BP23
Prediction of Summer Stratus in San Francisco Bay
Aided by Low-Level HYSPLIT Trajectories
Gary P. Ellrod
EWxC, LLC, Granby, CT
1. Introduction
The prediction of stratus and fog in the San Francisco Bay region is
challenging and important for anticipating conditions that may result in
extensive delays for inbound and departing air traffic. San Francisco
International Airport (SFO) for example, is typically in the top ten among
airports in the United States with the longest average arrival and departure
delays, almost solely due to stratus. The closely spaced runways at SFO
require visual contact for arriving aircraft, which cannot be observed with
low clouds present. (Reynolds et al. 2012)
A critical element in these forecasts is low level flow from the Pacific Ocean
into the Bay Area. Trajectories allow forecasters to visualize the origin of the
air mass over the area. The National Oceanic and Atmospheric
Administration (NOAA) Air Resources Laboratory's (ARL) Hybrid SingleParticle Lagrangian Integrated Trajectory (HYSPLIT) model can provide
forecasts of low level trajectories using data from a variety of numerical
prediction models generated by the National Centers for Environmental
Prediction (NCEP). These trajectories can help forecasters visualize the
Inflow of marine air into the Bay region.
4. Mean Trajectories
HYSPLIT18h trajectories from an initial time of 1800 UTC were
obtained for cases of non-frontal stratus and no stratus for San
Francisco Bay, based on aviation observations at SFO and Oakland
(OAK) during the summers of 2013-14. The trajectories used data
from the Global Data Assimiliation System (GDAS) at either 1.0 deg
or 0.5 deg resolution. Starting point of the trajectories was at 37.5N
123W at a height of 50m. The dialog boxes below show the
procedures for generating a 6 x 6 matrix of trajectories.
In Northern California coastal stratus events, HYSPLIT trajectories typically show
pronounced flow into coastal bays such as San Francisco and Monterey. Days with no
fog showed predominately alongshore trajectories toward Monterey or points west. The
onshore flow is forced by the formation of a heat low in the interior Central Valley (lower
right), which leads to a strong pressure gradient from the coast to the interior. Sea
surface temperatures along the coast are very cool (top right) due to upwelling caused
by the low level northerly jet, leading to condensation in the boundary layer.
Website URL for NOAA HYSPLIT model is: http://ready.arl.noaa.gov/HYSPLIT.php
NOAA AVHRR SST, 1 month
mean centered on 1 Jun
2013 (NOAA CoastWatch)
Sample dialog boxes for running 6x6 matrix of HYSPLIT trajectories
along the South Atlantic Coast (see panel 5) using 2-deg grid
spacing. HYSPLIT runs in <1min on a PC.
5. Forecasting Approach
Mean HYSPLIT trajectories for 38 cases of stratus and 30
cases of no-stratus were obtained in May-Sep of 2013-14, for
a starting point west of SFO (37.5N -123W). Mean vectors
below (red lines) show pronounced onshore flow for stratus
days, but stronger along-shore flow on cloud-free days. Thus,
HYSPLIT may help with afternoon forecasts of stratus at
SFO/OAK for the following morning.
Mean 18h HYSPLIT trajectories for 30 cases of stratus (yellow) and no stratus
(white) at SFO and OAK
3. Sample Cases
2. Procedures
Using the combined 6h HYSPLIT
points for stratus at both SFO and
OAK and no-stratus cases (red
dots), the likelihood of stratus at
both SFO and OAK the following
morning was estimated by
calculating the percentage of
stratus points within three sectors
(NE, ESE, and SSE). Most of the
points in the NE sector (which
includes SFO/OAK and approach
zones) were stratus events, while
no stratus occurred in the SSE
sector.
18h HYSPLIT forecast from 18Z, 8 Jul 14
(left), and GOES-15 visible image at 14Z,
9 Jul 14 (right)
18h HYSPLIT forecast from 18Z, 29 Jun 14
(left), and GOES-15 visible image at 14Z, 30
Jun ‘14 (right)
NCEP Surface analysis for stratus case (L) and no stratus case (R)
6. Verification
7. Findings
HYSPLIT 6h forecast positions were obtained each day
during the summer (Jun-Aug) of 2015 and verified using
aviation observations at SFO and OAK. The NE and ESE
sector locations for stratus prediction (see panel 5) verified
best at OAK (blue boxes), while the no-stratus SSE sector
forecast worked well at SFO. A correct stratus prediction
required ceilings observed at least two consecutive hours
between 1200 and 1800 UTC (4AM – 10AM PST).
In the San Francisco Bay area, mean 18h HYSPLIT
trajectories from 1800 UTC show promise in predicting
stratus for the following morning at SFO or OAK. A
preliminary forecast system was developed to show the
likelihood of stratus within three sectors using the
combined 6h trajectory points. Verification during
summer 2015 indicated that the sector method for
stratus prediction works best at OAK, and results for no
stratus (SSE sector) verify best at SFO.
8. References
Draxler, R.R., and G.D. Hess, 1998: An overview of the
HYSPLIT_4 modeling system of trajectories, dispersion, and
deposition. Aust. Meteor. Mag., 47, 295-308. (Available at:
http://www.arl.noaa.gov/documents/reports/MetMag.pdf
Reynolds, D. W., D. A. Clark, F. W. Wilson, and L. Cook, 2012:
Forecast-based decision support for San Francisco
International Airport. Bull. Amer. Meteor. Soc., 93, 1503-1518.
Author’s Email Address:
Gary.Ellrod@gmail.com
Web site: www.ellrodweather.com
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