Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117

MM5 simulations ofa SCOS97-NARSTO episode

D. Boucouvalaa,b, R. Bornsteina,*, J. Wilkinsonc, D. Millerd

aDepartment of Meteorology, San Jose State University, San Jose, CA 95192, USA bDepartment of Applied Physics, University of Athens, Athens, Greece c

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, USA dDepartment of Meteorology, Naval Postgraduate School, Monterey, CA, USA

Received 6 February 2002; accepted 31 March 2003

Abstract

MM5 simulations ofthe 4–7 August SCOS97-NARSTO ozone episode in the SoCAB showed that a relatively rare

easterly upper-level background flow influenced surface wind flow directions at in-land sites. An inland-moving surface

convergence zone resulted where the offshore background flow met an inland moving westerly (sea breeze plus thermal

upslope) onshore flow. Maximum inland penetration ofthe zone was to the San Gabriel Mountains, where it produced

peak observed ozone concentrations. MM5 reproduced the main qualitative features ofthe evolution ofthe diurnal sea

breeze cycle, as it reproduced the opposing easterly flow, inland movement ofthe sea breeze front, and retreat ofthe

land breeze front. The accuracy of predicted surface winds and temperatures were improved by modifications of MM5

and/or its input parameters, e.g., by use ofthe PSU MBLI scheme, analysis nudging with NCAR/NCEP model output,

accurate specification ofdeep-soil temperature, observational nudging with SCOS97-NARSTO surface and upper level

winds and temperatures, and use ofupdated urban land-use patterns.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: SCOS97-NARSTO; SoCAB; MM5; Meteorological conditions

1. Introduction

South Coast Air Basin (SoCAB) ozone arises from a

combination ofcomplex precursor emissions, topography, meteorology, and photochemistry, as described in

the companion observational study by Boucouvala and

Bornstein (2003), hereafter referred to as BB03. The

various observational studies mentioned in BB03 have

provided a general understanding ofthe complex

meteorological and chemical processes producing high

ozone concentrations in the SoCAB. Additionally, 3-D

mesoscale meteorological modeling studies using the

various databases have provided additional details.

Ulrickson and Mass (1990a) used the prognostic

hydrostatic Colorado State University Mesoscale Model

(CSUMM) to simulate summertime flows in SoCAB.

Simulated daytime onshore, upslope winds were generally more accurately reproduced than were nighttime

low speed, spatially variable wind patterns. Output from

additional winter and summer simulations by Ulrickson

and Mass (1990b) were input into a pollution trajectory

model, which showed entrainment and upward motion

producing planetary boundary layer (PBL) growth that

mixed pollutants into the elevated subsidence inversion

layer.

The mass-consistent wind model of Douglas and

Kessler (1991) used SCCCAMP data to analyze SoCAB

mesoscale flow patterns associated with the Catalina

Eddy, but such diagnostic models cannot reproduce its

evolution. Better results were obtained with the

CSUMM (Kessler and Douglas, 1991), but off shore

area winds were sometimes overestimated, as the

uniform initial wind fields did not include many largescale details. Taha (1997) also used CSUMM to simulate

effects from urban-induced albedo increases on SoCAB

flow patterns, with results input into the photochemical

Urban Airshed Model (UAM). Results showed that an

increased albedo lowered; surface temperatures and

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AE International – North America

*Corresponding author. Tel.: +1-408-924-5205; fax: +1-

408-9234-5191.

E-mail address: pblmodel@hotmail.com (R. Bornstein).

1352-2310/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1352-2310(03)00384-4
mixing depths; emission and reaction rates; and peak

ozone concentrations. Mesometeorolgical output from

the hydrostatic UCLA model was input into a passive

tracer model by Lu and Turco (1995) to qualitatively

reproduce many observed features of the polluted

marine boundary layer (MBL) over the SoCAB. The

San Fernando convergence zone was shown to lift

pollutants into the elevated inversion, resulting in a

residual elevated reservoir ofdaytime polluted air that

survived throughout the night.

The non-hydrostatic fifth generation mesoscale meteorological model (MM5) has been used to investigate

meteorological conditions over California during ozone

episodes. Hegarty et al. (1998) used it to simulate South

Coast air quality study (SCAQS) data with two-way

nesting in three domains, initial conditions from

ECMWF-TOGA analyses, and average US Navy sea

surface temperatures (SSTs). Predictions after 48 h were

not completely accurate, however, due to a lack of

analysis nudging and four-dimensional data assimilation

(FDDA).

Stauffer et al. (2000) simulated a six-day SCAQS

ozone period using MM5 with analysis nudging and

FDDA. Although general agreement existed with the

observations, the model underestimated afternoon overland wind maxima by 1–2 m s1

. These MM5 options

were also used by Tanrikulu et al. (2000) to design the

SoCAB measurement program for the 1997 summer

time Southern California ozone study (SCOS97-NARSTO). Results showed that MM5 with FDDA generally

reproduced observed mixing depth and PBL wind, but

that simulated mesoscale return flows that recirculate

SoCAB air back over the sea were, however, too weak

and intermittent.

Among the objectives ofthe California air resources

board (CARB) sponsored SCOS97-NARSTO (Fujita

et al., 2000) were the following: produce an extensive

upper air database (Wolfe and Weber, 1999; CARB,

2001); improve understanding ofrelationships between

emissions, transport, and ozone exceedances; and

develop plans for further emission reductions to attain

the US air quality ozone standards. Objectives ofthe

current effort include use of SCOS97-NARSTO data

and MM5 simulations to understand meteorological

factors in the formation of high SoCAB ozone

concentrations. The period selected by CARB for the

study was 4–7 August, during which maximum SCOS97-

NARSTO surface ozone (187 ppb) concentrations

occurred at 2200UTC (1500PDT, Pacific Daylight Time)

on 5 August.

2. Methodology

A series ofsix MM5 simulations was carried out, and

this section describes the formulation of the final one.

Results from this simulation are described in Sections

4.2 and 4.3, while the successive steps (and their impacts)

are given in Section 4.1.

MM5 Version 2.12+ is used in the current simulation, where the plus indicates that several options used

in the current simulation are not normally available in

2.12. MM5 is described by Grell et al. (1994) and

Haagenson et al. (1994), and options currently selected

include: Deardorff (1972) force-restore surface temperature, Dudhia (1989) simple ice, Kain and Fritch (1993)

cumulus parameterization in the outer three domains

(none in inner), Gayno-Seaman (Ganyo, 1994) 1.5 order

turbulent kinetic energy (TKE) (only normally available

in MM5 Version 3), and one-way continuous nesting.

With this nesting option, only one MM5 simulation is

required, information is fed to the inner domains at each

time step, and no feedback exists from inner to outer

domains (Lozej and Bornstein, 1999). With ‘‘normal’’

one way nesting, each domain is simulated in a separate

run, outputs are kept at hourly intervals, and information is fed to inner domains. Two-way nesting is like

continuous one-way nesting, but feedback exists from

inner to outer domains. Note that the counter-gradient

heat flux correction term in the Gayno–Seaman TKE

scheme allowed for an unreasonable maximum diagnostic mixing depth of5 km, but this resulted in a

numerical instability; reduction to a value of3 km

corrected the problem.

Version 2.12+ also uses the Leidner et al. (2000) Penn

State University (PSU) marine boundary layer initialization (MBLI) scheme (not yet in NCAR MM5 releases)

during a 13 h spin-up period, which quickly establishes

realistic MBLs in data-sparse marine regions. Establishment ofsuch a simulated layer during summertime

Pacific High conditions normally requires about 48 h, as

cool marine surfaces produce only weak near-surface

shear-induced turbulence mostly suppressed by subsidence-induced stability.

Quadruple nested grid domains were used in the

current simulation (Fig. 1). Domain 1 (outermost) is

centered at 34.5N, 118W, and has a large upwind fetch

that includes the Pacific High, a horizontal resolution of

135 km, and 41 33 grid points in the E–W and N–S

directions, respectively. Domain 2 has a grid resolution

of45 km and dimensions of67 61; Domain 3 has a

resolution of15 km and dimensions of91 85; and the

inner domain (Domain 4) has a grid resolution of5 km

and dimensions of121 85. All domains have 32

vertical layers, with a minimum sigma level closest to

the surface that corresponds to about 14 m, with wind

and temperatures at halfsigma levels.

The 25-category US Geological Survey (USGS)

global land-use distribution in MM5 was used with

10 min latitude and longitude resolution in Domains 1

and 2, 5 min in Domain 3, and 30 s in Domain 4. Grid

averaged topographic heights for the SoCAB were

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S96 D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117
obtained from: PSU/NCAR for Domain 1, Geophysical

Data Center for Domains 2 and 3, and Defense

Mapping Agency for Domain 4. SST and NCEP/NCAR

global gridded model analyses from were both obtained

from the NCAR Mass Storage System at a resolution of

2.5 2.5. Both fields provided initial and boundary

conditions, the latter at 12 or 6 h intervals, respectively,

with linear interpolation for intermediate time steps.

Low resolution Domain 1 input analysis fields were

improved by assimilation ofdata (via the MM5

RAWINS preprocessor) from about 70 upper air and

several hundred surface sites within Domain 1. Initial

conditions for the inner three domains are obtained via

sequential interpolation ofvalues from each encompassing coarser domain.

The FDDA technique used in the current simulations

is based on Newtonian relaxation (or nudging), as

described by Stauffer and Seaman (1990), Seaman et al.

(1995), and Shafran et al. (2000). Nudging is a

continuous form of FDDA that ‘‘relaxes’’ model results

toward the ‘‘correct solution’’ by artificial tendency

terms in the prognostic equations. In the current

application these terms depend on differences between

MM5 values and linearly interpolated (in time and

space) global analyses. As these are only available at 12

or 6 h intervals, smaller scale upper-level forcings may

not be captured. The process was only carried out above

the PBL (assumed at 850 mb) in temperature and wind

only in Domain 4.

The current observational nudging, similar to analysis

nudging, used SCOS97-NARSTO observations within a

preset radius ofinfluence and a predetermined 2 h time

window. Observational nudging for (surface and upper

air) winds was done throughout the vertical model

extent in Domain 4, while rawinsonde and radar

atmospheric sounding system (RASS) temperature and

moisture were again only nudged above the PBL. Based

on an average distance of20 km between SCOS97-

NARSTO surface observational sites, Stauffer (2001)

recommended a 75 km surface radius of influence. The

value was doubled above the surface, as the number of

SoCAB measurement sites decreases and gradients

become smoother with increasing height. Data from

almost all the 110 surface, 12 rawinsonde, and 26

profilers discussed in BB03 were included in the

observational nudging. Point Loma (PLM) rawinsonde

and El Centro (ECO) profiler data (see BB03) were,

however, excluded due to their irregular winds. No data

were held back, however, as the goal was production of

the most accurate possible input meteorological fields

for the photochemical air quality simulation model.

The input deep-soil temperature in the ‘‘force-restore’’

surface temperature prediction equation was reduced by

4 K to correct a systematic bias in domain-averaged

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150 W

140 W

140 W 130 W 120 W 110 W 100 W 90 W 80 W

40 N

30 N

20 N

130 W 120 W 110 W 100 W

40 N

30 N

20 N

DØ2

DØ1

DØ3

DØr4

Fig. 1. Four MM5 domains, with 135, 45, 15, and 5 km mesh sizes, respectively.

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S97
nighttime minimum temperatures, as this parameter is

the ‘‘restore’’ that balances daytime solar heating

(Bornstein et al., 1996). First half-sigma level (7 m)

simulated temperatures were adjusted for statistical

comparisons with 2 m observed values by use ofa log

law profile that used MM5 simulated friction potentialtemperature values. The MAPS statistical package of

McNally and Tesche (1994) were used for all statistical

analyses.

The following modifications to MM5 input parameters were also made to produce accurate 7 m

simulated winds: (1) extension ofLos Angeles urbanized

area according to updated land use maps, (2) increase of

urban roughness value z0 from 0.5 to 1.5 m, (3) increase

of z0 values for other land use classes by about a factor

oftwo, and (4) replacement ofsea-surface wind speed

dependent z0 formulation by a constant value.

The initial sea surface z0 value in MM5 (0.01 cm) is

normally internally increased as a function of increasing

wind speed using the formulation of Charnock (1955).

This formulation, however, is only valid under ‘‘complete equilibrium’’ conditions, which rarely ‘‘exist over

lakes and oceans’’ (Arya, 1988). As the more general

formulation of Kitaigorodski (1970), which relates sea

surface z0 values to wave height, yields values ofup to

10 cm, a value of1 cm was thus used in the current

simulations. The 50 cm MM5 urban z0 value is too low

for a city the size of Los Angeles, as literature values

range up to 4–5 m (Arya, 1988). A value of1.5 m was

thus selected, and given the generally rough topography

ofthe SoCAB domain, several other z0 values were

likewise increased. Additional details are given in

Bornstein et al. (2001).

All simulations started at 1200UTC on 2 August to

include a 2-day spin-up period. Only results from inner

domain (centered on the SoCAB) are presented.

3. Observational summary

For more complete synoptic and mesoscale observational summaries ofthis case than are given below (see

Sterbis (2000) and BB03, respectively). For readability,

SCOS97-NARSTO date–hour specifications generally

will be abbreviated. Thus 0500UTC on 6 August 1997

will be noted as 06/05UTC, and its PDT equivalent of

2200PDT on 5 August 1997 will be given by 05/22PDT.

The first observed synoptic wind shift over the SoCAB

(before the ozone peak) resulted from a northwesterly

along-shore movement ofa 700 mb synoptic ridge. The

SoCAB was thus no longer on the eastern side ofthe

ridge (in a generally northwesterly onshore flow), but

was now on its southeastern side (with a generally

northeasterly offshore flow). The second observed shift

(after the ozone peak) resulted from an eastward

onshore movement ofthe ridge. The SOCAB was thus

no longer on the southeastern side ofthe high, but on its

southern side (with a generally southeasterly along-shore

flow).

By 05/22UTC, the onshore (sea breeze plus upslope)

flow reached its maximum intensity and had an inland

penetration that generally extended to the west side of

the San Bernardino Mountains (SBM). As winds on the

eastern mountain slopes were from the north–northeast

(opposite to onshore flow), a strong convergence zone

formed. As the onshore flow developed, precursor

pollutants were carried eastward (inland) to the mountains, and the low wind speeds within the convergence

zone allowed ozone concentrations to increase by

photochemical processes driven by oxides ofnitrogen

and non-methane hydrocarbon emissions. The mountains acted as a barrier, however, and thus five sites on

the western slopes ofthe SGM reported ozone

concentrations at 05/22UTC (or 05/15PDT) at 120–

187 ppb. After the peak was reached southeast of SGM,

pollution advected slowly eastward and values decreased, and by 06/03UTC the sea breeze had retreated

back to the coast.

4. Results

4.1. Simulations

As accurate temperatures are required to produce

realistic thermal mesoscale flows (Pielke, 1984), attention should first focus on the simulation of accurate

surface temperature values (Bornstein et al., 1996).

Changes were thus successively made (and applied to

subsequent simulations) until accurate MM5 surface

temperatures (Table 1).

The first simulation (Fig. 2) did not include either

analysis or observational nudging, and thus observed

versus first-level MM5 domain-averaged diurnal temperatures show a maximum under prediction ofdaytime

temperatures of5 K and a maximum over prediction of

nighttime values by 2 K (Run 1 curve). The cyclic

equilibrium in the predicted diurnal temperature waves

is an expected result, given both the lack ofcloud cover

and small day-to-day variation in solar forcing during

the short simulation period. The observations, however,

show a warming on the 5 August (peak ozone day) due

to the previously discussed intensification ofthe GC

upper level high.

The addition in Run 2 ofanalysis nudging every 12 h

from NCEP GDAS model fields in the three outer

domains reproduced the observed warming on 5 August.

The maximum daytime under prediction was reduced to

3 K, while the maximum nighttime over prediction was

still about 2 K (Run 2a curve). While temperatures are

observed at about 2 m AGL, MM5 first-level values

were calculated at 23 m. A post-simulation log-law

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S98 D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117
height correction to 2 m was thus made by use of

computed surface boundary layer (SBL) scaling friction

potential temperatures to the simulated values. While

the correction worked well for daytime values (it almost

completely eliminated all differences), it had virtually no

effect on nighttime values (Run 2b curve).

In Run 3, the input constant deep soil temperature

(discussed in Section 2) was reduced by 4 K. While this

change produced excellent nighttime results, daytime

maximum values on the peak-ozone day were reduced to

a level slightly below observed values. Run 4 (for the

first time) also included observational nudging in

Domains 3 and 4 by the SCOS97-NARSTO measurements. As surface and PBL temperatures were not

nudged, domain-average predicted temperatures for

Run 4 did not change much from the good results of

Run 3.

An overestimation ofdomain-averaged surface nighttime winds by about 1 m s1 existed in Run 1 (not

shown), with the analysis nudging ofRun 2 producing

an even larger error (about 1.5 m s1

). Even inclusion of

observational nudging in Run 4 in the two outer

domains did not improve the results. Runs 5 and 6 thus

focused on improved surface wind values and thus did

not significantly alter the accurate Run 4 first half-sigma

level temperatures. As most SCOS97-NARSTO observations were within the innermost domain, observational nudging was thus performed only for this domain

in this final simulation. The Los Angeles urban area was

also updated (extended), z0 values were updated for

several land-use classes (Table 2), and the new seasurface z0 formulation was introduced. These changes

generally decreased nighttime speeds, with the new sea

surface formulation in particular reducing the excessive

speeds on both sides ofthe Ventura coast (at the SoCAB

northern edge).

To increase near-surface wind accuracy, the number

ofvertical grid levels was increased by two (to total of

32) in Run 6 (final case), with the first half-sigma level

thus reduced from 23 to 7 m. This eliminated the need

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Fig. 2. Domain averaged 2 m AGL predicted (different symbols for each Run) and observed () temperatures (C), where 0 h is

0700UTC or 0000PDT.

Table 1

MM5 simulations; see text for description

Runs Analysis

nudging:

domains

Log-law

temp.

correction

Lower

deep soil

temp.

Obs.

FDDA

domains

Updated

urban zone

New Sfc

rougness

First halfsigma level

(m)

1 None No No None No No 23

2 GDAS: 1–3 No/yes No None No No 23

3 GDAS: 1–3 Yes Yes None No No 23

4 GDAS: 1 Yes Yes 3+4 No No 23

5 GDAS: 1 Yes Yes 3+4 Yes Yes 23

6 NCAR: 1 Yes Yes 4 Yes Yes 7

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S99
for a wind-speed height correction. In addition, to

obtain more frequent global analysis data, 6 h NCEPNCAR analysis-nudging fields were used in this simulation.

4.2. Final-run temperatures

MM5 2 m temperatures at the 05/09UTC (05/02PDT)

nighttime period prior to the daytime peak ozone

(Fig. 3a) generally reproduced average observed features. These include an onshore gradient, with a cool

(onshore side of) coastline (18C), warm coastal plain

(24C), warmest inland southeast desert (30C), and

cool mountains (few observations available) (18C).

Concurrent observed 2 m values (Fig. 13a in BB03) are

about 20C, 22C, 30C, and 20C, respectively,

although accurate assessment ofobserved coastal values

is difficult due to the lack of over-water observations.

Similar problems exist with mountain top observations,

and a more complete statistical evaluation for the entire

simulation follows.

Daytime 2 m MM5 temperatures (Fig. 3b) 12 h later at

05/21UTC (05/14PDT) (hour before peak ozone) also

generally reproduce the concurrent observations, with

an onshore coastal gradient larger than 12 h before.

Values have increased during the previous 12 h (as

expected near the surface) to an average of about 33C

at the (onshore side of) coast, 36C on the coastal plane,

40C at inland southeast desert, and 24C on the

mountains. Concurrent observed values (Fig. 13b in

BB03) are about 33C, 38C, 43C, and 35C,

respectively, with the above mentioned mountaintop

data-gap again rendering a comparison useless.

MM5 800 m temperatures (a level generally close

to the coastal subsidence inversion top) at 04/12UTC

(04/05PDT), two nights before the ozone peak, also

generally reproduced average observed features

(Fig. 4a). This field is displaced by 3 h from that in the

above surface comparison to utilize the denser upperlevel observations at this time. A weaker (compared to

surface) horizontal temperature gradient exists, as

nighttime cooling over land areas generally decreases

with height. The gradient is also reversed from that at

the surface, with a warm coast (30C), cool coastal plain

(27C), cooler inland southeast desert area (25C), and

cold mountains (12C). Observed values (Fig. 15a in

BB03) also show a weaker and reversed gradient

(relative to surface), with values of about 30C along

the coast and 28C on the coastal plain; no data are

available in inland desert and mountain areas. As

discussed in BB03, the observed (and now also the

simulated) reversed gradient at 800 m AGL is due to the

existence ofthe subsidence inversion at coastal (but not

inland) sites.

Daytime 800 m MM5 temperatures 12 h later (day

before ozone peak) at 05/00UTC (Fig. 4b) do not show

the (strong) onshore coastal gradient seen at the surface,

as inland surface heating decreases with height. Temperature values have also not greatly changed from the

previous 12 h (as expected aloft, where diurnal variations are reduced), and thus the weak reversed gradient

simulated 12 h before still exists. Concurrent observations (Fig. 15b in BB03), however, do show a weak (nonreversed) onshore gradient, with coastal temperatures

about 2 K cooler than coastal plain values. Given the

sparse offshore observations, it is difficult to say which is

correct, but in either case, 800 m AGL coastal gradients

are weak.

MM5 800 m temperatures (Fig. 4c) during the night

before the ozone peak (05/09UTC or 05/02PDT) are

concurrent with the 2 m values in Fig. 3a, but are 4 h

after the 800 m observed values of Fig. 15b of BB03.

They show virtual elimination ofthe reversed horizontal

gradient ofthe previous night, consistent with the largescale upper-level warming discussed above and in BB03.

While coastline temperatures have remained at about

30C, coastal plain values increased (by about 1C) to

30C. Values did not significantly change over the next

3 h (not shown), and thus the 4 h time difference

mentioned above is not a problem.

Daytime 800 m MM5 temperatures 12 h later (1 h

before ozone peak) at 05/21UTC (05/14PDT) (Fig. 4d)

have not generally changed from the previous 12 h. The

exception is a 3C warming along the coast, which

produces a well-defined maximum. This maximum is

consistent with the concurrent observations BB03, which

showed that an eastward movement ofthe upper level

ridge produced offshore flow that advected warm

subsiding air over the coastline during this period.

MM5 has thus captured the most important features

ofthe observed temperature fields during the peak ozone

period. Both analysis and observational nudging positively influenced both the 7 and 800 m AGL results.

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Table 2

MM5 roughness lengths

MM5 land

use class

Land use

description

Roughness length (cm)

Old New

1 Urban 50 150

7 Grassland 12 10

8 Shrubland 10 20

9 Shrubland and

grass mix

11 30

10 Savanna 15 40

11 Deciduous

broadleaf

50 100

14 Evergreen 50 100

16 Water bodies 0.01 1

Erroneously in Haagenson et al. (1994) as 0.12.

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Fig. 3. MM5 terrain heights (dashed lines, 500 m increment) and 7 m AGL temperatures (solid lines, 3C interval) on 5 August at:

(a) 0900 and (b) 2100UTC, with relatively warm (W) and cold (C) areas indicated.

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Fig. 4. MM5 terrain heights (dashed lines, 500 m increment) and 800 m AGL temperatures (solid lines, 3C interval) at: (a) 1200UTC

4 August; (b) 0000UTC 5 August; (c) 0900UTC 5 August; and (d) 2100UTC 5 August, with relatively warm (W) and cold (C) areas

indicated.

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Fig. 4 (continued).

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S103
4.3. Final-run winds

MM5 winds at the sigma level closest to 3800 m AGL

(s 1⁄4 0:575) shows patterns that reflect the changing

position ofthe upper-level ridge discussed above. At

04/18UTC (Fig. 5a) the high is located offshore of the

northern SoCAB (with a smaller that will move out of

the domain one to the north) and thus produces onshore

(generally westerly) flow over the northern SoCAB,

along shore flow (northwesterly) over the central

SoCAB coast, and offshore (generally easterly) flow

over the southern SoCAB. Concurrent observations

(Fig. 8a in BB03) also imply that the high is offshore of

the northern SoCAB, but the lack of offshore observations makes it impossible to determine its exact location.

The observations do, however, show onshore (generally

westerly) flow over the northern SoCAB and along shore

flow (northwesterly) over the central SoCAB coast. The

observed onshore flow over the southern SoCAB coast,

which disagrees with the offshore MM5 results in that

area, is based only on one site.

Twelve hours later at 05/06UTC (Fig. 5b), the MM5

high is now further northwest (west of the SoCAB, and

south ofthe Santa Monica Mountains; SMM) in

association with (offshore) northerly to easterly flow

over the Basin. This pattern is consistent with the

concurrent observations (Fig. 8b in BB03). By the peak

ozone time of05/20UTC (Fig. 5c), the MM5 high is

more northward (along coast), and thus the flow is

mostly easterly offshore over most of the SoCAB, except

for a narrow zone of northerly flow along the coast in

the northwest part ofthe domain. This pattern is again

consistent with the concurrent observations (Fig. 8c in

BB03), which shows northeasterly flow over the entire

Basin. The lack ofan observed coastal high is due to a

lack ofobservations.

By 06/12UTC (Fig. 5d), the high has moved even

further eastward and inland, maintaining offshore flow

in the southern SoCAB, but producing southeasterly

along-shore flow over the northern SoCAB and a small

area ofonshore southerly flow north ofthe SoCAB.

This pattern is again consistent with the concurrent

observations (Fig. 8d in BB03), which show offshore

flow in the southern SoCAB and southeasterly along

shore flow over the northern SoCAB. The lack ofany

observed onshore southerly flow north ofthe SoCAB is

again due to a lack ofobservations.

The changing upper-level synoptic flows also impacted MM5 flows nearer the surface. While the MM5

7 m flow at 04/12UTC (04/05PDT) (Fig. 6a) (two nights

before ozone peak) shows strong onshore flow in

offshore areas, a complex combination of low-speed

flows exists over the SoCAB. Although inland areas

show organized down-slope flows towards the coast, no

clearly defined coastal-plain land breeze is seen. A

coastal zone ofnear-calm flow between the westerly flow

over the sea and the down slope flow off of the mountain

peaks narrows into a convergence line along the coast in

the northern part ofthe domain. The concurrent

observed flow (Fig. 9a in BB03) also shows a complex

combination oflow-speed coastal-plain chaotic flows, a

coastal convergence zone, and no clearly defined coastal

land breeze. Inland down-slope flows, however, are not

as strong as those from MM5 (about 2.5 versus 4 m s1

).

By 04/15UTC or 04/08PDT (Fig. 6b), the coastal

convergence zone and coastal plain near-calm area are

still present, but the latter is decreased in size. The

down-slope flows, however, have weakened, as this time

is within the ‘‘transition to upslope flow’’ period. The

concurrent observations (Fig. 9b in BB03) show

generally lower coastal plain speeds and thus a more

extensive chaotic flow area, but about the same amount

ofthermally driven mountain flows as the MM5 results.

Three hours later at 04/18UTC (Fig. 6c), a sea breeze

flow (in the same direction as the large-scale westerly

flow) and a thermal upslope flow have formed. Between

the two resulting westerly flow areas, easterly flow still

exists. Two distinct thermal forcings (i.e., land–sea and

topographic) have thus each produced separate westerly

flow areas (e.g., coastal and on the westward facing

slopes). The observations (an hour before) also show

similar developing sea breeze and opposing easterly

upslope flows (Fig. 9c in BB03), but instead ofan area

easterly flow between the two westerly flow regions, only

a narrow poorly documented calm (or along-coast) flow

region exists.

By 04/23UTC (04/16PDT) (Fig. 6d), 1 day before the

ozone peak, MM5 has produced local northeasterly

upslope thermal flows on the eastern side ofthe SBM.

Simulated sea breeze speeds reached maximum intensity,

and both westerly flows have merged into one strong

inland-directed breeze that reaches to the peaks ofthe

SBM. Concurrent observations (Fig. 9d in BB03) also

show the merging ofthe two westerly flows at about the

same location. By 05/04UTC (not shown), MM5 and

observed coastal winds both have started to become

offshore. Two hours later (not shown), the sea breeze

flow has been replaced by a chaotic flow over most of

the coastal plane, and upslope flows have become downslope, as on the previous night.

The 05/12UTC MM5 surface flow field (Fig. 7a) is

again complex, similar to that on the 4th, except that the

flow along the southern SoCAB has shifted from

westerly to northerly. In addition, a somewhat faster

(more organized) easterly flow is now seen in the

northeastern part ofthe domain. The latter change,

related to the observed shift in the upper level flow (Fig.

9c in BB03), is not yet apparent in the one surface site in

that region (Fig. 10a ofBB03). By 05/15UTC (Fig. 7b),

neither organized sea breeze nor westerly upslope

thermal flows on the western side ofthe inland

mountains have begun. A stronger and more organized

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Fig. 5. MM5 terrain heights (solid lines, 500 m increment) and 3800 m AGL winds (barb is 1 m s1) at: (a) 1800UTC 4 August;

(b) 0600UTC 5 August; (c) 2000UTC 5 August; and (d) 1200UTC 6 August, with anticyclone center(s) (H) based on general flow

directions (large subjective arrows).

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Fig. 5 (continued).

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Fig. 6. MM5 terrain heights (light solid lines, 500 m increment) and 7 m AGL winds (barb is 1 m s1) on 4 August at: (a) 1200, (b) 1500,

(c) 1800, and (d) 2300UTC, with subjective general flow directions (large arrows), near-calm chaotic areas (outlined areas), and

convergence-zones (dark solid lines).

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S107
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Fig. 6 (continued).

S108 D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117
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Fig. 7. Same as Fig. 6, but for 5 August and with convergence-zone zoom-in area (box).

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S109
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Fig. 7 (continued).

S110 D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117
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Fig. 8. MM5 terrain heights (light solid lines, 500 m increment) and 7 m AGL winds (barb is 1 m s1) within zoom-in area for Run 6

for: (a) 2100UTC 4 August; (b) 0100UTC 5 August; (c) 2100UTC 5 August; (d) 0100UTC 6 August; and for (e) Run 1 at 2100UTC 5

August, with subjective general flow directions (large arrows) and convergence-zones (dark solid lines).

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S111
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Fig. 8 (continued).

S112 D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117
easterly flow has formed on the eastern site of SGM and

SBM due to the now easterly flow from the new position

ofthe upper-level high. Concurrent observations (Fig.

10b in BB03) show somewhat more onshore sea breeze

flow, but also (as in the MM5 results) no westerly

upslope thermal flows.

By three hours later at 05/18UTC (Fig. 7c), the sea

breeze and thermal upslope flows have begun, and the

boundary between the two westerly flow areas is again

easy to discern. Because ofthe influence ofthe now

easterly upper level background flow, the sea breeze is

less developed than 24 h before (Fig. 6c). This agrees

well with the observations (Fig. 10c in BB03), but they

are not dense enough to discern the calm region between

two MM5 westerly flow regions (just inland and at

SGM-SBM).

By 05/23UTC (time ofozone peak), the MM5

westerly flow again reaches maximum intensity and the

convergence zone is again between SGM and SBM

(Fig. 7d). Convergence zone movement on the peak

ozone day relative to that ofthe previous day is

significant for ozone transport determination, and thus

the winds in key SGM-SBM convergence zone area are

discussed in greater detail. While Figs. 8a and c show

only slight differences in inland penetration at 2100UTC

(1400PDT) between the 4th and 5th, the opposing

background in the Cajon Pass between SGM and SBM

on the peak ozone day (5th) is more organized. While 4 h

later (Figs. 8b and d), the inland directed flow had

penetrated through the Pass on the 4th, it could not do

so on the 5th, as the flow north ofthe Pass on the 4th

was then westerly, as opposed to northerly on the 5th.

The northerly flow on the 5th resulted from the changes

in the upper level flow discussed above, and the lack of

penetration through the pass on the 5th allowed ozone

peak formation in that area. The higher peak ozone on

the 5th (relative to the 4th) at 2100UTC probably in part

resulted from the increased subsidence warming on the

5th. Organized opposing flow was only correctly

simulated with analysis nudging, as the Run 1 (no

analysis nudging) 05/21UTC results (Fig. 8e) indicate

only weak opposing flow and hence the convergence

zone pasted SGM-SBM.

MM5 surface winds in this critical area at this time

compared well to the observations over most ofthe

domain (Fig 10d in BB03). The maximum westerly flow

penetration in the gap region on the 5th was thus less

than on the previous day, and by 06/03UTC (05/20PDT)

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Fig. 8 (continued).

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Fig. 9. MM5 terrain heights (light solid lines, 500 m increment) and 800 m AGL winds (barb is 1 m s1) on 4 August at (a) 1200 and

(b) 2300UTC and on 5 August at (c) 0500 and (d) 2200UTC, with subjective general flow directions (large arrows) and convergencezones (dark solid lines).

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Fig. 9 (continued).

D. Boucouvala et al. / Atmospheric Environment 37 Supplement No. 2 (2003) S95–S117 S115
(not shown) the sea breeze again (as on the previous

day) had retreated back to the coast. On 06/23UTC

(afternoon after peak ozone, not shown), the sea breeze

was more from the south and penetrated further inland

near SBM (as compared to previous days). Increased

penetration ofthe modeled and observed sea-breeze

front resulted from the turning of the synoptic flow aloft

from an opposing northeasterly flow on the 5th to a

southeasterly flow on the 6th in association with the

formation of a surface Catalina Eddy (Rosenthal et al.,

2003).

Nighttime 800 m AGL MM5 winds at 04/12UTC

(Fig. 9a) show a somewhat less complex combination of

faster speeds than the concurrent 7 m MM5 results due

to reduced surface influences aloft. Simulated down

slope MM5 flows at inland sites agree with the

observations in Fig. 11a in BB03. In addition, no clearly

defined land breeze has been simulated, as MM5 coastal

winds correctly show generally along-coast flows. One

area ofdisagreement, however, is that the observed

southeasterly along-coast flow in the northern SOCAB

has not been simulated. By 04/23UTC (Fig. 9b), the

onshore sea breeze flow reached maximum intensity,

with speeds faster than the corresponding 7 m MM5

speeds (Fig. 6d). The offshore flow at inland sites is

likewise faster and more organized than at 7 m. MM5

results in the coastal plane agree with the concurrent

observed flow field (Fig. 11b in BB03), but inland flow

directions cannot be determined due to a dearth of

observational sites.

By 05/05UTC or 04/22PDT, offshore flow exists over

the northern halfofthe SoCAB coastline, while along

shore flow exists over its southern half(Fig. 9c). The

concurrent observations (Fig. 11c in BB03) do not show

such a well defined along shore flow, but few observations are available right on the coast. By 05/22UTC

(Fig. 9d), the onshore MM5 flow had reached maximum

intensity and penetration, as with the concurrent

observations ofFig. 11d in BB03. The MM5 coastal

flow over the SoCAB was, however, more onshore than

the observations. Its inland penetration was reduced

over that simulated on the previous day (Fig. 9b) due to

the now stronger opposing easterly flow at inland sites.

5. Conclusion

Simulations ofthe 4–7 August SCOS97-NARSTO

ozone episode in the SoCAB with MM5 showed that a

relatively rare easterly upper-level synoptic background

flow influenced surface wind flow directions at inland

sites. An inland-moving surface convergence zone

resulted where the offshore background flow at inland

sites met an inland moving westerly (sea breeze plus

thermal upslope) onshore flow. Maximum inland

penetration ofthe zone was to the SGM, where it

produced peak observed ozone concentrations. MM5

reproduced the main qualitative features of the evolution ofthe diurnal sea breeze cycle with reasonable

accuracy, as it first simulated the diurnal variation of

coastal temperature gradients that produce the spatial

wind velocity distributions associated with the diurnal

sea breeze cycle. It also reproduced the opposing easterly

flow, inland movement ofthe sea breeze front, and the

retreat ofthe land breeze front.

The accuracy ofpredicted surface winds and temperatures were improved by the following sequential

modifications ofMM5 and/or its input parameters:

analysis nudging, accurate specification ofdeep-soil

temperature, observational nudging with SCOS97-

NARSTO surface and upper level coarse RWP and

rawinsonde winds and RASS temperatures, use of

updated urban land-use patterns, use ofcorrected input

ocean and urban surface roughness parameter values,

and proper limitation ofMM5 diagnosed mixing-depth

values.

Quantitative aspects ofthe diurnal evolution ofthe

observed sea breeze cycle less accurately simulated

include too low MM5 nighttime wind speeds (1.5 vs.

2.5 m s1

), which could result from: errors in the early

version ofthe observational data set (version MET-0),

high threshold anemometer wind speed values, errors in

global-model winds, over-nudging in the vertical of

(profiler and rawinsonde) winds, an inner-domain

lateral-boundary too close to the Ventura coast, and/

or a too large inner-domain horizontal grid spacing.

Although comparisons between the SCOS97-NARSTO

observations and MM5 model results are mostly

positive, complete understanding ofthe meteorology of

the 4–7 August peak ozone event requires additional

data analysis and model simulations.

Acknowledgements

This research was sponsored under contract No. 97–

310 from the California Air Resources Board (CARB).

Thanks to Jonas Haile and Ruri Imamura for help with

the graphics.

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