452 Precipitation
2016
Prediction of Clouds and
Precipitation often Poor
• Most precipitation is inherently mesoscale
and many models lack sufficient resolution.
• H20 vapor is poorly observed on both the
mesoscale and synoptic scale
• Microphysics problems, with other physics
problems (e.g., PBL) adding to the
difficulty.
Model Microphysics
Model Microphysics
• Many schemes use bulk parameterizations
that only predicted mixing ratio of various
cloud/precipitation species (like cloud
water). Called single-moment schemes
• Newer schemes are double moment,
predicting mixing ratio and the number of
each precipitation/cloud type.
• Both single and double moment schemes
assume size distributions, often some kind
of exponential.
Cloud Droplet Distribution
Bin Schemes are even more
sophisticated
• Predicts the details of the size distribution
by dividing into size bins, each of which is
predicted…but far too computationally
expensive for operations.
• Some new schemes offer variable riming:
coating of ice crystals by water that freezes.
Precipitation Forecast Skill
• Only modest improvement during the past
30 years
• Skill declines with intensity
• Higher in winter when precipitation is
more synoptic and stratiform
• Less skillful in summer when more
convective
• Humans add less skill than for most other
parameters
Human: WPC
Mechanisms producing saturation, clouds and
precipitation
• Dynamical forcing as expressed by QG
dynamics: broad frontal scale clouds.
• Topographic: upslope precip, mountain
waves clouds.
• Convection: produced by destablization of
the atmosphere.
Total precipitation is the product of all three
Orographic
precipitation
enhancement
Annual
Precipitation
Orographic Enhancement of Precipitation
12
Convective
precipitation
Precipitation Rate
Precipitation Rate (m m /hr)
(mm/hr)
Surface
frontal
rainband
Post-frontal
convection
Upper-level
frontal band
10
Upslope site
(elev. 1067m)
Pre-frontal
precipitation
8
Valley site
Upslope site at 1067m
6
Valley site
4
2
0
-600
-400
-200
0
Distance (km
)
Distance
(km)
200
400
600
Windward Enhancement
• For relatively stable flow and typical
barrier, tends to be on upper slopes of
barrier.
• But does depend on height of mountains,
stability, shear, freezing level, and location.
• Precip eventually starts to DECREASE at
higher elevations. (moisture availability,
terrain cross section)
Feeder-seeder mechanism
For low stability conditions,
convection can be released on the
lower foothills of terrain,
resulting in max precipitation
there.
The Lake Whatcom Flood and
Debris Flow Event of January 9,
1983
• Moist, unstable flow climbed the
mountains around Lake Whatcom
• 6-10 inches feel over 36 hr
• Substantial flooding and major debris
flows down Smith and other local
streams.
Picture Courtesy of Don Easterbrook,
How large is orographic
windward enhancement?
• 2-10x for major mountain barriers
• 1-3x for smaller barrier (hundreds of
meters)
• Orographic effects are strongest when
winds are perpendicular to barrier
• Around here, most profound postfrontal
with convection.
Rainshadows
• Lack of precipitation and clouds on lee
sides of barriers.
Rainshadows Shift with
Approaching Flow Directions
and Depend on Stability
• As the flow approaching a barrier changes
direction, so does the orientation of the rain
shadow.
• Less rainshadow during warm frontal period, more
post-frontal.
Rainshadow
Rainshadow in westerly flow
Annual
Precipitation
SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1
Sequim: roughly 15 inches per year.
.
Progress in Precipitation Prediction in Terrain
NGM,
80 km,
1995
NGM, 1995
2001: Eta Model, 22 km
36-km
12-km
NWS WRF-NMM (12-km)
2007-2008
4-km MM5
Real-time
Smaller Scale Terrain Modulates
Precipitation
10-km
Small-Scale Spatial Gradients in Climatological Precipitation on the Olym
Alison M. Anders, Gerard H. Roe, Dale R. Durran, and Justin R. Minder
Journal of Hydrometeorology
Volume 8, Issue 5 (October 2007) pp. 1068–1081
Annual Climatologies of MM5 4km domain
Verification of Small-Scale
Orographic Effects
Convective Precipitation
• Need at least 2-4 km resolution to get most
convection even half right.
• If grid spacing is more coarse must have cumulus
parameterization--Kain-Fritsch is used for
MM5/WRF.
• Convection is least skillful precipitation in
virtually all models.
• High resolution models can potentially give a
heads on type of precipitation a day ahead (squall
line, supercell, etc.)
Coastal Wind Convergence
Coastal Destabilization
Convection
• More later in course
• Historically, subject approaches have
dominated (CAPE, shear, sounding
interpretation, synoptic typing, etc)
• Models lacked the necessary resolution to
explicitly forecast (2-4 km)
• Convective parameterizations terrible.
• Things are changing now…model
resolution improving.
Terminology Review
• Explicit convection: model has enough
resolution to do it itself (less than 4 km grid
spacing)
• Parameterized convection: model
parameterization uses large scale variables
to determine precipitation and other impacts
of convection. Example: Kain-Fritsch,
Grell, Kuo.
What convective
parameterizations need to do
Real-time WRF 4 km BAMEX Forecast
Valid 6/10/03 12Z
4 km BAMEX forecast
36 h Reflectivity
4 km BAMEX forecast
12 h Reflectivity
Composite
NEXRAD Radar
Real-time 12 h WRF Reflectivity Forecast
Valid 6/10/03 12Z
4 km BAMEX
forecast
10 km BAMEX
forecast
22 km CONUS
forecast
Composite
NEXRAD Radar
Hurricane Isabel Reflectivity at Landfall
18 Sep 2003 1700 Z
Radar Composite
41 h forecast from 4 km WRF
Atmospheric Rivers
• Meridional flow of moisture is often limited
to relatively narrow currents of moisture
and usually warm temperatures.
Most West Coast heavy precip events are associated with
“atmospheric rivers”, a.k.a. the “Pineapple Express”
A relatively narrow current of warm, moist air from the
subtropics…often starting near or just north of Hawaii.
A Recent Devastating Pineapple Express:
November 6-7, 2006
November 6-9,
2006
Dark Green: about 20 inches
We know quite a bit about
atmospheric rivers and heavy NW
precipitation events, although there
are still gaps in our knowledge
Synoptic Set-Up for Top Fifty
Events at Forks
Courtesy of Michael Warner
Precipitable Water
500 mb height
SLP
850 mb Temp
Extreme Precipitation Events
• The current of warm,
moist air associated with
atmospheric rivers are
found in the warm sector,
parallel, near, and in front
of the cold front.
• Thus, atmospheric rivers
are closely associated
with the jet core and the
region of large
baroclinicity.
Orographic Enhancement
• Upslope flow
greatly increases
precipitation rates
on terrain.
• Thus, wind speed
and angle of attach
can greatly modify
the extreme nature
of the precipitation.
Diurnal Effects on Precipitation
• Sea breeze/land breeze effects
• Peninsulas
• Terrain effects
Predicting Clouds
• Zero-order approach: use relative humidity
• 70% 1000-500 mb RH often corresponds
with thicker, middle-level clouds that have a
serious impact on radiation.
• 700 and 850 mb RH is also used by some.
Direct Use of Model Clouds
• All modern models predict clouds,
specifically mixing ratios of cloud liquid
water and cloud ice.
• The quality varies…and keep in mind there
are serious deficiencies of even the best
microphysical schemes.
• And problems with other physical
parameterizations: boundary layer turbulence
or radiation can also mess up model clouds.
Direct Use of Model Clouds
• Most problematic: stratus, stratocumulus,
and FOG.
• Remember, some models have spin-up
issues: precipitation and clouds improve
during the first 3-9 hours. Particularly true
of UW WRF which is now cold-started (no
clouds at initialization!)
Cloud/Precip Forecasting
Strategy
• Very short-term (0-3 hr): temporal
extrapolation (informed by human
judgement) is HARD to beat. Nowcasting.
– Use radar and satellite animation. Models
generally not that useful.
– New data assimilation/modeling systems: e.g.,
EnKF, RUC/RR/HRRR are becoming viable
and eventually will take this on successfully.
Strategy
• Short-term (2-6 hr): Satellite extrapolation
becomes central. Model becomes more
dominant at the end
• Daily (6-24 h): Satellite extrapolation and
model, weighing model more at longer
range.
• Remember model spin up issues.
Strategy
• A wise forecaster ALWAYS evaluates
model’s initial moisture fields—often a
failure mode and a good indicator of
potential model failures.
• How? Compare initial model fields and RH
to satellite imagery.
850
700
Strategy
• Know regional precip/cloud climatology
– Diurnal and geographic features tend to be very
repeatable, particularly during the warm season
– Example: front range convection
Human Forecaster Issues
• Precipitation is a parameter where in
general forecasters add the least skill
compared to objective guidance.
• There are examples: like Monday’s
convection in eastern WA/OR
• Psychological issues for high and low
precip probabilities
Human Versus
Objective Skill for
Precipitation
Forecasts: NWS
Offices Around the
US
Brier Scores for Precipitation for all stations for the entire
study period.
Brier Score for all stations, 1 August 2003 – 1 August
2004. 3-day smoothing is performed on the data.
Precipitation
Brier Score for all stations, 1 August 2003 – 1 August
2004, sorted by geographic region.
Reliability diagrams for period 1 (a), period 2 (b),
period 3 (c) and period 4 (d).
Rain Psychology
More than a day out….Human Forecasters
Tend to Overpredict 10-30%
--exaggerating the threat of rain when it is not
likely.
“Its probably not going to rain…but I will
throw in 10-30% anyway to cover myself”
And underpredict rain when it is fairly definite.
“Well it looks like it will rain…but I am unsure…so I will
knock down the probabilities to 60-80% to cover myself”
Humans Help with
Heavier Precipitation
The End