Diapositivo 1

1
GM versus LAM one case of deep
convection (27 August 2001) Manuel João
Lopes Margarida Belo Pereira Pedro Miranda
Instituto de Meteorologia Faculdade de Ciências
da Universidade de Lisboa
25th EWGLAM 10th SRNWP meetings Lisbon, 6-9
October 2003
2

• Motivation
• 1. Comparison of ECMWF and ALADIN/Portugal
  precipitation forecasts in deep
• convection situations
• 2. Verification of the usefulness of some of the
  diagnostic tools once calculated
• from the analysis and from forecast of each model
• 3. Determination of thresholds of stability
  indices for deep convection
• 4. Application of the diagnostic tools on weather
  forecast Centre of Instituto de
• Meteorologia

3

• Three important physical mechanisms for
  convection
• - Convective Instability
• - Orography
• Humidity convergence in low troposphere

Stability Indices
Convective Instability Index 0.5(?e2m ?e925 )
- ?e500 (possible threshold for
deep convection 10ºC)
Jefferson Index 1.6?sw850 0.5 (T700
Td700) T500 - 8 (possible threshold for deep
convection 32ºC)
This thresholds of indices were determined by
Belo Pereira in her first study of deep
convection.
4
Radar image at 1520UTC reflectivity (dBZ)
regions with stronger convective activity
Observed lightening strikes for the period
00-23UTC of 27 Aug 2001 (Information from site
http//www.wetterzentrale.de/topkarten/)
regions with weaker convective activity
The precipitation was associated to MCS that
developed in the North of Portugal and to smaller
scale convective cells located in the western
part of Central region of Portugal. Most of the
convective precipitation occurred in 1 hour
interval.
5
Forecasted precipitation on the 12UTC run of 26
August - ECMWF and ALADIN
Step H30 -gt Accumulated precipitation in 6 h
12-18UTC (contour interval is 1 mm)
ECMWF model
ALADIN model
Observations
24
32
10
9
13
7
2
1

• (Note The contour of Peninsula Iberia shows a
  displacement to NE of 0.25º (approximately) due
  to a software problem ! )
• Both models underestimate the precipitation but
  ALADIN forecasts a larger area of precipitation
  extended to the southeast of Spain. According to
  the convective developments observed on NOAA
  satellite images, this appears to be realistic.
• Neither ECMWF or ALADIN forecasts the
  precipitation associated to the convective cells
  that actually developed in some regions of the
  Centre of Portugal (Estremadura, Ribatejo and
  Beira Litoral) during this period.

6
Forecasted precipitation on 12UTC run of 26
August - ECMWF and ALADIN
Step H36 -gt Accumulated precipitation in 6h
18-24UTC (contour interval is 1 mm)
ECMWF model
ALADIN model
Observations

1. ECMWF forecasts no rain in Portugal !!!
2. But ALADIN forecasts more than 10mm in the area
   of northeast border of Portugal warning
   forecasters to the possibility of heavy showers
   in this region . The reflectivity pattern in
   MAX(Z) reveals the presence of reflectivity cores
   with reflectivity levels above 54 dBZ which may
   indicates strong convective activity.
3. However, ALADIN underestimates the occurred
   precipitation (for instance, forecasts only 3mm
   in Vila Real area, where it was recorded 26mm/1h
   at 19UTC).

7
Forecasted precipitation on 00UTC run of 27
August - ECMWF and ALADIN
Step H18 -gt Accumulated precipitation in 6h
12-18UTC (contour interval is 1 mm)
ECMWF model
ALADIN model
Observations
24
32
10
9
13
7
2
1

• The ALADIN forecast from 00UTC is better than the
  one of 12UTC (from the day before), because the
  area and the amount of forecasted precipitation
  is more in accordance with the observations. For
  instance, in the region of Serra da Estrela it
  forecasts 11.5mm in this period alerting to the
  possibility of heavy showers.
• On the contrary, ECMWF forecast from 00UTC is
  worse than the one of 12UTC (from the day
  before), because the amount of forecasted
  precipitation is more underestimated in the North
  of Portugal.
• Again, both models forecast no rain in the
  western part of Central region of Portugal (
  Beira Litoral, Estremadura and Ribatejo).

8
Forecasted precipitation on 00UTC run of 27
August - ECMWF and ALADIN
Step H24 -gt Accumulated precipitation in 6h
18-24UTC (contour interval is 1 mm)
ECMWF model
ALADIN model
Observations
1
2
26

1. ECMWF forecasts no rain over Portugal, during
   this period.
2. For this period, the ALADIN forecast from 00UTC
   is also better than the one of 12UTC (from the
   day before), because the area and the amount of
   forecasted precipitation is more in accordance
   with the observations.

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Physical mechanisms to convection in ECMWF
analyses at 12UTC
Vertical vel. (?) at 700hPa (contour interval is
0,2 Pa s-1)
?700 lt 0
?700 gt 0
?700 lt 0
Dif. Vorticity advec.(1000-500hPa) contour
interval is 0,3?10-11s-2Pa-1
Temperature advec.(850hPa) contour interval is
0,3ºC/h
Radar image at 12UTC reflectivity (dBZ)
According with the surface analysis of ECMWF the
situation is determined by an high pressure
located in Ireland and a low (1013hPa) located
southwest of Portugal. The differential
vorticity advection contributes for upward motion
in the areas of convection, except in the region
of river Tejo Valley. The warm advection (WA)
contributes for upward motion in the areas of
convection except in the Northwest of Portugal.
WA
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Physical mechanisms to convection in ECMWF
analyses compared with ALADIN analyses 12UTC
-gt Humidity convergence
ALADIN model
ECMWF model
Radar image at 12UTC reflectivity (dBZ)
Specific humidity converg. at 850hPa (contour
interval is 2?10-4g kg-1s-1)

• The analysis of ALADIN are consistent with ECMWF
  analysis but there are some important differences
  in terms of humidity and stability
• In the areas with convection, the analysis of
  humidity convergence (at 850hPa) of ECMWF
  indicates convergence in the North and the South
  of Portugal but indicates divergence in the
  Central part of Portugal.
• The analysis of humidity convergence of ALADIN
  indicates convergence in the areas with
  convection over Portugal and indicates divergence
  in Alentejo, where there was no convection at
  this time.

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Physical mechanisms to convection in ECMWF
analyses compared with ALADIN analyses 12UTC
-gt Instability
Radar image at 12UTC reflectivity (dBZ)
ECMWF model
ALADIN model
Convective Instability Index (contour interval is
1ºC)

• Considering threshold of 10ºC for convective
  instability index and comparing with observations
  (precipitation radar), analysis of ECMWF and
  ALADIN are reasonable in the northern regions
  where was already convection.
• None of the analysis indicates conditions for
  deep convection in the region of river Tejo
  Valley and in the western part of the South
  regions where convection also occurred.
• ECMWF analysis indicates also conditions for deep
  convection in the area of the Atlantic Ocean
  adjacent to the northwest of Portugal, where
  convection occurred only after 15UTC.
• In both models, the forecast for 12UTC are not
  significantly different from the analysis of
  12UTC. However, humidity convergence forecasts
  are much worse. For instance, in the areas with
  convection, ECMWF forecasts divergence for
  southern regions while, in the areas with no
  convection, ALADIN forecasts convergence for the
  region of Serra da Estrela.

12
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H15
Forcing to upward motion
Radar image at 15UTC reflectivity (dBZ)
ECMWF model
ALADIN model
Vertical velocity (?) at 700hPa (contour interval
is 0,2 Pa s-1)

1. ECMWF forecasts upward vertical motion (at
   700hPa) in the whole country.
2. In the regions with convection, ALADIN forecasts
   upward vertical motion except in the western part
   of the Central region of Portugal.

13
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H15
-gt Humidity Convergence
Radar image at 15UTC reflectivity (dBZ)
ALADIN model
ECMWF model
Specific humidity converg. at 850hPa (contour
interval is 2?10-4g kg-1s-1)

1. In the regions with convection, ECMWF forecasts
   humidity convergence (at 850hPa) in a part of the
   northern region and divergence in the western
   part of the Central region.
2. ALADIN forecast indicates humidity convergence in
   the regions where convection occurred.

14
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H15
-gt Instability
ALADIN model
ECMWF model
Radar image at 15UTC reflectivity (dBZ)
Convective Instability Index (contour interval is
1ºC)

• Both models forecasts convective instability in
  the northern region where MCS developed.
• ECMWF also forecasts convective instability in
  the area of the Atlantic ocean adjacent to the
  northwest of Portugal, where convection occurred
  after 15UTC.
• ECMWF forecast indicates conditions for deep
  convection (e.g., the presence of upward vertical
  motion, humidity convergence and instability) in
  a part of the northern region where convection
  developed.
• ALADIN forecast indicates conditions for deep
  convection except in the western part of Central
  region of Portugal

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Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H15
-gt Instability
Radar image at 15UTC reflectivity (dBZ)
ALADIN model
ECMWF model
Jefferson Index (contour interval is 1ºC)
In both models, the forecasted of Jefferson
index leads to the same results as the forecast
of convective instability index (e.g., the
forecasted area with conditions for deep
convection is identical).
16
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H18
Forcing to upward motion
Radar image at 18UTC reflectivity (dBZ)
ECMWF model
ALADIN model
Vertical velocity (?) at 700hPa (contour interval
is 0,2 Pa s-1)

1. In the regions with convection, ECMWF only
   forecasts vertical upward motion in the area of
   the northeast border of Portugal.
2. ALADIN forecasts upward vertical motion in whole
   area with convection.

17
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H18
-gt Humidity Convergence
ALADIN model
ECMWF model
Radar image at 1750UTC reflectivity (dBZ)
Specific humidity converg. at 850hPa (contour
interval is 2?10-4g kg-1s-1)
Specific humidity convergence at 850hPa (isoline
of 2?10-4g kg-1s-1)

1. ECMWF only forecasts humidity convergence in the
   area of the northeast border of Portugal.
2. ALADIN forecasts humidity convergence in the
   regions with convection, except in a small area
   of the northeast regions.

18
Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H18
-gt Instability
ALADIN model
ECMWF model
Radar image at 1750UTC reflectivity (dBZ)
Convective Instability Index (contour interval is
1ºC)

• Both models forecasts convective instability in
  the northern regions where MCS developed.
• While ECMWF forecasts only indicates conditions
  for deep convection (e.g., the presence of the
  three mechanisms to convection) in the area of
  the northeast border of Portugal, ALADIN forecast
  indicates the same conditions in the regions with
  convection except in a small area of the
  northeast regions.

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Physical mechanisms to convection in ECMWF
forecast compared with ALADIN forecast H18
-gt Instability
ALADIN model
ECMWF model
Radar image at 1750UTC reflectivity (dBZ)
Jefferson Index (contour interval is 1ºC)

• In the regions with deep convection, ECMWF
  forecast of Jefferson index doesnt indicates
  instability in the area of Serra da Estrela.
• ALADIN forecast for this index includes the whole
  region with deep convection.
• Again, in both models the forecasted of
  Jefferson index leads to the same results as the
  forecast of convective instability index (e.g.,
  the forecasted area with conditions for deep
  convection is identical).

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• Conclusions
• Both models underestimate precipitation but
  ALADIN forecasts more than 10mm/6h in the periods
  12-18UTC and 18-24UTC, which in convective
  situations can mean heavy showers due the
  concentration of precipitation in very short
  periods.
• Opposite to ECMWF, ALADIN improves its
  performance from 12UTC run of 26 August to 00UTC
  run of 27 August increasing the amount and the
  area of precipitation in agreement with the
  highest instability region (e.g., the region with
  heavy showers and thunderstorm).
• Nevertheless, none of the models forecasts
  precipitation associated to convective cells
  which actually developed in some areas of the
  Central and Southern regions of Portugal. This
  may be possible for instance if the LSM and LAM
  underestimate the instability in these regions or
  if the convective phenomena were completely
  filtrated due to their scale.
• In this situation, the use of diagnostic tools
  was very important to indicate conditions of deep
  convection, specially in the case of ECMWF where
  the forecast of rain didnt give an alert.
• We hope that with the validation of diagnostic
  tools and the adjustment of the indices
  thresholds the regions with deep convection can
  be better defined.