Dr Andrew Martin 1 Dr Allison Michaelis 2 Also Collaborating Dr F Martin Ralph 2 Dr Paul Loikith 1 Funded by USACE FIRO Lake Mendocino Project 1 Department of Geography Portland State University ID: 1025637
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1. Mesoscale Modification of Precipitation during Landfalling Atmospheric Rivers by Frontal CyclogenesisDr. Andrew Martin1, Dr. Allison Michaelis2Also Collaborating: Dr. F. Martin Ralph2, Dr. Paul Loikith1 Funded by: USACE FIRO @ Lake Mendocino Project1Department of Geography, Portland State University; 2Center for Western Weather and Water Extremes at Scripps Institution of Oceanography
2. OutlineBackgroundHow are Secondary Cyclones related to ARs?Impact on Hydrometeorological Forecasting: Dec 2014 on the Russian RiverThe St. Valentines’ Day, 2019, ARStudy: Impact of Secondary Cyclone on Orographic Precipitation EnhancementReview: Broader Lessons About Hydrometeorological Forecasting During ARs and Cyclogenetic EnvironmentsPV StreamerARCycloneSec. CycloneGOES-W Water Vapor Valid 21 UTC on 12 Feb, 2019FIRO2
3. ARs and the Cyclone Lifecycle“Genesis”Distinct Warm SectorRepeat?3
4. December 2014 Russian River FloodSecondary Cyclone Developing on the AR trailing (cold) front:modified the landfalling AR led to a streamflow forecast challengeLLLLLX1234Martin et al. (2019 – J. Hydromet.) found two modes of reduced predictability. At the shortest lead times, forecast errors were caused by rapid modification of orographic precipitation.Hydrographs @ Guerneville, CA4
5. Warm Conveyor Belt – UCAR/COMETARs and Lifting Mechanisms Baroclinic Instability – Rauber et al. (2003)Orographic Lift – Neiman et al. (2009)5
6. Orographic Precipitation Per Alpert (1986), in the absence of water vapor storage: P1P2P3 Where P1, P2, P3 capture orographic, all other lift, evaporation, respectively.The Bodega Bay Coastal Atmospheric River Observatory can empirically compare the strength of P1 to resulting precipitation. ~ 70% variance explained.Mountain Accumulation (mm)Upslope Flux of Vapor (mm m s-1 hr)Ralph et al. (2013 – J. Hydromet)6
7. A Sheltering of the AR Vapor FluxOPC Surface Analysis Valid 06 UTC 11 Dec, 2014, Just after Secondary Cyclogenesis.7An incipient warm front offshore briefly sheltered the ranges from vapor flux, leading to a hiatus in precipitation. (Martin et al., 2019 – J. Hydromet).
8. Valentines Day (2019) ARLLLLLL1234Secondary Cyclone Developing on the AR leading (warm) front:Strengthened and turned the AR northward Again led to a short lead time streamflow forecast challengeBecause of event chronology, differing source of overforecast8
9. At the ARO (K) Rain (in hr-1)upslope flux (mm m s-1)Altitude (km MSL)Change in Oro. RatioSondeSondeAdapted fromesrl.noaa.gov9
10. Offshore and SondesConditions sampled offshore in the WCB, AR (dashed blue, red); correspond closely to conditions over the ARO during heavy precipitation on 02/13 (solid blue), 02/14 (solid red). Sonde Profiles. Aircraft = dashed, ARO = solid.MERRA-2 SLP, IVT > 500 kg m-1 s-1 (orange), and isotherms spanning fronts (upslope)WCB (P2 > P1)AR (P1 > P2)WCB, 02/13AR, 02/14bold where saturated10mtn. top
11. Impact on Orographic Precipitation500 mSea LevelThe ratio of precipitation rates at the mountaintop (CZC – 500 m) to the coastal (BBY) ARO sites can yield insight into the relative strengths of P1, P2. Typically, this ratio is 2-3.Low ratios are indicative of a weak P1 term. Feb 13, 2019 brought heavy rain with record low ratios.11
12. What Role Did Cyclogenesis Have?GEFS ensemble member forecasts containing a stronger secondary cyclone also showed properties consistent with lower P1/P2 ratio. Red = control member.48 hour forecasts, valid 00 UTC on 13 Feb, 2019.12
13. DiscussionDiffering cyclogenesis mechanism (2014/2019) but similar short-term flood forecast outcome.Valentines’ Day event did not dramatically alter P1 contribution (no “sheltering”), but did dramatically lower the P1/P2 ratio.Potential short-term flood forecast challenge related to runoff routing rather than to accumulating precip.Secondary cyclones and other nearby circulation features may also be modified by the presence and strength of an AR. Allison and Reuben have more..13