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AtmosphericRiversAtmos 5210:Synoptic–DynamicMeteorologyII

JimSteenburghDepartmentofAtmosphericSciences

UniversityofUtahjim.steenburgh@utah.edu

MajorcontributionsfromDr.JonathanRutz

LearningObjectives

• Afterthisclassyoushould:

• Beabletoidentifyatmosphericriversandtheirpotentialimpactsusingatmosphericanalysesandnumericalforecasts

• UnderstandtheprocessesthatcontributetoARdecayormaintenanceduringpenetrationintotheinteriorwesternU.S.

• Beabletoforecastpotentiallyhigh-impactAReventsincludingcomparisonswithpastevents

Introduction

KeyMoisture-RelatedVariables

• Integratedwatervapor(IWV)– theamountofwatervaporinanatmosphericcolumnexpressedasthedepthofwaterifthatvaporwerecondensed• a.k.a.precipitable waterortotalprecipitable water

• Integratedwatervaportransport(IVT)– thetotalamountofwatervaportransportinanatmosphericcolumn

b. Objective identification of atmospheric rivers

ARs are objectively identified in the ERA-Interimusing two definitions. The first, IVT250, defines an AR asa contiguous region $2000 km in length with IVT $250 kgm21 s21. Here, IVT is defined as

IVT51

g

ð100 hPa

psfc

qV dp , (1)

where g is the gravitational acceleration, q is the specifichumidity, V is the total vector wind, p is pressure, and psfcis the surface pressure. The integration is done using dataat the surface, 50-hPa intervals from the surface to 500hPa,and 100-hPa intervals from 500 to 100hPa. The second,IWV20, defines anAR as a contiguous region$2000km inlength with IWV $ 20mm. Here, IWV is defined as

IWV51

g

ð100 hPa

psfc

q dp , (2)

with the integration done as described above. Both IVTand IWV are then interpolated to the CPC analysis gridto facilitate the attribution of precipitation to ARs, asdescribed in section 2c. In both cases, the AR lengthcriterion is evaluated as the greatest distance betweentwo points within each contiguous feature.Previous work based on SSM/I satellite data uses

IWV20 as a proxy for AR identification in lieu of the

wind observations necessary to calculate IVT (e.g., Ralphet al. 2004; Neiman et al. 2008b; Dettinger et al. 2011).Whereas some results are presented based on IWV20, wefocus on IVT250 for two reasons. First, IVT is stronglyrelated to precipitation over complex terrain (Junker et al.2008; Neiman et al. 2002, 2013; Ralph et al. 2013a) andmore strongly correlated with cool-season precipitationover most of the westernUnited States than IWV (Fig. 2).Second, subjective evaluation of numerous AR eventsreveals that areas of IVT $ 250kgm21 s21 crossing theWest Coast of North America penetrate farther into theinterior than coinciding areas of IWV $ 20mm and cor-respond well with the spatial extent of heavy precipitation(e.g., Fig. 3). The use of IVT also aids in overcoming thelimitations associated with the reduction of overall at-mospheric thickness and IWV over elevated terrain.In contrast to earlier studies (e.g., Ralph et al. 2004;

Neiman et al. 2008b; Dettinger et al. 2011; Wick et al.2013), AR width is not considered in the identificationprocess. ARs identified using the IVT250 criteria rarelyexceed 1000 km in width, whereas those that do still tendto possess AR characteristics (e.g., large length to widthratio and intense lower-tropospheric water vapor flux).

c. Attribution of precipitation to atmospheric rivers

The precipitation from the CPC analysis is defined asAR related if an AR is identified at a given grid pointduring any of the five 6-hourly analysis times duringthe 24-h accumulation period (1200–1200 UTC). For

FIG. 2. (a) Correlation coefficient between the daily-mean (1200–1200 UTC) IVT and 24-h precipitation. (b) As in (a), but for daily meanIWV. (c) (a) minus (b).

908 MONTHLY WEATHER REV IEW VOLUME 142

b. Objective identification of atmospheric rivers

ARs are objectively identified in the ERA-Interimusing two definitions. The first, IVT250, defines an AR asa contiguous region $2000 km in length with IVT $250 kgm21 s21. Here, IVT is defined as

IVT51

g

ð100 hPa

psfc

qV dp , (1)

where g is the gravitational acceleration, q is the specifichumidity, V is the total vector wind, p is pressure, and psfcis the surface pressure. The integration is done using dataat the surface, 50-hPa intervals from the surface to 500hPa,and 100-hPa intervals from 500 to 100hPa. The second,IWV20, defines anAR as a contiguous region$2000km inlength with IWV $ 20mm. Here, IWV is defined as

IWV51

g

ð100 hPa

psfc

q dp , (2)

with the integration done as described above. Both IVTand IWV are then interpolated to the CPC analysis gridto facilitate the attribution of precipitation to ARs, asdescribed in section 2c. In both cases, the AR lengthcriterion is evaluated as the greatest distance betweentwo points within each contiguous feature.Previous work based on SSM/I satellite data uses

IWV20 as a proxy for AR identification in lieu of the

wind observations necessary to calculate IVT (e.g., Ralphet al. 2004; Neiman et al. 2008b; Dettinger et al. 2011).Whereas some results are presented based on IWV20, wefocus on IVT250 for two reasons. First, IVT is stronglyrelated to precipitation over complex terrain (Junker et al.2008; Neiman et al. 2002, 2013; Ralph et al. 2013a) andmore strongly correlated with cool-season precipitationover most of the westernUnited States than IWV (Fig. 2).Second, subjective evaluation of numerous AR eventsreveals that areas of IVT $ 250kgm21 s21 crossing theWest Coast of North America penetrate farther into theinterior than coinciding areas of IWV $ 20mm and cor-respond well with the spatial extent of heavy precipitation(e.g., Fig. 3). The use of IVT also aids in overcoming thelimitations associated with the reduction of overall at-mospheric thickness and IWV over elevated terrain.In contrast to earlier studies (e.g., Ralph et al. 2004;

Neiman et al. 2008b; Dettinger et al. 2011; Wick et al.2013), AR width is not considered in the identificationprocess. ARs identified using the IVT250 criteria rarelyexceed 1000 km in width, whereas those that do still tendto possess AR characteristics (e.g., large length to widthratio and intense lower-tropospheric water vapor flux).

c. Attribution of precipitation to atmospheric rivers

The precipitation from the CPC analysis is defined asAR related if an AR is identified at a given grid pointduring any of the five 6-hourly analysis times duringthe 24-h accumulation period (1200–1200 UTC). For

FIG. 2. (a) Correlation coefficient between the daily-mean (1200–1200 UTC) IVT and 24-h precipitation. (b) As in (a), but for daily meanIWV. (c) (a) minus (b).

908 MONTHLY WEATHER REV IEW VOLUME 142

KeyMoisture-RelatedVariables

IWV IVT

IWV&IVTarenotequivalent

KeyMoisture-RelatedVariables

IWV IVT

HighIWV,LowIVT

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KeyMoisture-RelatedVariables

IWV IVT

IWVdecreasespoleward,IVTincreases

AtmosphericRivers(ARs)

• Narrowcorridors(i.e.,filaments)ofstrongverticallyintegratedwatervaportransport(Newelletal.1992;NewellandZhu1994;ZhuandNewell1998)

• Oftenfoundalongthepre-cold-frontalLLJandmaycontributetothemoisture-richportionofthebroader,ascendingwarmconveyorbelt(Ralphetal.2004;Sodemann andStohl 2013)

• Achievetheirhighwatervaporcontentthroughtransportfromthetropics[i.e.,tropicalmoistureexports(TMEs)]and/ormoistureconvergence(Knippertz etal.2013;Cordeira etal.2013)

• Associatedwithmidlatitude hydrologicextremes

ImportanceofPre-FrontalLLJJULY 2004 1743R A L P H E T A L .

FIG. 23. Conceptual representation of an atmospheric river over the northeastern Pacific Ocean. (a) Plan-view schematic of concentratedIWV (IWV � 2 cm; dark green) and associated rain-rate enhancement (RR � 0.5 mm h�1; red) along a polar cold front. The tropical IWVreservoir (⌃3 cm; light green) is also shown. The bold line AA⇧ is a cross-section projection for (b). (b) Cross-section schematic throughan atmospheric river [along AA⇧ in (a)] highlighting the vertical structure of the alongfront isotachs (blue contours; m s�1), water vaporspecific humidity (dotted green contours; g kg�1), and horizontal alongfront moisture flux (red contours and shading; ⇥105 kg s�1). Schematicclouds and precipitation are also shown, as are the locations of the mean width scales of the 75% cumulative fraction of perturbation IWV(widest), CLW, and RR (narrowest) across the 1500-km cross-section baseline (bottom).

the width scale of IWV� 2 cm decreased systematicallyfrom south to north, thus suggesting cold-frontal con-fluence may act to contract the lateral scale of the IWVplumes with increasing distance downstream of thesouthern terminus of the fronts. In contrast, the peakvalues of CLW and RR in the core region increased bynearly a factor of 2 and an order of magnitude, respec-tively, from south to north, while their width scales alsoincreased with increasing latitude. The latitudinal var-iation of CLW and RR reflects the fact that frontal cir-culations are generally stronger and broader in the mid-latitude storm track than they are in the subtropics. Thisvariation may also reflect Hadley cell subsidence, whichcan extend northward to ⇤30⌅N in the Northern Hemi-spheric winter mean. It is intriguing to note that thenorthward decrease of total IWV (factoring in that thearea of IWV � 2 cm narrows yet the core value staysconstant) appears to be offset by the increasing CLWand RR. While this is suggestive of a conservation ofwater mass in a Lagrangian sense, future efforts focusedon a full water budget study are required to demonstratethis.The case study and composite results are summarized

schematically in Fig. 23. In Fig. 23a the mean positionof the composite baseline is shown with respect to thecomposite cold front and atmospheric river. The verticalstructure is shown in Fig. 23b, including the key featuresidentified in the case study and in the satellite and syn-optic composites at the position of the composite base-

line where RR ⌃ 0. Overall, Fig. 23 highlights the nar-row and shallow nature of atmospheric rivers as wellas the elevated position of their core near 1 km MSL.Their relationship to upper- and lower-level jets andvertical frontal circulations and their associated con-vection, are also shown.An additional goal of the study was to objectively

evaluate the capabilities and limitations of current ob-serving systems to detect atmospheric rivers over theeastern Pacific for operational weather prediction andclimate monitoring. SSM/I-observed IWV data are re-liable in clouds and light to moderate rain, while GOESIWV measurements cannot be obtained in cloudy re-gions. The GOES IWV data on the edges of cloud bandsassociated with atmospheric rivers were not correlatedwith the SSM/I-observed IWV within these bands. Infact, the IWV values outside a 300-km-wide swath cen-tered on the IWV core are nearly uncorrelated with thecore values (i.e., r2 ¯ 0.1), which suggests difficultiesin using current data assimilation systems applied toGOES IWV data to try to fill the gap. Nonetheless, newensemble-based data assimilation systems hold promisefor using the GOES data outside the rivers to refine thepositioning (but not amplitude) of IWV plumes throughuse of an appropriate flow-dependent model of back-ground error covariances (Evenson 2003). Additionally,Scofield et al. (2000) have developed a promising com-positing technique using both SSM/I and GOES IWVdata in combination with operational numerical model

Ralphetal.(2004)

Identification

• Satellitebased(Ralphetal.2004)• IWVreadilyavailable;IVTnotreadilyavailable• UseIWVasanIVTproxy(OK,butnotgreat)• ARsidentifiedascontiguousregionsofIWV≥20mmthatare≥2000kminlengthand≤1000kminwidth

• AnalysisorNWPbased• IVTmagnitude

• e.g.,contiguousregionsofIVT≥250kgm-1 s-1 ≥2000kmlong(Rutz andSteenburgh 2012;Rutz etal.2014)

• PercentileIVTapproaches• e.g.,seasonallyvarying85th percentileIVT(GuanandWaliser2015)

GlobalLandfallDistribution

GuanandWaliser 2015

ImportanceofIVT

BettercorrelationwithprecipitationRutz etal.(2014)

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Mt.Rainier:6–7Nov2006450mm(17.7in)in36h

ParkevacuatedDamageledto5-monthclosure

Neimanetal.(2008),NPS,https://www.nps.gov/mora/learn/management/upload/2006%20Flood%2012_17_09.pdf

ExampleAREvents ExampleAREvents

MammothMountain,CA:232cmsnow

Sundance,UT:191cmsnow,313mmSWE

December2010InlandpenetratingAR

232cm(91in)snow@MammothMt,CA191cm(75in)snow@Sundance,UT

MammothMountain,BillNalli,Rutz etal.(2014),Steenburgh (2014)

CharacteristicsofARsoverWesternU.S.

Discussion

• Whereandwhydoyouthinkatmosphericriversaremostcommon• AlongtheUSwestcoast?• InthewesternUSinterior?

• WhatprocessesfavorARdecayduringpenetrationintothewesternUS?

• WhatprocessesmightcontributetoARmaintenanceorintensification?

ARCharacteristics:WesternU.S.

• Reanlayis data:• ERA-Interim• Cool-season(Nov-Apr)• Nov1988–Apr2011

• ARdefinition:• ≥2000-kminlength• IVT≥250kgm-1 s-1

• Precip:• NOAA/CPCunifieddailyprecip analysis(0.25º)• SNOTELgauge

Rutz etal.(2014)

ARFrequency

Rutz etal.(2014)

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ARDuration

Rutz etal.(2014)

FractionofCool-SeasonPrecip

Rutz etal.(2014)

SignificanceofInfrequentARs

Rutz etal.(2014)

TopDecile24-hEvents

Rutz etal.(2014)

ARPathways/SierraInfluences

Composite500-mbheightandconditionalARfrequencies

Rutz etal.(2014)

ARPathways/SierraInfluences

Composite500-mbheightandconditionalARfrequencies

Rutz etal.(2014)

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Aspect,Exposure,WVDepletion

Rutz etal.(2014)

Lagrangian Perspective

• Launch950-hPatrajectoryfromT1whenARispresent• CoastallyDecaying:ReachesT2,butnotinanAR• InlandPenetrating:ReachesT2 inanAR• InteriorPenetrating:ReachesT2 inanAR

Rutz etal.(2015)

Overview

Rutz etal.(2015)

OverviewNorthSouth

MaximaTotalARs

TotalinlandpenetratingTotalinteriorpenetrating

HighSierradecay

MaximaFractionofARsthatbecomeinlandorinteriorpenetrating

Rutz etal.(2015)

Characteristics@InitiationBlue:CoastalDecayingRed:InteriorPenetrating

HighSierradecay

ThebestwaytogetaninteriorpenetratingARistostartwitha“big”ARatthecoast

Rutz etal.(2015)

PressureChanges

p

∂p/∂t

Rutz etal.(2015)

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MoistureFluxChanges

qu

∂qu/∂t

Rutz etal.(2015)

WaterVaporChanges

q

∂q/∂t

Rutz etal.(2015)

DryingRatio

TotalChangeinq(~“dryingratio”)

Rutz etal.(2015)

MomentumChange

u

∂u/∂t

Rutz etal.(2015)

IntegratedMomentumChange

Rutz etal.(2015)

Summary

Rutz etal.(2015)

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ARPrediction

UsefulWebSites

• AtmosphericRiverPortal,Center forWesternWeatherandWaterExtremes• http://mead.ucsd.edu/

• Manymanyproducts– goodforIVTidentification,intensity,structure,probability,etc.

• NWS/WREnsemblegraphics• http://ssd.wrh.noaa.gov/naefs/

• GEFSIVT

• NWSSituationalAwarenessTable• http://ssd.wrh.noaa.gov/satable/• IVTstandardizedanomaliesandreturnperiods

Real-TimeExamples&Exploration

GroupActivity

• EvaluatethecharacteristicsofafutureAReventalongthewestcoastofNorthAmericaoverthenext10days• Whatistherangeofpotentialintensitiesandlandfalllocations?• Howunusualarethelowestandhighestintensitiesrelativetopastevents?• Howlongmighttheeventpersistataspecificlocation?• Whatsortofforecast,watch,orwarningactiondoestheeventwarrantatthepresenttime?