High resolution models: Tropical Convection and Transport through the Tropical Tropopause Layer

Maria Russo, Scott Hosking, Peter Braesicke, John Pyle

Convection and the Tropical Tropopause Layer (TTL)

Picture based on Gettelman and Forster, 2002

Height km (K)

17

15

12

10

380

360

350

340

Active convective region Less active convective region

Cold point Tropopause

Q=0

Lapse rate minimum

TROPICS

TTL

P (hPa)

90

130

200

280

Troposphere-Stratosphere transport: what do we know?

Processes responsible for Troposphere-Stratosphere Transport: 1. Direct injection by convection penetrating into the lower stratosphere

2. Mixing in UTLS due to the breaking of convectively generated gravity waves

3. Slow radiative ascent above Q=0 level

4. Isoentropic transport from TTL to extra tropical lower stratosphere

These processes have different timescales: their relative contribution to the stratospheric composition will vary for species with different lifetimes.

Troposphere-Stratosphere transport: what do we know?

Newell & Gould-Stuart,1981: ‘Stratospheric fountain’ hypothesis (preferential entry into the stratosphere at specific times and places characterised by lowest CPT temperatures, e.g. West Pacific)

Gettelman et al., 2002; Liu and Zipser, 2005; Rossow and Pearl, 2007: frequency of convection penetrating the stratosphere is estimated from satellite data to be low (~0.5-1% of total number of storms)

Gettelman et al., 2004: detailed study of radiative heating rate (Q). Q=0 level found ~15km, 3km above convective outflow and 2km below CPT

Bonazzola & Haynes, 2003; Flueglistaler et al. 2005: air enters the TTL over West Pacific, is dehydrated, and subsequently moved by fast horizontal transport into extra tropical lower stratosphere

Ricaud et al., 2007: impact of convection found in satellite trace gas distributions up to ~16-17km

Scientific Objectives

Identify seasonal/regional preferences for air entering the stratosphere

Estimate the relative contribution of convective transport vs large scale transport in determining the composition of the lower stratosphere

Simulate transport of short-lived ( ~1-3weeks) halogenated hydrocarbons (bromoform, methyl iodide); biogenic emissions from tropical shallow waters.

Investigate the ability of convective parametrization in models (e.g. UM) to give a reasonable representation of convective transport of water and trace gases. Effect of model resolution on convective transort.

ToolsGlobal UM at various resolutions (N216, N96)

Mesoscale UM, relocated to tropical domains (Australia) at various resolutions down to convective permitting simulations (~1kmx1km)

Recent Tropical Campaigns:

SCOUT-O3: Nov-Dec ’05(Darwin, Australia)

ACTIVE:Nov’05-Feb’06(Darwin, Australia)

SCOUT-AMMA: Jul-Aug ’06(West Africa)

HIBISCUS: Feb ’05(Bauru, S. America)

Model Setup

N216 L38 (~60km): 1 month global run for November 2005

• Physics setup based on UKMO operational forecast

• Initial conditions from UKMO data-assimilated dump for Nov’05

• Forced by climatological, SST & sea ice

• Ozone, soil moisture and soil temperature constrained by climatological values

Added idealised tracer with different source regions, and exponential decay rates.

Idealised tracers

Tropical Convection: comparison with satellite

UM montly mean OLR NOA monthly mean OLR

Nov’05: Outgoing Longwave Radiation (OLR)

Tropical Convection: comparison with satellite

Model Satellite

Nov ’05: Total Precipitation Rate

UM montly mean precip. rate CMAP monthly mean precip. rate

Tropical Convection penetrating Q=0 level

Monthly mean convective cloud top height % of days with CCT penetrating Q=0

Tracer transport by tropical convection

Monthly mean surface tracer, τ=6hMonthly mean model OLR

Tracer transport by tropical convection

Timeseries, surface tracer, τ=6h

Tracer transport by tropical convection

Monthly mean surface tracer, infinite lifetime

Tracer with convective transport Tracer with no convective transport

FISH data from SCOUT-O3

Water vertical distribution in the UTLS

TOTAL WATER FROM ‘FISH’ INSTRUMENT

Summary

• We setup and run the UM (N216 L38) for 1 month (Nov ’05)

• A number of tracers with different lifetimes and source regions have been included in the simulation to investigate convective transport to the UTLS

• The location and intensity of tropical convection in the model compares well with satellite data when averaged over a month

• We are investigating the effect of convective transport on idealised tracers and water. This has implication for chemistry climate models and climate runs.

• Model simulations suggest that short lived species can be transported above the Q=0 level within a day.

Further work

• Run for other months (August 2006 – AMMA period)

• Analysis of convection and convective transport at other resolutions: lower (N96L38 global), higher (~12km mesoscale)

• Validate the model against campaign data

• Identify regional preferences for air entering the TTL and lower stratosphere (tracers budgets)

• Estimate the relative contribution of convection and large scale transport in determining the composition of the TTL and lower stratosphere

Extra Slides

Unified Model details:

UK Met Office Unified Model version 6.1

• Non hydrostatic model with hybrid (height) vertical coordinate.

• Charney-Philips grid-staggering in the vertical, Arakawa C-grid staggering in the horizontal.

• Edwards-Slingo radiation scheme with non-spherical ice spectral files.

• CAPE closure convection scheme, including tracer and momentum transports and convective anvils.

• Parametrized entrainment and detrainment rates for shallow convection (Grant and Brown, 1999).

• Large-scale precipitation scheme by Wilson and Ballard, 1999).

• The MOSES (Met Office Surface Exchange Scheme) surface hydrology and soil model scheme (Cox et al., 1999).

Monthly mean OLRCGAM current Climate for Nov ‘05

UM 1 month global run constrained by Nov climatological SST and sea ice

NOA monthly mean satellite OLR for Nov ‘05

Monthly mean precipitationCGAM current Climate for Nov ’05

UM 1 month global run constrained by Nov climatological SST and sea ice

CMAP precipitation for Nov ‘05

Convective cloud top height (km)

60km MODEL 12km MODEL

Overview of the TTL and possible mechanisms for troposphere-stratosphere exchange

Overworld

ELS ELS

Picture courtesy of James Levine (modified from Holton et al. 1995)