The Seasonal Cycle of Polar Specific Humidity Inversions in Four Reanalyses

Specific humidity is generally thought to decrease with height in the troposphere, but specific humidity inversions, layers in which specific humidity increases with height, have been previously observed. Here, we document the climatology of these inversions in four reanalyses [the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Prediction second reanalysis (NCEP-2), the European Centre for Medium-Range Forecasts (ECMWF) 40-year reanalysis (ERA-40), the National Aeronautics and Space Administration (NASA) Modern Era Retrospective Analysis for Research Applications (MERRA), and NOAA's Climate Forecast System Reanalysis (CFSR)]. Humidity inversions are most frequent in the polar regions, including the Arctic. Polar inversions are the most persistent in winter and the strongest (as defined by the humidity difference across the inversion divided by the pressure difference across the inversion) in summer or autumn with low bases (at pressures > 900 hPa in the Arctic). These inversions are confirmed by rawinsonde data, but there are discrepancies between the observed annual and diurnal cycle in inversion frequency and those portrayed in the reanalyses. The reanalyses will also be used to explore the dominant conditions under which these humidity inversions form in the polar regions. Such an analysis can be used to evaluate the performance of global coupled climate system models in producing such humidity inversions. For instance, several models from the latest phase of the Coupled Model Intercomparison Project (CMIP5) [the National Center for Atmospheric Research (NCAR) Community Climate System Model version 4 (CCSM4), and Community Earth System Model (CESM1), the National Aeronautic and Space Administration (NASA) Goddard Institute for Space Studies (GISS) Model E version 2 (GISS-E2), the Japanese Model for Interdisciplinary Research on Climate version 4h (MIROC4h), and the Centre National de Recherches Météorologiques (CNRM) Climate Model version 5 (CNRM-CM5)] simulate similar specific humidity inversions in the polar regions to those documented in the reanalyses.