Transcript Eta Model Overview

Heavy Cold-Season Precipitation in British
Columbia, Washington and Oregon
MSC/COMET Presentation, 23 February 2001
Gary M. Lackmann
Department of Marine, Earth, and Atmospheric Sciences
North Carolina State University
1
The “Pineapple Express”: A Worst-Case
Scenario for West Coast Flooding
• What is the “Pineapple Express” (PE)?
– Characterized by
• anomalous subtropical moisture transport
• warm temperatures, heavy precipitation
• rapid snowmelt, lowland flooding
– Directly affects
• British Columbia
• Washington, Oregon, Northern California
– Indirectly affects much of North America?
2
Outline
I. A Brief Climatology: The Pineapple Express
 Methodology: stream and rain gauge data
 Limitations of compositing
 Composite patterns and implications
II. Case Study: Flood of 16-18 January 1986
 Methodology: Piecewise moisture transport
 A moisture transport feedback
 Anticipation of model biases
3
I. A Brief Pineapple Express Climatology
• Objectives:
– Identify planetary- and synoptic-scale common
denominators for cold-season heavy precipitation
– Seek identifiable precursors
– Determine “character” of moisture transport
– Provide context for more detailed case studies
• Methodology:
– Use daily precipitation data and stream gauge data to
identify events
– Examine individual events, stratify case sample
– Generate composites for 6-day period bracketing event
4
Methodology
• A. Atmospheric Composite:
– 27-year data sets from
•
•
•
•
Olympia (OLM),
Seattle-Tacoma Apt (SEA),
Stampede Pass (SMP), WA
Astoria (AST), OR
– Case selection criteria:
• Daily precipitation > 12.5 mm (0.5”) 24 h -1 and
• Maximum Temp. > 10 C (lowland) or > 5 C (mountain)
• B. Runoff Composite:
– Tolt River discharge values > 4,000 ft3 s-1.
5
Methodology and Case Selection Results
• Six-day composites generated from NCEP CD
• Anomalies: deviations from 27-year weighted climo
• 46 cold-season events from 1962-1988:
–
–
–
–
–
November
December
January
February
March
18
12
8
5
2
• Tolt: Less sensitivity to temperature criterion
–
–
–
–
–
November
December
January
February
March
3
11
17
5
2
65
Composite 500 height and SLP evolution
7
Composite 500 height anomaly evolution
8
Tolt Composite 500 height anomaly evolution
9
Composite SLP anomaly evolution
10
Composite 850 height anomaly evolution: Part I
11
Composite 850 Temp anomaly evolution: Part II
Large-scale Chinook effect?
Are Pineapple Express events precursors
to large-scale warming trends east of the
Rocky Mountains?
12
Case Study Methodology
•Representative case selected from 46-case sample: The flood
of 17-18 January 1986
•Series of cyclones moved from eastern Pacific towards
Washington and British Columbia
•Severe flooding occurred as result of snowmelt, heavy rain
•Questions:
– Which flow anomalies are responsible for moisture
transport?
– QG dynamics versus orographic lifting?
– Piecewise moisture transport via PV inversion
13
Precipitation Totals, 17-18 January 1986
14
Case Study Methodology: PV
Piecewise moisture transport:
• Quasigeostrophic form of potential vorticity (PV) is given by
q*  QGPV  f 
n
q
i 1
*i
•q partitioned, piecewise geopotential obtained via inversion
 '   1 q*i 
where
1 2
  1 

    f 0 
f0
p   r p 
15
00 UTC 17 January 1986
January 1986 Case Study Results:
• Moisture transport due to transient, cyclonic systems
• Lower-tropospheric, diabatically produced PV
anomalies dominate transport
• Feedback hypothesized involving LLJ, diabatic PV
redistribution, and warm-sector moisture transport
• Models must accurately represent cold-frontal
precipitation in order to account for this feedback