Tape Recorder in the UM

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1. The Tropical Tape Recorder of the Unified Model
Ross Bannister [1], Andrew Gregory [1], Alan O'Neill [1], Julia Slingo[1], William Lahoz [1] and Katrin Nissen [2].
[1] Department of Meteorology,University of Reading, UK.
[2] Department of Meteorology,University of Edinburgh, UK.
The `tape recorder' water vapour signal in the tropical stratosphere has been observed for a number of years by instruments aboard the UARS satellite. Its character (upward advection speed, signal amplitude, etc.) may depend upon a number of atmospheric processes. The ability of a GCM to reproduce a realistic tape recorder signal provides an important test of the model. The GCM may also be used as a tool to help understand the mechanisms most important for water vapour control.

Analysis of the AMIP-II run of the 58-level Unified Model (UM) has shown that the UM can simulate a clear tape recorder-style variation in water vapour. An assessment is made of key aspects of this tape recorder, which are compared to observations. Crucial to the tape recorder is the underlying structure of the water vapour field in the upper troposphere and its seasonal variation. The UM has been used to help identify the regions most relevant to the troposphere-stratosphere exchange of water vapour. These are conventionally thought to be the localities of `colder than average' tropopause temperatures, where the air is `freeze-dried' (temperature controlled). Results from the UM challenges the view that only temperature has the essential influence of the entry values of stratospheric water vapour.

2. What is the 'tape recorder' signal?
The troposphere and stratosphere are environments of distinctly different character. The processes which are important for the exchange of mass between the two (called strat./trop. exchange or STE) are generally not well understood. In many observational and modelling studies, the distributions and time evolution of tracer concentrations are examined to help shed light on this problem. Water vapour in the lower and middle atmosphere is a particularly useful tracer, but is difficult to model well owing to the variety of mechanisms which it is potentially influenced by. We highlight two classes of mechanism: (i) sources and sinks (e.g. condensation and convection), and (ii) 'resolved-flow' advective processes.

In the troposphere, the water vapour field is affected principally by diabatic 'source-and-sink' processes. Above the temperature-minimum of the tropopause however, there are few sources and sinks, and water vapour behaves as a conserved tracer (advection-dominated regime). Like for any long-lived species, transport of water vapour here is governed by the upward, and poleward residual Brewer-Dobson (BD) circulation.

The conditions described above conspire to yield the so-called 'tape recorder' signal in the tropical middle atmosphere. Air enters the stratosphere in the tropics. Here the water vapour delivered is seasonally varying ('moist' (4.5 mg/kg) in JJA and 'dry' (3.5 mg/kg) in DJF (This is the 'record head' of the tape recorder analogy. Diabatic heating, accompanying the BD circulation, lifts the signal systematically. The BD circulation is slow (30 m/day) and vertical mixing of tracer is weak, ensuring that the retarded signal persists at elevated levels of the middle atmosphere and after many months. This memory of tropopause history is the 'magnetic tape'. Some pictures of observed and modelled tape recorder signals are shown in our poster (found on the UGAMP website - look on the main page or under "previous hot topics").

3. Aims of our Work
In order to gain a clearer picture of the tape recorder and its related processes, we have used the 58-level, stratosphere-troposphere configuration of the UK Met. Office Unified Model (UM). We aim to assess the main factors which affect the water vapour in the upper troposphere and lower stratosphere. In order to represent the water vapour values and their seasonality near the tropopause 'source' region correctly, the model must simulate diabatic processes realistically. Although the BD circulation itself requires diabatic heating for mass to cross the quasi-horizontal isentropes, an aspect of the model crucial to transport is the advection scheme performance. The slow motion, and the relatively crude vertical level spacing presents a very severe test of any advection scheme. A problem common to most general circulation models is that tracers are advected more quickly than the wind speed (hence a faster tape recorder than observed).

Some of our aims:

To assess the tape recorder in the UM.
To determine the source regions of the 'dry' and 'moist' air.
To check the relevance of our hypothesis that the entry of water vapour is restricted thermodynamically (ie via temperature) during northern winter, but is controlled by transport during the summer. These are two fundamentally different regimes. Conventional belief is that the thermodynamic mechanism is active through all seasons.

4. Further Information
We have a poster on the UGAMP website - look on the main page or under "previous hot topics"

Our paper "Tropical Cross-Tropoause Transport of Water Vapour: Part 2, Diagnosis of Entry Regions and Mechanisms Affecting Water Vapour in the Unified Model" is in preparation.