This work was funded by NSF grant ATM-0340602, NOAA grant NA17RJ1228, and by a one-year AMS Graduate Fellowship.

1
An Observational Analysis of The 13 July Gulf
Surge Event During The 2004 North American
Monsoon Experiment
Peter J. Rogers, Richard H. Johnson, Paul E.
Ciesielski, and Brian D. McNoldy Department of
Atmospheric Science, Colorado State University,
Fort Collins, CO
DYNAMICAL MECHANISMS
OBSERVATIONAL RESULTS AND DISCUSSION
INTRODUCTION
The synoptic environment from two days
before (11 July, t -2 d) to two days after (15
July, t 2 d) gulf surge onset at Puerto
Peñasco was influenced by several features. ?
At 700 hPa (Fig. 2a), the subtropical high
weakened and broadened as it built north
and west from the LA/MS border to the
southern Great Plains. Tropical Storm Blas
propagated northwest from the eastern
tropical Pacific Ocean to the south and
west of Baja CA. ? At 200 hPa (Fig. 2b), a
southwest- northeast aligned inverted trough
was located over central Mexico and quickly
weakened as it moved northwestward.
Gulf surges are northward propagating
disturbances along the Gulf of California (GOC)
that advect large quantities of cool,
moisture-laden air from the GOC or eastern
tropical Pacific Ocean into the low deserts of
the southwest United States and northwest Mexico
during the North American Monsoon (NAM).
Previous attempts to understand the underlying
dynamical mechanisms for these surges have been
hampered by a lack of data in this region. High
temporal and spatial observational measurements
collected during the 2004 North American Monsoon
Experiment field campaign (1 July 15 August)
are used to describe the structure and probable
dynamical mechanisms of two gulf surge events,
one of which is discussed in this presentation.
Of greatest interest was the deployment of three
National Center for Atmospheric Research (NCAR)
Integrated Sounding Systems (ISSs) along the east
coast of the GOC, in addition to a relatively
dense network of rawinsonde sites across the NAM
geographical domain.
The convective environment on 13 July
consisted of many thunderstorm complexes along
the Sierra Madre Occidental (SMO) and GOC coastal
plain (Fig. 3a) that later merged together to
form a large mesoscale convective system (MCS)
between Bahia Kino and Los Mochis (Fig. 3b).
Strong evaporational cooling associated with
convective rainfall over the SMO foothills and
GOC coastal plain aid in the development of a
surface mesohigh (Fig. 8a). The anticyclonic
flow around this system and Coriolis deflection
piles cooler air along the north-south barrier of
the SMO, inducing south-southeasterly flow and
a linear Kelvin wave-like disturbance that
propagates north. As the surge moves north, it
develops nonlinearities along its leading edge
due to the greater depth of the surge compared to
that of the low-level nighttime stable layer into
which it is propagating (Fig. 8b). These
nonlinearities are evident in the surface (Fig.
6) and boundary layer (Fig. 7) observations as
characteristics similar to those of atmospheric
bores or solitary waves. The GOC LLJ may also
enhance gulf surge flow along the northern gulf.
(a)
(b)
(a)
(b)
Figure 3 Infrared GOES-10 Satellite imagery at
(a) 0146 UTC 13 July and (b) 0800 UTC 13 July.
Colored contours represent cloud top temperatures
(C).
Figure 2 T2A gridded dataset 700 (a) and 200 hPa
(b) heights (10 m intervals) (black solid),
temperatures (2C intervals) (red dotted), mixing
ratios (g kg-1) (colored contours), and wind
vectors (m s-1) at 1200 UTC from t -2 d (11
July) to t 2 d (15 July) where t 0 (13 July)
corresponds to day of gulf surge onset at Puerto
Peñasco. White regions represent areas below the
surface.
Gulf surge onset occurred at 1000 UTC 13
July (0600 UTC 13 July) at Puerto Peñasco (Bahia
Kino) as evident in the strong south-southeasterli
es that deepened and strengthened with time
(Figs. 4a-b, top panels). This pattern may be
enhanced by the GOC low-level jet (LLJ) and lasts
less than 12 hours. Maximum wind speeds
approached 20.0 m s-1 about two hours after
initial onset, but at a higher altitude at Bahia
Kino. Surge onset at Los Mochis (Fig. 4c) was
more difficult to discern, but was perhaps
related to the south-southeasterly flow that
began around 0000 UTC 13 July up to 2.0 km. Surge
surface signals at the northern sites (Figs.
4a-b, bottom panels) included a weakening of the
diurnal thermal signature, substantial pressure
rises, and a wind shift to the south-southeast.
The south-southeasterly flow on 14 July at each
site was most likely associated with Tropical
Storm Blas.
Figure 5 Half-hour interpolated rawinsonde
potential temperature and moisture flux
(meridional wind times mixing ratio) anomalies at
Yuma (a), Puerto Peñasco (b), Bahia Kino (c),
Empalme (d), Los Mochis (e), and Mazatlan (f)
from 0000 UTC 10 July to 0000 UTC 17 July.
Anomalies are calculated using 5 July-16 August
means. Colored contours represent potential
temperature (K). Solid (dotted) thin black
curves represent positive (negative) moisture
flux (m g s-1 kg-1). Thick solid black curve
equals zero, with each successive contour at (?)
25 m g s-1 kg-1 intervals.
(a)
(b)
DATA AND METHODS
(c)
Preceded by anomalous warming, gulf surge
passage was also accompanied by strong anomalous
low-level (below 800 hPa) cooling ( 4-8C) and
moisture flux (gt 175 m g s-1 kg-1), most
evident from Empalme northward centered about
1200 UTC 13 July (Figs. 5a-d). The impressive
cooling/moistening signals on 14 July at the
northern sites were most likely due to southerly
advection of cool, moist air via Tropical Storm
Blas from the east tropical Pacific Ocean.
The NCAR ISS sites were located at Puerto
Peñasco, Bahia Kino, and Los Mochis (Fig. 1), and
consisted of a Vaisala GPS sounding system, an
enhanced surface observing station, a 915 MHz
Doppler clear-air wind profiling radar, and a
radio acoustic sounding system (not used in this
study). Rawinsonde data from these sites and
numerous others (Fig. 1) were objectively
analyzed onto a 1x1 horizontal grid with 25 hPa
vertical resolution two (four) times per day over
the T2A (T1A) domain of 90-120W and 15-40N
(100-115W and 22-35N).
(d)
(a)
(b)
(c)
(e)
Figure 8 Conceptual schematic detailing the
evolution of proposed gulf surge dynamic
mechanism at times t0 (a) and t1 (b). Dotted
contour represents surge onset and perpendicular
yellow arrows represent gulf surge propagation
speed. Colored contours represent altitude above
sea level (m). Bottom panels depict gulf surge
horizontal structure (O hundreds of km).
(f)
CONCLUSIONS
Figure 4 Puerto Peñasco (a), Bahia Kino (b), and
Los Mochis (c) wind profiler and surface data
from 0000 UTC 11 July 0000 UTC 16 July. Wind
speed (colored contours) (m s-1) is plotted every
half hour and wind barbs every two hours. One
full barb equals 5 m s-1. Surface temperature
(red) (C), dewpoint temperature (blue) (C), and
pressure (green) (hPa) are averaged over and
plotted every half hour.
SURGE PROGRESSION, 20 m s-1
? Synoptic circulation patterns (subtropical
ridges, tropical cyclones, mid-tropospheric
easterly waves, and inverted troughs) can
enhance the development of NAM-related
convection. ? Preceded by strong anomalous
warming, the gulf surge exhibited
cooling, moistening, and increased
south-southeasterly flow from the surface
up to approximately 800 hPa. ? Surge passage
(in the early morning) was accompanied by a
rapid surface pressure rise. Surface winds
shifted to the southeast and strengthened
slightly. ? Gulf surge was amplified by the
nocturnal northern GOC LLJ. ? Surge
propagation speed along the northern GOC
was approximately 20.0 m s-1. ? Observations
suggest a nonlinear Kelvin wave-like
disturbance with atmospheric bore or
solitary wave-like characteristics along its
leading edge.
Initial gulf surge onset as identified from
Figs. 4a-b was accompanied by two distinct
anomalous surface pressure and temperature pulses
at both Puerto Peñasco (Figs. 6a and d) and Bahia
Kino (Figs. 6b and c). During the two hours
following surge onset, the pressure (temperature)
increased approximately 1.25 hPa (1.0C) at
Puerto Peñasco and 2.25 hPa (0.7C) at Bahia
Kino. Similar signals at Los Mochis were not
evident. Tracking these features between the
northern sites resulted in a propagation speed of
20.5 m s-1, assuming an initial surge front
perpendicular to 160. Several hours after
onset, the pressure anomaly curves remained at or
above their newly achieved levels and the
temperature anomaly curves decreased.
Prior to gulf surge passage, (Fig. 7, 0600
UTC) the local thermodynamic environment at
Puerto Peñasco consisted of a low-level thermal
inversion and nighttime stable layer. Conditions
above the inversion were near neutral and dry.
By 1200 UTC, the well-mixed boundary layer depth
substantially increased as evident in the lifted
height of the thermal inversion, and was a result
of surge passage.
(a)
Figure 6 Surface pressure (black) (hPa) and
temperature (red) (C) anomalies at Puerto
Peñasco (a) and Bahia Kino (b) for 13 July.
Anomalies are calculated using 0000 UTC 7 July
2359 UTC 14 August means. Panels (c) and (d) are
three hour subsets surrounding the time of gulf
surge onset at Bahia Kino and Puerto Peñasco,
respectively as noted by the dotted vertical
curves in panels (a) and (b).
(c)
(d)
Figure 7 Puerto Peñasco rawinsonde temperature
(red) (C), dewpoint temperature (green) (C),
and wind (1 full barb 5 m s-1) profiles at
0000, 0600, 1200, and 1800 UTC 13 July. Black
dotted curve represents reference dry adiabat.
(b)
Strong cooling, moistening, and increased
south- southeasterly flow also occurred both
above and below the thermal inversion.
Figure 1 2004 NAME rawinsonde sites. Site color
code refers to number of launches per day during
the extended observing period (EOP) and intensive
observing periods (IOP). Red arrows denote ISS
sites. Colored contours represent altitude above
sea level (m).
This work was funded by NSF grant ATM-0340602,
NOAA grant NA17RJ1228, and by a one-year AMS
Graduate Fellowship. For further information,
please visit http//tornado.atmos.colostate.edu/na
me/ or email progers_at_atmos.colostate.edu.