Modification of Precipitation by Coastal Orography in Storms Crossing Northern California

23 Pages·2015·3.78 MB·English

Applied Aviation Sciences - Prescott College of Aviation
11-2005
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CCrroossssiinngg NNoorrtthheerrnn CCaalliiffoorrnniiaa
Curtis N. James
Embry-Riddle Aeronautical University, [email protected]
Robert A. Houze Jr.
University of Washington - Seattle Campus
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James, C. N., & Houze, R. A. (2005). Modification of Precipitation by Coastal Orography in Storms
Crossing Northern California. Monthly Weather Review, 133(11). Retrieved from
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3110 MONTHLY WEATHER REVIEW VOLUME133
Modification of Precipitation by Coastal Orography in Storms Crossing
Northern California
CURTIS N. JAMES
DepartmentofMeteorology,Embry-RiddleAeronauticalUniversity,Prescott,Arizona
ROBERT A. HOUZE JR.
DepartmentofAtmosphericSciences,UniversityofWashington,Seattle,Washington
(Manuscriptreceived4October2004,infinalform14March2005)
ABSTRACT
Thisstudycompilesandinterpretsthree-dimensionalWeatherSurveillanceRadar-1988Doppler(WSR-
88D) data during a 2.5-yr period and examines the typical orographic effects on precipitation mainly
associatedwithwinterstormspassingovercoastalnorthernCalifornia.
Thethree-dimensionalmeanreflectivitypatternsshowechostructurethatwasgenerallystratiformfrom
over the ocean to inland over the mountains. The flow above the 1-km level was strong enough to be
unblockedbytheterrain,andthemeanechopatternoverlandhadcertaincharacteristicsnormallyasso-
ciatedwithanunblockedcross-barrierflow,bothonthebroadscaleofthewindwardslopesofthecoastal
mountainsandonthescaleofindividualpeaksoftheterrainonthewindwardside.Upward-slopingecho
contoursonthescaleoftheoverallregionofcoastalmountainsindicatedbroadscaleupslopeorographic
enhancement.Onasmallerscale,themeanstratiformechopatternoverthemountainscontainedastrong
embeddedcoreofmaximumreflectivityoverthefirstmajorpeakofterrainencounteredbytheunblocked
flowandasecondaryechocoreoverthesecondmajorriseofthecoastalmountainterrain.
Offshore,upstreamofthecoastalmountains,thereflectivitypatternshowedaregionofenhancedmainly
stratiformechowithin(cid:1)100kmofthecoast,withanembeddedechocore,similartothoseovertheinland
mountainpeaks,alongitsleadingedge.Itissuggestedthattheoffshoreenhancementiscausedbyinten-
sifiedfrontogenesisintheoffshorecoastalzoneand/orbytheonshoredirectedlow-levelflowrisingover
athinlayerofcool,stableairdammedagainstthecoastalmountains.
Theorographicallyenhancedprecipitationoffshoreandoverthecoastalmountainswaspresenttosome
degreeinallthelandfallingstorms.However,thedegreetowhicheachfeaturewaspresentvaried.Allthe
featuresweremorepronouncedwhenthe500–700-hPaflowwasstrong,themidlevelhumiditywashigh,and
the low-level cross-barrier wind component was strong. When the low-level stability was greater, the
offshoreenhancementofprecipitationwasproportionatelyincreased,andthegeneralbroadscaleenhance-
mentinlandwasreduced.
1. Introduction (e.g., Nagle and Serebreny 1962; Elliott and Hovind
1964;Hobbsetal.1975,1980;Houzeetal.1976;Braun
CoastalnorthernCaliforniaisanideallaboratoryfor
etal.1997;Doyle1997;Colleetal.1999,2002;Yuand
observingstabletoweaklyunstableorographicprecipi-
Smull2000;Neimanetal.2004;Ralphetal.2004).The
tation. Deep convection is rare along the West Coast,
terrain in the region contains quasi-two-dimensional
andheavyprecipitationisusuallyassociatedwithland-
mountain ridges that are analogous to traditional ide-
fallingbaroclinicsystemsthatdirectstrong,moistlow-
alizedmodelstudiesofflowoverterrain(e.g.,Queney
level flow against the terrain from the Pacific Ocean
1948).Theridgesareorientedfromnorth-northwestto
south-southeast and are approximately orthogonal to
theprevailinglow-levelflowduringheavy-precipitation
Correspondingauthoraddress:Prof.C.N.James,Dept.ofMe-
events(Fig.1).TheSouthForkMountainandtheKing
teorology, Embry-Riddle Aeronautical University, 3700 Willow
Range ridges will be prominent in the subsequent dis-
CreekRoad,Prescott,AZ86301–3720.
E-mail:[email protected] cussions of this paper.
©2005AmericanMeteorologicalSociety
MWR3019
NOVEMBER2005 JAMES AND HOUZE 3111
quasi-perpendicular to the two-dimensional ridges of
the coastal mountains of northern California and thus
inanoptimalorientationfororographicmodificationof
theprecipitationassociatedwiththepassingbaroclinic
system.
The modification of the precipitation that occurs as
the LLJ impinges on a mountain range is a multiscale
process. The flow pattern in the baroclinic system is
modified on the mesoscale, depending on the stratifi-
cation of the temperature, moisture, and wind stratifi-
cation of the impinging flow. Over the Alps, Houze et
al. (2001, hereafter HJM) found that the basic pattern
ofprecipitationenhancementdependedonwhetherthe
upstream flow was blocked or unblocked. In the
blockedcases,theenhancementbeganupstreamofthe
barrier (as suggested, e.g., by Grossman and Durran
1984). In the unblocked cases, the enhancement oc-
curredalmostentirelyoverthelowerslopesofthebar-
FIG.1.DigitalterrainmapofcoastalnorthernCaliforniawith
rier with little activity upstream. Major Alpine floods
terrain elevation shaded. The locations of important geographic
occurprimarilywiththeunblockedcases,inwhichthe
features and the Eureka WSR-88D are labeled. Black-filled
circlesoverlandrepresentautomatedraingaugestations;black near-surface boundary layer air rises directly over the
dotsoveroceandepictthelocationsofEtaModelgridpoints.The barrier, with little tendency to turn parallel to the bar-
fourpointsenclosedbytheellipsewerehorizontallyaveragedto
rier or flow around individual peaks. The unblocked
produceasyntheticsoundingrepresentativeoftheupstreamflow.
cases are marked by rapid growth and fallout of pre-
cipitation over the lower windward slopes and espe-
ciallyoverthefirststeepriseoftheterrain(Buzzietal.
Whenstrong,moist,low-levelwindinteractswiththe
1998; Doswell et al. 1998; HJM; Yuter and Houze
California coastal orography, heavy rainfall accumula-
2003). In the combined case where lower-level flow is
tion (up to 250 mm in a day) can occur. The rain may
blockedandanupperlayerisunblocked,itisalsoevi-
combine with rapid snowmelt and produce extreme
dent from the Mesoscale Alpine Program (MAP) that
flooding of local rivers and streams. During one flood
orographic precipitation enhancement can occur both
inDecember1964,theEelRiver(Fig.1)roseapproxi-
upstreamfromandoveramountainbarrier[theinten-
mately 25 m above flood stage and reached a peak
discharge rate of 21 300 m3s(cid:2)1 (Sommerfield et al. sive observation period 8 (IOP8) case studied by Me-
dinaandHouze(2003,2005)andMedinaetal.(2005)].
2002). These extreme events can occur when the low-
Inthispaper,wesuggestananalogousbehavioroccurs
level jet (LLJ) required by hydrostatic and semigeo-
overcoastalnorthernCalifornia,wheretheflowabove
strophicbalanceaheadofafrontconveysmoistureand
1kmisgenerallystrongandlargelyunblockedbutmay
heat into the region of the mountains for a prolonged
periodoftime,sometimesforseveraldays(Ralphetal. have a lower layer of blocked flow also affecting the
2004, 2005). Floods occur in the European Alps for precipitation processes.
muchthesamereason,asthelow-levelmoistjetahead Small-scale processes also participate in orographic
of a front impinges on the barrier (Buzzi et al. 1998; precipitation enhancement. Smith (1979) pointed out
Doswell et al. 1998; Rotunno and Ferretti 2001). thelikelyimportanceofconvective-scalecellularityfor
Browning (1986) emphasized the strong poleward enhancing orographic precipitation particle growth by
sensibleandlatentheatfluxintheLLJtotheeastofan coalescence or riming. Elliott and Hovind (1964) sug-
approaching cyclone by calling it a “warm conveyor gestedthatprecipitationassociatedwithfrontspassing
belt.” LLJs approaching the California coastline origi- over California indeed manifested embedded small
nateinthenorthernfringesoftheTropicsandarecor- convectioncells,whichtheythoughtenhancedthetotal
respondinglyquitemoist.Ralphetal.(2004)referredto rainfall.AlsoexaminingCaliforniaprecipitation,White
the narrow belt of moisture flux within a warm con- etal.(2003)suggestedthatsmallconvectivecellsmight
veyor belt as an “atmospheric river.” When fronts ap- enhance the rainout. They also suggested, on the mi-
proachthenorthernCaliforniacoast,thisnarrowzone crophysicalscale,thatthecoalescenceofliquiddropsin
of moist flow is nearly always southwesterly, oriented thecloudlayerbelowthe0°Clevel(sometimesreferred
3112 MONTHLY WEATHER REVIEW VOLUME133
to as “warm rain”) is an important microphysical tivity)andairmotions(indicatedbyradialvelocity).By
mechanism producing precipitation fallout in storms furtherrelatingtheradarreflectivityandvelocityfields
passingoverthecoastalmountainsofCalifornia.White to the details of the topography of the terrain, we fur-
etal.(2003)alsoshowed,aswillbeshowninthispaper, therinferhowtheorographymodifiesthemicrophysics
thatmajorprecipitationsystemsaccountingformostof and dynamics.
the rain in the northern California coastal region ex- In section 2 of this paper, we describe the data and
tendwellabovethe0°Clevelandexhibitapronounced methodsusedinthisstudy.Section3presentsthelarge-
bright band, signaling that the ice phase is also impor- scale setting of major rain events over the northern
tant.Browningetal.(1975)suggestedthatbothliquid- Californiacoastalregion.Section4describestheover-
water coalescence and ice-phase processes contribute all average three-dimensional radar echo climatology.
to high precipitation efficiencies in orographic precipi- Section5explainshowwestratifythedatasetinto“ep-
tation. Yuter and Houze (2003) found that small cells ochs” defined by combinations of environmental vari-
embeddedinorographicprecipitationinbaroclinicsys- ables. Section 6 discusses the variability of the radar
tems over the European Alps were important in en- echoclimatologywithrespecttomidlevelflow.Section
hancing the growth of precipitation particles both 7 analyzes the orographic precipitation processes with
aboveandbelowthe0°Clevel.Theratesofcoalescence respect to the wind velocity and stability of the low-
belowandrimingabovethe0°Clevelwerecomparable level flow impinging on the mountains. Section 8 inte-
within small-scale embedded updrafts. Medina and grates all the results.
Houze (2005) have found that strong turbulent over-
turning in the shear layer separating a lower layer of
2. Data and methods
retarded or blocked flow from an upper layer of un-
blocked flow may enhance the growth and fallout of The basic dataset for this study is a two-and-a-half-
precipitationparticlesonthewindwardslopes.Larger, yeararchiveofmajorprecipitationeventsfromtheEu-
deepconvectivecellscanoverwhelminglydominatethe reka,California,WSR-88D.Thiscoastalradar,located
precipitationoverthewindwardslopeiftheflowovera nearCapeMendocino,hasarelativelyunimpededview
mountainbarrierissufficientlyunstable(e.g.,Caracena ofprecipitationoverthemountainstoitseastandover
etal.1979);however,thenorthernCaliforniaregionin theoceantoitswest.Thislocation(Fig.1)allowssam-
winter is not a favored location for deep convection, pling of precipitation systems as they approach the
and the weak instability leads primarily to embedded coast, make landfall, and move over the mountains.
shallowconvectioninabasicallystratiformcloudlayer. Terrainclutterandshadowingarelessofaproblemfor
In this study, we seek insight into the multiscale the Eureka WSR-88D than other West Coast WSR-
physicalprocessesinvolvedinorographicprecipitation 88Dsites(Westricketal.1999).ArchivedlevelIIdata
enhancement over the coastal mountains of northern of reflectivity and radial velocity (Crum and Alberty
Californiaandhowthenatureoftheenhancementvar- 1993)wereobtainedformostoftheheavy-precipitation
ies with the strength, stability, and layering of the up- days during 1 October 1995–31 March 1998, a period
stream flow. Achieving this objective will help deter- that included the California Landfalling Jets Experi-
mine whether processes occurring over this mountain- ment (CALJET; Ralph et al. 1999).
ous region are similar to or different from processes Amajorprecipitationeventwasdefinedasadayon
over the Alps and other major mountain ranges. We which at least 25% of the 73 automated rain gauges in
use the Weather Surveillance Radar-1988 Doppler the region bounded by 39°N, 42°N, 122°W, and the
(WSR-88D) operational radar data collected over a Californiacoastlinerecorded25mm(1in.)ormoreof
two-and-a-half-year period at Eureka, California (Fig. precipitation.Theblack-filledcirclesinFig.1showthe
1).Thisradarcoversprecipitationbothovertheocean locations of the gauges. Radar archives were available
and as it crosses the windward slopes of the mountain for 61 of the 67 heavy-precipitation days identified
barrier. This is not a study of rain measurement by (Table 1). The basic unit of radar data was the three-
radar. Rather, we aim to understand how the coastal dimensional volume scanned by the WSR-88D eleva-
mountainsaffectthephysicalprocessesofprecipitation tion angle sequence. To reduce autocorrelation and
growthandfallout.Toaccomplishthisaim,wecompile minimize data storage requirements, we reduced the
time-mean three-dimensional patterns of radar reflec- timeresolutionoftheradardatabyusingonlythedata
tivityandradialvelocity.Byexaminingsimultaneously volumes obtained at 1-h time intervals.
the detailed mean spatial structure of both the reflec- Since the radar processor’s clutter suppression algo-
tivityandradialvelocityfields,wededuceaspectsofthe rithmwasinsufficienttoremoveallterraincontamina-
interplay of microphysics (indicated grossly by reflec- tion, we developed a digital terrain mask. The terrain
NOVEMBER2005 JAMES AND HOUZE 3113
TABLE1.Heavy-raindaysidentifiedduring1Oct1995–31Mar1998whenWSR-88Darchiveswereavailable.
1995 1996 1997 1998
11,12,14,15Dec 15,16,18,27Jan 1Jan 2,3,11,12,14,16,18,25,26Jan
4,17,18,19,20Feb 16Mar 1,2,3,5,6,7,14,16,19,21Feb
4Mar 18Apr 12,21,22,23Mar
21May 3Jun
17,18,19Nov 8Oct
4,7,8,9,10,26,29,30,31Dec 15,16,26,29Nov
7,14Dec
mask used an equivalent-earth-radius ray-propagation Meanreflectivityandradialvelocitywerecomputed
modeltoapproximatethealtitudeoftheradar’slowest using the interpolated radar volumes. For reflectivity
tilt (0.5°) at each radar gate (Doviak and Zrnic 1993, averaging,amissingvalueindicatesabsenceofprecipi-
14–23). The equivalent earth radius corresponding to tation. Therefore, the reflectivity factor (mm6 m(cid:2)3) at
the strongest vertical refractivity gradient of all 61 the corresponding grid point was set to zero. Missing
heavy-precipitation days was used to give a liberal es- radial velocity information, on the other hand, merely
timate of the amount of beam refraction. Then, a ter- indicatestheabsenceofscatterers.Therefore,themean
rain elevation dataset with 30-s spatial resolution was reflectivity factor Z and radial velocity V at each grid
used to determine whether any terrain intersected the point were computed as
bottomoftheradarbeamwithinahorizontallatitude–
longitude element of dimensions 1(cid:3) (cid:4) 1(cid:3). If the main (cid:6)N
Z
lobe of the radar (width 0.94°) was intersected by ter- i
rain,thenthatrangebinandallbinsatfartherrangein Z(cid:5)i(cid:5)1 (cid:7)1(cid:8)
N
thatradialwereremoved.Toreducesidelobecontami-
nation,ifterrainwaslocatedwithin0.5°ofthebottom and
ofthemainlobe,thenthecorrespondingradarbinwas
deleted. This technique removed virtually all terrain (cid:6)n
V
clutterandshadowingfromthedatasetandallowedall i
remaining radar reflectivity bins over terrain to be in- V(cid:5)i(cid:5)1 , (cid:7)2(cid:8)
n
terpreted as precipitation rather than clutter contami-
nation.
To remove noise, radial velocity data were removed
if their corresponding reflectivity values were deleted
orbelowathresholdof(cid:2)10.0dBZ.Theradialvelocity
data were then dealiased using a University of Wash-
ington algorithm similar in construction to the WSR-
88Dalgorithm(EiltsandSmith1990).Asmallfraction
of the volumes that were not successfully dealiased by
thealgorithmwererejected,leavingatotalof1176for
analysis.Thevolumeswerebilinearlyinterpolatedtoa
three-dimensional Cartesian grid with 2-km horizontal
spacing and 0.5-km vertical spacing using the National
Center for Atmospheric Research’s (NCAR) SPRINT
software (Mohr and Vaughan 1979) and finally con-
verted to Unidata’s Network Common Data Format
(NetCDF)foranalysisusingMountainZebra(Jameset
al.2000),whichisaversionofNCAR’sZebrasoftware
(Corbetetal.1994)inwhichthedetailedterrainfieldis
included.Theinterpolationgrid,superposedwithaver- FIG.2.HeightvsrangerepresentationoftheEurekaWSR-88D
tilt sequence looking east from the radar site (indicated by the
tical cross section of the most commonly used WSR-
opencircleat767-maltitudeand0-kmrange).Eachradartiltis
88D volume coverage pattern (VCP 21), is shown in shaded, and interpolation grid points are indicated by “(cid:9).”
Fig. 2. (AdaptedfromJamesetal.2000.)
3114 MONTHLY WEATHER REVIEW VOLUME133
whereNisthetotalnumberofvolumes((cid:5)1176),andn
((cid:1)N)isthenumberofvolumeswithnonmissingradial
velocity values. From (1), it is evident that Z maxima
indicate regions where the precipitation was either
morefrequent,morereflective,orboth.Higherreflec-
tivity is generally correlated with heavier rainfall. To
reduce unwanted noise and radar artifacts, inverse
range-squared horizontal smoothing was applied at
eachinterpolationgridpointwithina16-kmhorizontal
radius of influence for all horizontal maps (6-km hori-
zontal radius for vertical cross sections).
To investigate the sensitivity of rainfall to the up- FIG. 3. The interdependence of squared moist Brunt–Väisälä
stream flow and stability, superposed epoch analyses frequencyandwinddirectioninthe900–800-hPalayerovervir-
tuallyalloftheEta-derivedupstreamsoundings.NegativeBrunt–
(e.g., Reed and Recker 1971; HJM), or “composites,”
Väisäläfrequenciesdenoteconditionalinstabilityinthe900–800-
were constructed by dividing the radar volumes into
hPalayer.
subsetsorepochsdefinedbysomespecificwindorther-
modynamic condition. Then, the mean and standard
deviationwerecomputedateachgridpointinthesub- Väisälä frequency was small in magnitude (i.e., less
setofvolumes.Thestatisticalsignificanceofthesesub- than 10(cid:2)4 s(cid:2)2), indicating that deviation from moist
set means were indicated by two-sided Student’s t dif- neutrality was minimal, with some days being slightly
ference-of-means tests that were performed at each moist-unstable and others slightly moist-stable. Fur-
grid point using an a priori confidence level of 95% thermore, the 900–800-hPa stability was generally un-
(Spiegel 1972). As in HJM, the sample sizes in this correlated with wind direction, although northwest
study and the low autocorrelation between successive flows were seldom absolutely stable.
radarscansbothappeartobeadequatefordifference-
of-means tests (e.g., Wilks 1995).
3. Large-scale flow and stability
The superposed epoch analyses led to conclusions
about the sensitivity of the precipitation in the vicinity Figures4aand4bshowthemeanlarge-scalesynoptic
ofEurekatodynamicandthermodynamicvariablesof conditions indicated by 12-hourly NCEP global model
the large-scale flow offshore. We estimate these up- reanalysisoutputat2.5°(cid:4)2.5°resolutionforthemajor
streamvariablesusinganalysesand6-hforecastsfroma rainevents(Kalnayetal.1996).Thismapshowsthaton
90 km (cid:4) 90 km (cid:4) 50-hPa resolution National Centers average the 1000- and 500-hPa flows were generally
forEnvironmentalPrediction(NCEP)EtaModelgrid. southwesterly,perpendiculartothemountainrangesof
Themodeldataatthefourupstreammodelgridpoints coastalnorthernCalifornia,consistentwithRalphetal.
bounded by the ellipse in Fig. 1 were horizontally av- (2004) and other studies mentioned earlier. A baro-
eragedtoproducesmoothverticalsoundingsat6-hin- clinictroughwaslocatedoffshorewiththemaximumof
tervals. If no model grid was available within 3 h of a large-scale upward motion located over the northern
radar-volume time, the volume was not used for those Californiacoast.Figure4alsodepictsthemeansynop-
calculationsthatrequiredsoundinginformation.Model ticmapsfortwosubsetsofcases.Forreasonsdiscussed
gridswereavailablefor1116oftheradarvolumes.The in section 6, we focus on cases for which the 900–800-
700–500-hPa layer in the synthetic soundings was used hPa flow was west-southwesterly, which produces the
torepresent“midlevel”characteristicsoftheflow.The greatest orographic effect on the precipitation.
900–800-hPa layer ((cid:1)1–2 km MSL) in the synthetic The west-southwesterly events are subdivided into
soundings, corresponding to the LLJ altitude and the days in which the thermodynamic stratification of the
strongest correlation with precipitation (Neiman et al. 900–800-hPalayerwasslightlyunstableorneutralver-
2002), was used to estimate the static stability, wind sus absolutely stable. The large-scale mean synoptic
speed, wind direction, and dewpoint temperature up- patterns for the unstable/neutral cases (Figs. 4c,d) and
wind. The static stability was represented by the moist stable cases (Figs. 4e,f) were qualitatively very similar,
Brunt–Väisälä frequency (Durran and Klemp 1982) except that the stable events exhibited slightly weaker
and was computed using finite differences. southwesterly gradient wind. The midlevel height pat-
Figure 3 summarizes the static stability estimates in terns (Figs. 4d,f) were also very similar, with compa-
the 900–800-hPa layer of onshore flow, as computed rable 700-hPa upward motion between unstable and
fromtheEtaModeloutput.ThesquaredmoistBrunt– unstable events. The 700-hPa vertical motion patterns
NOVEMBER2005 JAMES AND HOUZE 3115
FIG.4.MeanNCEPreanalysisfields,averagedover60heavy-precipitationeventsduring1Oct1995–31Mar1998.(a)MSLpressure,
1000–500-hPathickness,andisotachsofgradientwind;(b)500-hPaheightand(cid:10);(c)and(d)compositesynopticpatternsforthesubset
ofheavy-raineventsthathadunstableorneutralwest-southwesterlyflowinthe900–800-hPalayer,withthe0°Clevelabove2.5km
MSL;and(e)and(f)sameas(c)and(d),respectively,exceptthatonlystableeventsareshown.
are both elongated in Figs. 4d and 4f, with apparent at 50-hPa vertical resolution derived from the Eta
frontal orientation from south-southwest to north- Modelanalysisand6-hforecastfields(section2)forthe
northeast.Inbothcases,theCaliforniacoastappeared 61 heavy-precipitation days. The average 0°C level is
to be located in prefrontal flow. around 780 hPa (2.1 km MSL), and the (cid:2)15°C level is
Figure5ashowstheoverallmeanupstreamsounding about560hPa((cid:1)4kmMSL).Thedewpointdepression
3116 MONTHLY WEATHER REVIEW VOLUME133
intheprofilegraduallyincreasesfromabout2°Catthe
surface to 8°C above 4 km MSL, indicating that
midlevel cloud was less frequent than at low levels. In
addition, the temperature profile reveals the preva-
lenceofweakconditionalinstabilitybelowthe850-hPa
level, combined with abundant near-surface moisture.
Within the layer of conditional instability below 850
hPa, the wind profile veered with height, indicative of
warmadvectionand/orEkmanturning,withanaverage
900–800-hPa wind speed of 30–35 kt ((cid:1)15–18 ms(cid:2)1)
fromthewest-southwest.ThemeanEta-derivedprofile
inFig.5aisconsistentwiththepointdropsondeobser-
vations of Ralph et al. (2005) within 17 different land-
fallingLLJs,exceptthatthelargerareasamplinginour
Eta Model profile depicts slight conditional instability
rather than moist neutrality in the 900–800-hPa layer
and dry air intrusion at midlevels.
Figure 5b shows the average sounding for the un-
stable/neutral events, while Fig. 5c shows the average
sounding for the stable events. A small but notable
difference between the stable and unstable events is
that the stable events (Fig. 5c) had higher humidity
(indicated by much smaller dewpoint depression val-
ues)atmidlevels.Aswillbeshown,theintensityofthe
orographicmodificationofthesynoptic-scaleprecipita-
tion pattern was sensitive to these slight differences in
stability and/or humidity.
4. Radar climatology
The horizontal cross sections of echo patterns de-
scribed in this section are mostly taken at the 2-km
level,whichishighenoughtoavoidmuchofthenear-
field blocking of the radar beam by the terrain while
being in the rain layer below the 0°C level (Fig. 2).
Vertical cross sections incorporate all available grid
levels.
Figure6ashowsthe2-km-altitudehorizontaldisplay
of reflectivity over coastal northern California, aver-
agedoverthe61heavy-precipitationdays.Partialbeam
shadowingoccurredbehindSouthForkMountain(Fig.
1) and other terrain features, and corresponding radar
gates beyond those obstacles to the beam were re-
movedfromthedataset.Removaloftheseblockedand
partially blocked beams results in the circle of radar
observations being much smaller to the east of the ra-
←
FIG. 5. Mean upstream soundings (location shown in Fig. 1) ofdayswithwest-southwesterlyflowat900–800-hPaflowand0°C
derived from Eta Model output. (a) The average over all 60 levelabove2.5kmMSL.Thetemperatureprofileissolid,andthe
heavy-precipitationdays.(b)and(c)Meansoundingsforsubsets dewpointtemperatureisdashed.
NOVEMBER2005 JAMES AND HOUZE 3117
FIG. 6. Radar-derived precipitation climatology obtained for all heavy-precipitation events observed by the Eureka WSR-88D,
comprisingatotalof1176hourlyradarvolumes.Constantaltitudeplotsof2kmdepict(a)meanreflectivity,(b)therainfallfrequency,
orpercentageofradarvolumesinwhichthereflectivitywasatleast13dBZ,and(c)meanDopplerradialvelocity.Negative(positive)
radialvelocityindicatesflowtoward(awayfrom)theradar.Rangeringspacingis20km,andazimuthlinesaredrawnevery45°.The
whitecontourrepresentsthecoastline.(d)Averticalcross-sectionplotofmeanreflectivityfromsouthwesttonortheastalongthesolid
redlinein(a),withtheunderlyingterrainshadedgreen.Thedashedredlinein(a)indicatesthepositionofverticalcrosssectionsin
Fig.7.
dar than to the west in Fig. 6 and similar figures reflectivityisroughly60kmfromthecoastline,whichis
throughout the paper. roughly 150 km from the crest of the Coastal Range.
TheoverallpatternofreflectivityinFig.6aindicates Estimates of the Rossby radius (L (cid:5) NH/f) using the
both enhancement of echo directly over the coastal representative dry Brunt–Väisälä frequency (N (cid:5) 0.01
mountainsandupstreamofthecoastline.Theenhance- s(cid:2)1), characteristic terrain height (H (cid:5) 1.5 km MSL),
mentovertheterrainisevidentfromtheechomaxima andCoriolisparameterf(cid:5)10(cid:2)4s(cid:2)1correspondtothis
overtheKingRangeandoverthecrestandwindward offshoredistance,suggestingthatconvergentliftingdue
slopes (southwest side) of South Fork Mountain and to geostrophic adjustment in subcloud air may be en-
theimmediatelyadjacentterraintoitssouth(locations hancing precipitation upstream, before the southwest-
in Fig. 1), while upstream enhancement of the precipi- erly airstream (Fig. 6c) directly encounters the coastal
tation processes is evident from the echo pattern over terrain. The Rossby radius estimated using a moist
the ocean. Brunt–Väisälä frequency (Durran and Klemp 1982)
Theechocontoursoffshoreareorientedroughlypar- characteristic of absolutely stable events (N (cid:5) 0.002
alleltothecoast,withechointensitygenerallyincreas- s(cid:2)1)isonlyabout30km,andwithinthisdistancefrom
ing toward shore. The strongest offshore gradient of the barrier the reflectivity was even higher, suggesting
Fig 6live4/C
3118 MONTHLY WEATHER REVIEW VOLUME133
additional adjustment within the cloud layer itself,
closer to shore. However, these are only rough esti-
mates. The Doppler velocity data do not provide con-
clusiveevidenceofthesuspectedupstreamflowadjust-
ment,andthecomplexityoftheterrainandvariability
in the static stability make scale analysis to obtain the
appropriate expression of the Rossby radius problem-
atic.
Aquasi-circularmaximumofreflectivityisapparent
offshoreataradarrangeofabout40kminFig.6a.This
curved maximum is associated with the brightband ef-
fect of melting ice particles. However, the ringed pat-
tern also mimics the shape of the coastline, which
bulges westward at Cape Mendocino, and the curved
reflectivity pattern could therefore also indicate up-
stream enhancement of the precipitation. Braun et al.
(1997) analyzed aircraft data and found that the up-
stream modification of echo patterns appeared as an
echo maximum paralleling the Pacific coastline.
Figure6ashowsthatmaximaofthemeanreflectivity
occurred over specific areas of the terrain, especially
overtheKingRangeandSouthForkMountain(Fig.1).
Thequestionariseswhethertheprecipitationwasmore
intense or more frequent in these locations. Figure 6b
depicts the percentage of radar volumes in which the
reflectivity equaled or exceeded 13 dBZ, which is
roughly equivalent to a rainfall rate of 0.2 mm h(cid:2)1. FIG.7.Verticalcrosssectionsofmeanradialvelocityduring(a)
Overall,thepatternsinFigs.6aand6barequalitatively unstable/neutraland(b)stableevents(analysisV)takenapproxi-
similar, suggesting that orographic forcing generally matelyparalleltothewindalongthedashedredlineinFig.6a.
Theterrainisshadedgreen,andnegative(positive)radialvelocity
makestheprecipitationmorefrequentratherthanmore
indicates flow toward (away from) the radar location shown at
intense. (An exception to this rule will be discussed in
140-kmdistanceand767-maltitude.
section 7b.) Calculations of mean conditional rainfall
rate (not shown), which is related to mean echo inten-
sity,confirmthisresult,withtheexceptionthatslightly ratedandunsaturatedair,indicatingthatflowblocking
higherechointensityoccurredbothoverthehigherter- was not occurring at these levels. Figure 7 shows the
rain and offshore within roughly 60 km of the coast. average Doppler radial velocity along a southwest–
Figure6cshowstheprevailingDopplervelocityat2 northeast cross section passing through the radar site
km MSL, averaged over all 1176 radar volumes. The forunstable/neutralandstableconditions(asdefinedin
mean flow was nearly perpendicular to the Coastal section 3). Both sections show strong southwesterly
Range from the southwest at speeds approaching 20 flow increasing gradually with height throughout the
ms(cid:2)1 at 2 km MSL. Maps of the Doppler velocities at layer observed by the radar. The flow was slightly less
otheraltitudesindicatedthatthewindwasveeringwith intenseinthestablecasesbutneverthelessrapidlymov-
height,especiallybelow3-km-MSLaltitude,consistent ing over the coast and up over the mountains. These
with frictional turning, flow blocking, and/or warm ad- sections are, again, consistent with unblocked flow
vection. The strong horizontal low-level wind toward above the 1-km level.
the coast in the storms examined in this study ((cid:1)20 The vertical cross section of average reflectivity in
ms(cid:2)1) was at least twice the strength of the cross- Fig. 6d, taken parallel to the prevailing southwesterly
barrier low-level wind component of (cid:1)8 ms(cid:2)1 ob- wind along the red line in Fig. 6a is consistent with
served during heavy-rainfall events in the Alps during unblockedflowoverthecoastalterrain.Amaximumof
MAP (see HJM). The mean soundings (Fig. 5) show echo intensity occurred over the first major peak of
that the strong flow toward the barrier in the 1–3-km terrain(i.e.,theKingRange,locatedatabout70kmon
(900–700 hPa) layer during heavy-precipitation events the horizontal scale in Fig. 6d). This maximum ex-
had Froude numbers greater than unity for both satu- tendedtothehigherlevelsasanupwardprotrusionof
Fig 7live4/C