Transcript Assessment of Environmental Stability and RUC10 Forecasts of Storm Initiation (Roberts)

Assessment of Environmental
Stability and RUC10
Forecasts of Storm Initiation
Rita Roberts and Jim Wilson
National Center for Atmospheric Research
Objective
To examine the capability of numerical
models (e.g. RUC 10) to provide 0-6 hr
prediction of precipitation initiation and
evolution?
Model Performance

Forecasting precipitation initiation and
evolution
Identify storm initiation episodes
(area of new storms initiated by common forcing mechanism)
Sample initiation episode
a)
b)
112 Identified
Statistics on RUC 10km Model (3hr)
Precipitation Initiation Forecasts
YES - Forecasts
No - Forecasts
18
Number of Events
16
14
12
10
Surface-Based
Elevated
8
6
4
2
0
No Offset
Good
Forecast
Spatial
Temporal
(50-250km) (1-5 hr)
Offset
Offset
Ability of RUC 10 km Model to Initiate Precipitation
Initiation
Mechanism
Fronts
Number
of Events
23
% Time
% Time
Precipitation Precip. Area
is forecast
too large
83
40
% Time
Fcast is late
30
Surface
Lows
100 with fronts
75 or near fronts
25
4 associated
Best forecasts
are
Elevated
(frontal)
11
83
56
56
Elevated
(isolated)
39
68
35
35
Convergence
Boundaries
32
62
40
50
Initiation Zone
16:40 – 18:00
FRONTS
4 June 2002
15:00
Valid 18:00
3hr RUC
Forecast
Yellow contours = 35 dBZ
echo at forecast time
12:00
Valid 18:00
6hr RUC
Forecast
Initiation
Cold Front
4 June 2002
RUC Analysis and
Observations at
15:00 UTC
RUC CAPE
RUC CIN
Reflectivity echoes
overlaid
Surface-Sounding CAPE
Surface-Sounding CIN
Evolution
Cold Front
4 June 2002
RUC Analysis and
Observations at
18:00 UTC
RUC CAPE
RUC CIN
Reflectivity echoes
overlaid
Surface-Sounding CAPE
Surface-Sounding CIN
Decay
Cold Front
4 June 2002
RUC Analysis and
Observations at
23:00 – 00:00 UTC
RUC CAPE
RUC CIN
Reflectivity echoes
overlaid
Surface-Sounding CAPE
Surface-Sounding CIN
Convergence
Reflectivity echoes
are overlaid
Ability of RUC 10 km Model to Initiate Precipitation
Initiation
Mechanism
Number
of Events
% Time
% Time
Precipitation Precip. Area
is forecast
too large
83
40
% Time
Fcast is late
Fronts
23
30
Surface Lows
4
100
75
25
Elevated
(frontal)
11
83
56
56
Elevated
(isolated)
39
68
35
35
Convergence
Boundaries
32
62
40
50
Surface Lows
28 May 2002
Initiation Zone
17:40-19:40
15:00
Valid at 18:00
Reflectivity
Slide 30
3 hr RUC10 Forecast
Yellow contours = 35 dBZ echo at forecast time
Surface Low
28 May 2002
18:00 UTC
Reflectivity
RUC CAPE
Convergence
RUC CIN
Ability of RUC 10 km Model to Initiate Precipitation
Initiation
Mechanism
Number
of Events
% Time
% Time
Precipitation Precip. Area
is forecast
too large
83
40
% Time
Fcast is late
Fronts
23
30
Surface Lows
4
100
75
25
Elevated
(frontal)
11
83
56
56
Elevated
(isolated)
39
68
35
35
Convergence
Boundaries
32
62
40
50
Surprise that the model forecasted 68% of these events
13 June 2002 – 3hr RUC10 Forecasts
6:00
Valid 9:00
9:00 UTC
Init Zone 3
0630-0900
12:00
Valid 15:00
9:00
Valid 12:00
10:50 UTC
12:40 UTC
Init Zone 5
0930-1050
Yellow contours = 35 dBZ echo at forecast time
Init Zone 6
1030-1240
13 June 2002
RUC analysis
and
observations
at 10:30 UTC
RUC CAPE
RUC CIN
Reflectivity
echoes overlaid
Surface-Sounding CAPE
Surface-Sounding CIN
Ability of RUC 10 km Model to Initiate Precipitation
Initiation
Mechanism
Number
of Events
% Time
% Time
Precipitation Precip. Area
is forecast
too large
83
40
% Time
Fcast is late
Fronts
23
30
Surface Lows
4
100
75
25
Elevated
(frontal)
11
83
56
56
Elevated
(isolated)
39
68
35
35
Convergence
Boundaries
32
62
40
50
Model doesn’t forecast these events well; 50% are late
22 May 2002 - Dry Line
23:00 – 00:00 UTC
RUC-CAPE
Surface-Sounding CAPE
GOES-Sounder CAPE
Surface-Sounding CIN
GOES-Sounder CIN
Dry line
Dry line
segment
RUC-CIN
Model Performance


Forecasting precipitation initiation and
evolution
Forecasting long-lived storm
complexes
Analysis 15-16 June 2002 case
8 hour loop
Synoptic low and
trough line
Evolution of
storm complex
dependent on
the emergence
and
characteristics
of the gust
fronts
L
Verification of
RUC 6hr fx
for 21:00
00:00
03:00
Does not capture accurate
evolution of storms
Does not propagate system
Need to do a better job of representing
downdrafts and outflows in models
12 – 13 June 2002
Validation of 3 hr RUC forecasts
a) 1800
b) 2100
c) 0000
d) 0300
e) 0600
Conclusions
RUC10 Initiation Forecasts:
Best - fronts and lows
Second - elevated, however often late
Worse – small scale convergence lines
Conclusions
Why is this so?

Comparison of RUC stability fields with observations
show that the model is producing realistic values
during the initiation period, although with somewhat
higher magnitudes than the observations.
Conclusions
RUC10 Storm Evolution Forecasts:
RUC10 is unable to forecast the evolution and
propagation of storm.
Why is this so?
Downdrafts and gust fronts have a major influence
on the evolution, lifetime and motion of convective
storm complexes.
Conclusions
RUC10 Storm Evolution Forecasts:
Most numerical models, including RUC10, run on grids
typically larger than the scale of convection and do not
represent convection explictly but rather employ a
cumulus parameterization scheme.
As a result, RUC10 has difficulty in either producing a
convective downdrafts or generating downdrafts and
outflows of the correct intensity seen in the
observations.
Research and Operational Challenge
Precipitation microphysics plays a key role in downdraft
production and characteristics. This suggests the use of
polarimetric radar to measure in-cloud precipitation
structure associated with downdrafts.
Assimilation of refractivity information into numerical
models and short-term forecast systems is critical for
improved accuracy in short-term thunderstorm forecasting.
Forecasting Challenge: Tracking the “pockets” of boundary
layer moisture so critical for convection initiation and growth.
Moisture
gradient
Convergence
boundaries
00:40
22:57
00:31
00:01
UTC
23:27UTC
UTC
Moisture
gradient
Convergence
boundaries
00:31
22:57
00:01
00:40
UTC
23:27UTC
UTC
Moisture gradients and convergence boundaries associated with thunderstorm
development, June 12, 2002
END
12 – 13 June 2002
Validation of 3 hr RUC forecasts
a) 1800
b) 2100
c) 0000
d) 0300
e) 0600
Fig. 15. June 12-13 RUC 6hr precipitation
forecasts (solid white contour) overlaid
on radar reflectivity (gray shade scale in
dBZ on right) at 3h intervals (a-e). The
forecasts are 3h accumulations ending
at the given time. The reflectivity is the
instantaneous field at the given time. The
first precipitation contour represents an
accumulation of 1 mm during the 3 h
period; the second contour (only reached
in b) is 10 mm. Boundaries are shown by
thick white lines.