Observational analysis of storms and flooding events in the Pacific Northwest. Introduction

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Precipitation Processes and Patterns Group Introduction Average Annual

Pacific Northwest are modified by the Coastal and Cascade Ranges.

1 Observational analysis of storms and flooding events in the Pacific Northwest Jake Crouch UNCA ATMS Class of 2007 April 18, 2009 Cloud and Precipitation Processes and Patterns Group Introduction Average Annual Rainfall Washington Oregon Land falling extra-tropical cyclones in the Pacific Northwest are modified by the Coastal and Cascade Ranges. Some of these storms can become very intense, yielding frequent rainfall and resulting in flooding and mudslides. 1

Orographic Precipitation Pacific Northwest

2 Orographic Precipitation Pacific Northwest Weather Impacts Heavy precipitation can occur in winter Strong, moist flow forced upslope of mountains (Cascades) Can lead to severe flooding Creates mudslides Ex: Feb. 8-9, 1996; Jan 8-11, 2006; Nov. 6-7, 2006 Associated with deep rain layer 2

3 Data Sources NWS weather radar Micro Rain Radar Upper-air sounding Gridded elevation data Radar Data Radar data interpolated to a Cartesian grid with 1km vertical and horizontal resolution 3

Doppler velocity provides information on rain layer depth Storm maximum rain layer depth

4 Radar Data In our analysis of radar data we must be aware of bright band effects due to melting Height (km) Source: Houze and Medina, 2004 MRR Derived Rain Layer Depth MRR Doppler velocity provides information on rain layer depth Storm maximum rain layer depth used because of varying freezing level heights associated with frontal passages m/s Time (hrs) 4

5 Rain Layer Depth Category Shallow Medium Deep Rain Layer Depth < 1.0 km < 2.0km and 1.5km 2.0 km Number of Occurrences Shallow Rain Layer Storms (34 total) Medium Storms (61 total) Deep Rain Layer Storms (45 total) Rain Layer Depth (m) Terrain Data Mt. Hood Mt. St. Helens Mt. Jefferson 75 th Percentile Rain Layer Depth Median Rain Layer Depth 25 th Percentile Rain Layer Depth Willamette Valley 5

6 Low- Level Wind Direction We will focus on storms with winddirections from the This narrows our storm subset to 70 storms. Number of Occurrences Wind Direction ( ) Wind Direction Influence <194 >194 and <227 >227 Less More Less More Less More Source: Yuter et al.,

7 IWV vs. Rain Layer Depth Persistence of precipitation is an important component of rainfall accumulation For each radar grid point, calculated persistence of precipitation by determining exceedence frequency of Z>13 dbz 7

8 Persistence Shallow Deep Flood CFDD Explanation CFDDs are a statistical tool used to identify regions along the slope that are favorable for precipitation enhancement. Visualize distribution of persistence as a function of distance from the Cascade crest. 8

9 CFDD Explanation 1 km Start with persistence plot. Examine data at each 1 km distance from the crest. Take the distribution of the data at that distance, and normalize by the number of non-zero data points. Compile each histogram into singular figure plot. Histogram at 80 km from Crest.4 Normalized Occurrence Persistence (%) 9

10 All Histograms Histograms Color Coded by Value Persistence Distributions Persistence (%) 0 10

11 CFDD Explanation.4 Persistence (%) Shallow Storms Persistence 2km Altitude Lower Slope Mid Slope Upper Slope Persistence (%) Distance from Crest (km) 0 11

Deep Non-Flood Storms Persistence 2km Altitude Lower Slope Mid Slope Upper Slope 90.

1 20130 110 90 70 50 30 10 0 Distance from Crest (km) 0 Flood Storms Persistence 2km

12 Deep Non-Flood Storms Persistence 2km Altitude Lower Slope Mid Slope Upper Slope Persistence (%) Distance from Crest (km) 0 Flood Storms Persistence 2km Altitude Lower Slope Mid Slope Upper Slope Persistence (%) Distance from Crest (km) 0 12

13 Results Holding the wind direction relatively constant, we can test the sensitivity of precipitation to other environmental parameters. Rain layer depth: - Positively correlated with IWV - Potential indicator of flooding events - Relationship with the spatial distribution and magnitude of precipitation persistence Lower Mid Upper 40% 50% 30% Lower Mid Upper 50% 60% 40% Shallow Storms: modal precipitation persistence peaks along the mid-slope with lesser values along the lower and upper slope. Deep Storms: modal precipitation persistence peaks along the midslope, with slight decrease near crest Lower Mid Upper 75% 70% 65% Flood Storms: modal precipitation persistence increases along the entire slope, reaching a maximum at the upper slope. Higher values for all regions. 13

The Future Analysis in progress: Analysis of higher altitude ice processes Relative intensity of precipitation Future work: Intermittent vs.

14 The Future Analysis in progress: Analysis of higher altitude ice processes Relative intensity of precipitation Future work: Intermittent vs. continuous rainfall Changing persistence and 3-D terrain structure needs to be represented in simulations to better understand physical aspects of these storms 14

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16 Aerosol Snorkel W-band Cloud Radar CIP Cloud Camera W-band Cloud Radar Mailbox MWR 197GHz MWR Motion Stabilized MWR CSD Lidar 16

17 PPI Regular Drizzle 13:33 UTC 28 OCT 08 RHI 35 1 km 2 km PHOTO IN DIRECTION OF RHI PPI Unexpectedly Strong Convection 12:00 UTC 26 OCT 08 RHI km 2 km PHOTO IN DIRECTION OF RHI 17

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L. McMurdie, R. Houze, J. Zagrodnik, W. Petersen, M. Schwaller

L. McMurdie, R. Houze, J. Zagrodnik, W. Petersen, M. Schwaller International Atmospheric Rivers Conference, San Diego, CA, 9 August 2016 Goals of OLYMPEX Validate GPM satellite radar and passive microwave