Characterization of intense precipitation events over small scale

Transcription

Characterization of Intense Precipitation Events
over a Small Scale, Southern Alpine River
Catchment
A. Barbi, A. Bonan, R. Millini, and A. Rossa
Centro Meteorologico di Teolo, ARPA Veneto
ICAM/MAP 2005, Zadar, Croatia, arossa@arpa.veneto.it
1
Motivation and goal

Importance of heavy precipitation for medium/small river
catchments

Demands on forecasting/nowcasting and warnings for
local authorities and public
Our aim…

Characterize precipitation and hydrological response in a
small north-eastern Italian Prealpine basin of during
intense precipitation episodes
Provide guidelines to operational forecasters for the
monitoring and warning process
2
Overview

Introduction

Data set and approach

Catchment description

Meteorology of interest

Pluvio- and hydrometric analysis

Operational guidelines
3
Data and Methodology

Study period 1992-2003

ARPAV surface station and raingauge network (ca. 200)

Hydrometric station (Posina-Stancari)

Mt. Cesen surface station (1’550masl)

No hydrological model

Simple statistical analysis
¾ Percentile analysis for daily precipitation
¾ Rainfall accumulation – duration diagram
¾ Flow level increase – duration diagram

Synoptic analyses
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Posina Catchment

Small (114km2) catchment
on the prealpine chain

Well monitored
¾ 5 rain gauges
¾ 3 flow level gauges

Concavity in local
topography Æ convergence

Precipitation climatology
5
Precipitation climatology
Schwarb et al. 2001: 1971-1990
raingauge network: Autumns 1961-1990
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7
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Criticality defined for Posina catchment

Choice of hydrometric threshold for critical events:
water level Hc =2m

Corresponding discharge: 20% of historical max. value
(Nov. 1966)

Rating curve for high stream flow levels not reliable
Î analysis done in terms of flow level
Î
13 cases identified as critical in the autumns 1992-2003

Average basin lag: 6-9h Î application for nowcasting

Intense phase of an event: <RR>basin ≥ 6mm/h
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Meteorological events
Marked upper-level trough dipped
into western Mediterranean
Intense warm-moist low-level surface
flow (Sirocco winds)
10
Orographic enhancement
Rain intensity distribution versus wind intensity classes (critical events)
¾ wind-precipitation
relation during intense
phases
22
20
18
¾ rank correlation
values between
0.4 and 0.9
16
Rain intensity (mm/h)
14
¾ higher correlations
when clear signal of
Med. cyclone
12
10
8
6
Î Wind evolution
indicative for precip
evolution and duration
of event Î monitoring
4
spread = IQR
2
0
0−2.5
2.5−5
5−7.5
7.5−10
10−12.5 12.5−15 15−17.5
Wind intensity classes (m/s)
17.5−20
20−22.5
22.5−25
11
25−27.5
Pluvio- and hydrometric analysis: criticality
500
450
Rainfall total (mm)
400
350
300
250
200
150
100
Prec. 1-2-3dd 95th perc.
50
Prec. 1-2-3dd 90th
perc.
0
12
24
36
48
60
72
84
96
Duration (hours)
Î QPF in terms of accumulation and duration: indicator for criticality
Î short-term forecasting, depends on QPF quality
12
Rise of hydrometric level (m)
Hydrometric analysis: flow level increase
3.0
Intensity 2-3 mm/h
Intensity 3-5 mm/h
Intensity 5-8 mm/h
Intensity 10 mm/h
Line (3-5 mm/h)
Line (2-3 mm/h)
Line (5-8 mm/h)
2.5
R2 = 0.9506
R2 = 0.7163
2.0
1.5
1.0
12
24
36
48
60
72
84
96
Duration (hours)
Î
Based on avg. intensity and duration: estimate of flow level increase
Î
Short-term forecasting and monitoring (lead times ~3h)
13
Summary

Period 1992-2003: 13 cases identified as critical

Avg. Values during intense phases: 11mm/h and 13m/s
for at least 9 consecutive hours

During intense phase mainly SSE-ly winds

Orographic enhancement seems to play important role

Pluviometric and hydrometric analyses reveal „some
predictable behaviour“ of the catchment in this regime
14
Operational application: Oct 2004 case
500
R is e o f h y d ro m e t ric le v e l ( m )
450
Rain fall to tal (mm)
400
350
300
250
200
150
100
50
0
3.0
Intensity 2-3 mm/h
Intensity 3-5 mm/h
Intensity 5-8 mm/h
Intensity 10 mm/h
Line (3-5 mm/h)
Line (2-3 mm/h)
Line (5-8 mm/h)
2.5
R2 = 0.9506
R2 = 0.7163
2.0
1.5
1.0
12
24
36
48
60
72
84
96
12
24
Duration (hours)
36
48
60
72
84
96
Duration (hours)
¾ 27 Oct: QPF 90mm/24h (~4mm/h):
initial level 0.4m + 1.1m = 1.5m < Hc
¾ 29 Oct: QPF 80mm/36h (~2mm/h):
initial level 0.7m + 1.0m = 1.7m < Hc
¾ 31 Oct: QPE 160mm/48h (~3mm/h):
initial level 0.9m + 1.6m = 2.5m > Hc !!
Effective peak flow level: 2.23m during phase 3
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Finally …

Characterized well monitored Posina catchment
Æ useful for operational forecasting

Robust wind-precip relation (orographic enhancement)

Redo analysis in other catchments?

Update analysis with new ‘critical’ events

Results may be transferred to other catchments with
similar characteristics

Use of hydrological model, both for real-time monitoring
and catchment characterization
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Thank you for
your attention!!
Pluviometric Analysis
Percentile analysis of very wet days over Posina basin
(1992-2003)
Period
90th
95th
99th
1 day
80 (83%)
112 (100%)
183 (100%)
2 days
106 (100%)
139 (100%)
204 (100%)
3 days
153 (100%)
184 (100%)
267 (100%)
e.g. for 80mm/1d 83% of the cases become critical, while 17% stay uncritical
Î QPF in terms of accumulation and duration: indicator for criticality
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