Stratospheric water vapour and tropical tropopause temperatures in ECMWF analyses and multi-year simulations

Abstract

Stratospheric humidity analyses produced operationally by the European Centre for Medium-Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near-tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar-night vortex. The lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Niiio are illustrated.Analysed temperatures near the tropical tropopause are generally in good agreement with corresponding radiosonde measurements, with standard-level biases of the order of 0.5 degC or less. The past two years are the coldest by about 1 degC in a series of tropical mean 100 hPa analyses extending back to 1979. A cooling trend of about 0.6 degC per decade is seen in the global means of the 100 hPa analyses.Moisture is spread zonally and upward from the tropical tropopause as the data assimilation proceeds, but the rate of upward transfer is much faster than observed. Substantial lateral mixing can occur within the stratosphere over the course of a season. Moistening at middle and high latitudes due to mixing with more humid tropospheric air is confined, realistically, to a shallow layer at the base of the stratosphere.The rate of upward transfer of tropical stratospheric moisture is much more realistic in a multi-year simulation using a version of the model that has finer stratospheric resolution than the version used for the operational data assimilation. Temperatures at the tropical tropopause and in the Antarctic polar night are accurately simulated, apart from excessive persistence of cold south-polar temperatures in late winter and early spring. The latter is conducive to drying the model stratosphere; lack of a parametrization of moistening due to methane oxidation is an obvious deficiency in this regard.

The role of the south‐east Asian monsoon and other seasonal features in creating the ‘tape‐recorder’ signal in the Unified Model

“…The ERA-40 dataset does exhibit a tape-recorder signal (Oikonomou, personal communication), but this was determined largely by the (60-level) forecast model used in the assimilation. Its tape speed is roughly twice that found in an earlier (50-level) free-running model of the ECMWF (Simmons et al 1999). The difference between the free-running model and the assimilation probably results from erroneous vertical wind increments imposed indirectly by the assimilation.…”

The ASSET intercomparison of stratosphere and lower mesosphere humidity analyses

“…Therefore, the assimilation of these humidity data would be of minimal use. Currently, the operational assimilation of humidity data in the stratosphere by Numerical Weather Prediction (NWP) centres is limited by the availability of suitable data (Simmons et al, 1999). For example, the quality of humidity observations from radiosondes decreases with decreasing water vapour and thus stratospheric water vapour observations from radiosondes are not very useful (Schmidlin and Ivanov, 1998), whilst operational satellite sounders tend to be only sensitive to tropospheric water vapour (e.g.…”

The Influences of TOVS Radiance Assimilation on Temperature and Moisture Tendencies in JRA-25 and ERA-40

“…Descriptions of ERA-40's forecast and DA systems are found in Uppala et al (2005), Jakob et al (2000), and Simmons et al (1999), and the system is based on Integrated Forecast System (IFS) cycle 21r4 (Jakob et al 2000). The semi-Lagrangian advection scheme (SL) was introduced at cycle l8r5 (the system introduced in April 1998).…”