systematic errors in medium-range forecasts of tropopause structure

www.met.reading.ac.uk/~sws98slg

Systematic errors in medium-range forecasts of tropopause

structure

Suzanne Gray, Caroline Dunning, John Methven, Giacomo Masato, Jeffrey

Chagnon. University of ReadingSeptember 2013

Potential vorticity

Potential vorticity is conserved following fluid parcels for adiabatic frictionless flow.

This makes it a good tracer for upper-tropospheric air over several days.

Climatology of PV (in PVU) and in NH winter (Hoskins,

1990)

tropopause

270K

330K

Potential vorticity

Rossby Waves(or planetary waves): PV conserving motion that owes its existence to the isentropic gradient of PV.

ECMWF analyses of PV on 315K isentrope.

Forecast errors in upper-level PV

Rossby wave amplitude at the extratropical tropopause inadequately developed in ECMWF operational forecasts: DJF 2001-2 so 10 years ago (Dirren et al. 2003).

PV on 320 K isentrope (analysis and 96h forecast): 1200 UTC 16 Jan 2002 (Davies and Didone 2013)

(Davies and Didone 2013)

Mechanisms of Rossby-wave growth

Initial time

Later time

(Davies and Didone 2013)

Mechanisms of Rossby-wave growth

Band of anomalously low PV on the equatorward side of the tropopause

Deflects tropopause equatorward

Schematic illustration of the diabatic PV dipole relative to the tropopause in Rossby wave – based on

extratropical cyclone case study

longitude

(Chagnon et al. 2012)

Mechanisms of Rossby-wave growth

Extratropical cyclone influence on Rossby wave growth

Intersecting WCB1 air parcels on 315K surface

Intersecting WCB2 air parcels on 305K surface

Martínez-Alvarado et al (submitted)

analysis

Long lead time forecast

Control run

Reduced LH

Forecast errors in upper-level PV

TIGGE forecasts for Nov 2009 case study (MSc thesis, Sideri 2013 supervised by Chagnon and Martínez-Alvarado)

Quantification of the systematic error in tropopause structure in

medium-range weather forecasts.

AIM

• Data extracted from the TIGGE (THORPEX Interactive Grand Global Ensemble) archive*.

• Daily (12 UTC) northern hemisphere fields of PV on the 320K isentrope used from the control runs

• Three operational centres: ECMWF, the Met Office, and NCEP

• Six winter seasons (December, January and February from 2006/7 (2008/9 for NCEP) to 2011/12).

Data

*The TIGGE archive consists of global model ensemble forecast data from ten NWP centres, starting from October 2006: see http://tigge.ecmwf.int/

Two types:

i.Amplitude error

ii.Location error

Example forecast errors

PV on 320K isentrope

Define using

i.PV value (greater or less that the assumed tropopause value: 2.24PVU assumed here).

ii.Equivalent latitude (north or south) where the equivalent latitude is the limiting latitude if the area, A, in which the PV on an isentope < tropopause PV is reshaped into a pole-centred circle.

Categorisation

Hemispheric errors

Average RMS forecast – analysis difference.

Hemispheric errors

Average RMS forecast – analysis difference scaled by mean analysis PV for forecast centre.

Ridge PV gradient forecast

Ridge area forecast

Reduced forecast resolution after day 10

2.55x107 km2

2006/72007/82008/92009/102010/112011/12

Ridge PV forecast

1. Upper-level PV forecast errors in operational global models saturate after about 9 days.

2. Met Office and ECMWF forecasts show a decrease in ridge area with forecast leadtime and increase in mean ridge PV out to 5 days.

3. NCEP results vary strongly with year but also show an increase in mean ridge PV with forecast lead time.

4. The forecast biases in PV in analysed ridges are consistent with a reduction in (i) the Rossby wave amplitude and

(ii) PV gradient across the tropopause5. Mechanism causing error growth is not proven here but ……

…errors are consistent with a systematic failure of forecasts in the representation of the outflow of air with a negative

anomalies of PV from diabatic processes in warm conveyor belts into ridges.

Conclusions

Example forecast errors

PV on 320K isentrope

Ridge area forecast:

tropopause PV = 3.35 PVU

Ridge PV forecast

tropopause PV = 3.35 PVU

Instantaneous heating

Steady heating

Sources and sinks

PV dipole arising from heating applied in a barotropic environment

20

0g g

NDPV

fDt z

H where H is the

heating rate

Tropopause erosion and/or upper-level divergence.

Associated with ‘type C’ cyclogenesis (Plant et al., 2003).

PV effect on cyclones: direct effect

PV effect on cyclones: indirect effect

Jet enhancement leading to modified Rossby wave propagation