper records. Here, the first systematic compilation of pre-IGY data is made, and a novel quality-assessment technique is applied, which reveals that much of the early data have uncorrected radiation and lag errors, especially in the former Soviet Union. Incorrect geopotential height units and problematic time stamps were also found. The authors propose corrections and present corrected hemispheric fields that show large changes and improved internal consistency in height and temperature across Eurasia compared with uncorrected data. The corrections are important, especially as they have a clear spatial structure that interferes with the planetary wave structure. These corrected data are useful for climate studies and considerably enhance the length and quality of the upper-air record but may not be suitable for trend analysis. Assimilation of the uncorrected data has led to a widespread warm bias in NCEP–NCAR reanalysis in the 1950s.

The 1935–2003 Air Temperature Record from the Summit of Mount Washington, New Hampshire

Abstract Meteorological observations have been taken continuously at the summit of Mount Washington since 1932. Results of an analysis of the air temperature record over the 1935–2003 period show a statistically significant increase in mean temperature of ∼0.3°C, while the diurnal temperature range has decreased by ∼0.15°C. The decadal structure evident in the record reveals that, in contrast to North American trends, the summit experienced relatively cool temperatures in the 1940s. The late 1980s and early 1990s were relatively warm on the summit, in agreement with North American decadal trends. The times of daily maximum and minimum temperatures show that the summit climate is dominantly influenced by boundary layer processes 30% of the time and free air circulation 50% of the time. No evidence of a “weekend effect” was found.

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.

Dewpoint and Humidity Measurements and Trends at the Summit of Mount Washington, New Hampshire, 1935–2004

Abstract Meteorological conditions have been recorded at the summit of Mount Washington, New Hampshire, (44°16′N, 71°18′W, 1914 m ASL) since November 1932. Use of consistent instrumentation allows analysis of humidity measurements as calculated from error-checked dry bulb temperature, wet bulb temperature, and pressure during the period 1935–2004. This paper presents seasonally and annually averaged dewpoint temperature, mixing ratio, and relative humidity means and trends, including clear-air and fog subsets and, beginning in 1939, day and night subsets. The majority of linear trends are negative over the full study period, although these decreases are not constant, with relatively large (small) values in the mid-1950s (late 1970s). Annual mean dewpoint (water vapor mixing ratio) over the 70-yr period has decreased by 0.06°C decade−1 (0.01 g kg−1 decade−1). During this period the annual frequency of fog increased by 0.5% decade−1. Dewpoint and mixing ratio trends, both generally decreasing, differ by sea...

A Monthly Upper-Air Dataset for North America Back to 1922 from the Monthly Weather Review

Abstract Upper-air observations with kites, aircraft, and radiosondes were performed in the United States operationally since the 1890s. In this paper, the authors present a reevaluation of newly digitized monthly mean values from the Monthly Weather Review back to 1922. Data from 46 U.S. weather stations are presented with a focus on early kite and aircraft observations during the 1922–38 period. A quality assessment of the data, based on reconstructed reference series, is carried out and the quality of the monthly mean data is found to be sufficient for analysis of variability in upper-level circulation. Anomalies of upper-level temperatures from the reevaluated station data, together with surface fields, are shown for selected periods.

NOAA CIRES Twentieth Century Global Reanalysis Version 2

The Twentieth Century Reanalysis Project, supported by the Earth System Research Laboratory Physical Sciences Division from NOAA and the University of Colorado CIRES Climate Diagnostics Center, is an effort to produce a global reanalysis dataset spanning a portion of the nineteenth century and the entire twentieth century (1871 - near present), assimilating only surface observations of synoptic pressure, monthly sea surface temperature and sea ice distribution. Products include 6-hourly ensemble mean and spread analysis fields on a 2 by 2 degree global latitude-longitude grid, and 3 and 6-hourly ensemble mean and spread forecast (first guess) fields on a global Gaussian T62 grid. Fields are accessible in yearly time series (1 file per parameter) and monthly synoptic time (all parameters per synoptic hour) files. Ensemble grids, spectral coefficients, and other information will available by offline request in the future.\n\n The Twentieth Century Reanalysis Project used resources of the National Energy Research Scientific Computing Center managed by Lawrence Berkeley National Laboratory and of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which are supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and Contract No. DE-AC05-00OR22725, respectively.\n\nNote: Version 2c of this reanalysis (running from 1851 - 2011) is the recommended research version. Please see ds131.2 to access Version 2c.