Author: Chris Castellano

CW3E Publication Notice: Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers

CW3E Publication Notice

Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers

December 2, 2024

A new article titled “Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers,” led by CW3E Director F. Martin Ralph and coauthored by Matthew Simpson (CW3E), Sam Iacobellis (CW3E), Jay Cordeira (CW3E), Forest Cannon (Tomorrow.io), Alison Cobb (ECMWF), Allison Michaelis (Northern Illinois University) and Luca Delle Monache (CW3E) was published in the American Meteorological Society’s Monthly Weather Review. This study investigates the vertical structure of the stable marine boundary layer (SMBL) in atmospheric river (AR) environments and key modulating processes using data from more than 1000 dropsondes collected during the AR Reconnaissance program. By analyzing an extensive database of dropsonde observations over the Northeast Pacific, we aim to document the occurrence of AR-SMBL conditions in mid-latitude ARs and explore their origins and the decoupling process between the lower AR atmosphere and the underlying ocean surface. This work is well aligned with CW3E’s goals to advance scientific understanding of ARs and to improve extreme precipitation forecasting for Western North America.

Water vapor transport associated with ARs is focused in the lower troposphere where the combination of strong low-level winds and increased moisture produces strong horizontal fluxes. As a result, the behavior of the SMBL is potentially important in modulating AR strength (Ralph et al. 2017). In addition, accurately representing SMBL dynamics beneath ARs by numerical weather prediction (NWP) models can be critical for data assimilation. For example, Lavers et al. (2018) found the greatest initial condition errors in European Centre for Medium-Range Weather Forecasts (ECMWF) forecasts of ARs are near the top of the MBL.

We examined the hypothesis that, as relatively warm air parcels are advected poleward towards and within an AR over progressively cooler ocean waters, sensible heat is transferred from the air mass defining the AR into the ocean surface, thereby cooling the lowest levels of the atmosphere. This atmospheric cooling subsequently increases static stability and vertical wind shear within the MBL, effectively decoupling the lower layer of the AR from the ocean surface.

Simulated backward air parcel trajectories originating from dropsonde locations within the AR core were used to calculate the 24-hour change in SST experienced by an air parcel (DSST24) beneath each AR. The DSST24 varies from –13°C to +2°C and is directly related to the strength of the AR and its orientation relative to the SST gradient. The DSST24, therefore, distinguishes weak and strong decoupling regimes (WDR, SDR). In SDR cases, relative to WDR cases, the SMBL is characterized by stronger static stability, low-level jet, vertical wind shear, horizontal water vapor transport, and moderately shallower SMBL depth (Figure 1).

Figure 1. Composite profiles within the AR core for strong (SDR, solid line) and weak decoupling regime (WDR, dashed line) cases for (a) potential temperature, (b) wind speed (m s–1), (c) water vapor flux (g kg-1 * m s–1), (d) Bulk Richardson number (Rib).

SDR cases were associated with a greater sensible heat loss to the ocean relative to the WDR cases (Figure 2), leading to a cooling of the lower atmosphere and the increase in static stability observed in composite profiles. Sensible heat flux and IVT values are highly correlated in SDR cases while the WDR sensible heat flux exhibits an irregular structure relative to IVT contours.

Figure 2. ERA5-derived spatially centered and rotated composite analyses of sensible heat flux (W m–2; shaded according to scale) and IVT magnitude (contoured every 100 kg m–1 s–1 starting at 200 kg m–1 s–1) for the subset of IOPs containing a change in SST (a) less than –4°C (N=74) and (b) between –4°C and +4°C (N=25).

The study highlights the complex structure of the SMBL beneath ARs and documents the increase in air-sea decoupling due to advection of the warm air mass in ARs over cooler SSTs. This observation-based paper will act as a foundation for future work to improve NWP model representation of AR-SMBL dynamics though enhanced boundary layer parameterizations.

Lavers, D. A., Rodwell, M. J., Richardson, D. S., Ralph, F. M., Doyle, J. D., Reynolds, C. A., Tallapragada, V., & Pappenberger, F. (2018). The gauging and modeling of rivers in the sky. Geophysical Research Letters, 45(15), 7828-7834. https://doi.org/10.1029/2018GL079019

Ralph, F. M., Iacobellis, S. F., Neiman, P. J., Cordeira, J. M., Spackman, J. R., Waliser, D. E., Wick, G. A., White, A. B., & Fairall, C. (2017). Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers. Journal of Hydrometeorology, 18(9), 2577-2596. https://doi.org/10.1175/JHM-D-17-0036.1

Ralph, F. M., Simpson, M., Iacobellis, S., Cordeira, J. M., Cannon, F., Cobb, A., Michaelis, A. C., & Delle Monache, L. (2024). Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers. Monthly Weather Review (published online ahead of print 2024). https://doi.org/10.1175/MWR-D-23-0207.1

CW3E AR Update: 18 November 2024 Outlook

CW3E AR Update: 18 November 2024 Outlook

November 18, 2024

Click here for a pdf of this information.

Long-Duration Atmospheric River Event to Bring Heavy Precipitation to California and Oregon

  • A strong atmospheric river (AR) is forecast to make landfall over the US West Coast tomorrow (19 Nov) in association with a rapidly intensifying low-pressure system.
  • After the initial AR landfall, the AR is forecast to stall over Northern California for several days.
  • The location of AR landfall has continued to trend southward in recent model runs, with the strongest AR conditions and heaviest precipitation now expected in Northern California.
  • Active weather is likely to continue across California this weekend into early next week, but there is considerable forecast uncertainty in the duration and exact location of AR activity beyond Fri 22 Nov.
  • AR 4 conditions (based on the Ralph et al. 2019 AR Scale) are likely in coastal Northern California.
  • Some locations may experience AR conditions for > 72 consecutive hours.
  • The NWS Weather Prediction Center (WPC) is forecasting > 10 inches of total precipitation in portions of Northern California during the next 7 days.
  • Some watersheds in Northern California could receive > 20% of their normal total water year precipitation during the next 10 days.
  • Heavy rainfall will likely result in hydrologic impacts over portions of Northern California and southern Oregon.
  • The WPC has issued a moderate risk excessive rainfall outlook for Del Norte and Humboldt Counties Thu 21 Nov into early Fri 22 Nov.
  • Major winter storm impacts are likely in the Klamath Mountains, Southern Cascades, and Northern Sierra Nevada.

Click images to see loops of West-WRF GFS IVT and IWV forecasts

Valid 0000 UTC 18 November 2024 – 0000 UTC 25 November 2024

Summary provided by C. Castellano, S. Bartlett, J. Cordeira, J. Kalansky, and M. Steen; 18 November 2024

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*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.

CW3E Event Summary: 17-20 October 2024

CW3E Event Summary: 17-20 October 2024

28 October 2024

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Complex Storm Produces Heavy Snow in CO and Record-Breaking Rainfall in NM

  • An amplifying mid-level shortwave trough over the western US evolved into a cutoff low over AZ, setting the stage for widespread precipitation in the Four Corners region.
  • The combination of strong synoptic-scale forcing, large instability, ample moisture, and strong vertical wind shear created favorable conditions for the development of organized convection over eastern NM on Sat 19 Oct.
  • These thunderstorms produced very heavy rainfall, particularly in the vicinity of Roswell, NM (> 6 inches).
  • Roswell received nearly 50% of its annual precipitation in a 6-hour period and set a new all-time daily precipitation record (5.78 inches).
  • Based on NOAA Atlas 14, the observed 6-hour rainfall at Roswell exceeded the 500-year storm.
  • Snowfall accumulations of 1–2 feet were also observed in the Uinta and San Juan Mountains.
  • Some locations received the equivalent of ~15% of their typical annual peak snowpack during this event.
  • Extremely heavy rainfall in southeastern NM caused flooding along the Pecos River south of Roswell.
  • Life-threatening flash flooding occurred in Roswell and Chaves County, where more than 300 people were rescued from floodwaters.
  • Nearly 40 people were hospitalized and two fatalities were reported.


 

 

 

 

 

 

 

 

 

Summary provided by C. Castellano, D. Nash, S. Bartlett, and J. Rutz; 28 Oct 2024

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CW3E Event Summary: 18-20 October 2024

CW3E Event Summary: 18-20 October 2024

25 October 2024

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Strong Atmospheric River Produced Heavy Rain in Southeast Alaska and British Columbia

  • An atmospheric river (AR) brought heavy precipitation to southwestern British Columbia and northwestern WA during 18–20 October.

The AR:

  • The AR formed within a plume of subtropical moisture over the central North Pacific Ocean and made landfall in British Columbia on Fri 18 Oct.
  • AR3–4 conditions (based on the Ralph et al. 2019 AR Scale) were observed in southern Vancouver Island and coastal WA.

Impacts:

  • At least 5-10 inches of precipitation fell over much of southwestern British Columbia, the Olympic Peninsula, and the North Cascades.
  • Heavy rain caused significant flooding on the Coquitlam River in Port Coquitlam, BC, with streamflow exceeding the previous record from March 2007.
  • Minor riverine flooding also occurred in northwestern WA.

MIMIC-TPW2 Total Precipitable Water

Valid: 1200 UTC 17 October – 1200 UTC 21 October 2024

Click images to see loops of GFS IVT/IWV analyses

Valid 1200 UTC 17 October – 1200 UTC 21 October 2024


 

 

 

 

 

 

 

Summary provided by C. Castellano J. Kalansky; 25 Oct 2024

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