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1. The Tropical Tape Recorder of the Unified Model
Ross Bannister , Andrew Gregory , Alan O'Neill
, Julia Slingo, William Lahoz  and Katrin Nissen .
 Department of Meteorology,University of Reading, UK.
 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'
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.