comet rfc/hpc hydrometeorology course 02-1 12/3/01 john cortinas, jr., ph.d. [email protected]...

COMET RFC/HPC Hydrometeorology Course 02-1 12/3/01

John Cortinas, Jr., Ph.D.

[email protected] of Oklahoma-NOAA Cooperative Institute for

Mesoscale Meteorological Studies

RFC/HPC Hydromet Course 02-1:Precipitation TypeRain/Snow Lines


Forecasting Precipitation Type

1000 mb850 mb700 mb500 mbCloud/Precipitation RegionQPF: A forecast of the location and the liquid equivalent amount ofprecipitation during a given period.

Since the phase of the precipitation affects the amount of liquid water that reaches the ground, these forecasts must considerprecipitation type during the coldseason.


Forecasting Rain/Snow Lines and Other Precipitation Types

* Have snowflakes formed aloft?

* Will the snowflakes melt before reaching the ground?

* Will the snowflakes sublimate before reaching the ground?

* Are the snowflakes going to melt totally or partially?

* Will frozen precipitation fall through layers of supercooled water?

1000 mb850 mb700 mb500 mbCloud/Precipitation Region


Outline

* Brief review of precipitation microphysics associated with producing:

Snow (ice) Freezing rain (supercooled droplets) Ice pellets

* Review forecasting techniques for forecasting precipitation type.

* Explain representation of precipitation in numerical models.

* Introduce SPC products that may be helpful in forecasting precipitation type.


Factors that Determine Precipitation Type at the Ground

* Ground temperature (melting at the ground?)

* Precipitation microphysics (snowflake/ice crystal characteristics)

* Thermodynamic stratification (temperature and moisture vertical profiles)


Why is an understanding of precipitation microphysics important for QPFs?

* Precipitation microphysics determine precipitation type!

* This knowledge reduces forecaster reliance on “rules of thumb” that may be inaccurate.

* Anticipate short-term changes in precipitation type and intensity.

* Understand shortcomings of precipitation-type numerical guidance.

* Knowledge of precipitation microphysics helps forecasters interpret numerical model guidance intelligently (apply adjustments to model forecast).


Ice Physics - Basic Concepts

* ice nuclei - microscopic particles that serve as nuclei for ice crystal formation

* ice (snow) crystal - a small ice particle that results from deposition of water vapor onto an ice nucleus

* snow crystal habit - the shape of an ice crystal

* snowflake - an aggregate of ice crystals

* snowflake ice density - ice density of snowflake (bulk ice = 0.9 g/cm3)


Ice Nucleation* Homogeneous nucleation - the spontaneous initiation of an

ice crystal caused by random collisions of water vapor theoretically occurs when temperature is less than -40˚C and RH is

several hundred percent!

* Heterogeneous nucleation - the spontaneous formation of ice upon ice nuclei

requires slightly supersaturated (with respect to ice) conditions and temperatures less than 0˚C, usually less than -10˚C

Three modes of nucleation:

» Contact

» Deposition

» Freezing


Ice Nuclei

Most ice nuclei are clay particlesthat originate in desert and arid regions.

The concentration of active ice nuclei is temperature dependent.

A very small concentration of active freezing nuclei can existin air with a temperature ashigh as -5˚C (compared to ~1 X 108 L-1

for cloud condensation nuclei.)


Heterogeneous Nucleation

How does an ice crystal form?

* Contact mode: ice nuclei initiates the ice phase at the moment of contact with a supercooled droplet (riming)

* Deposition mode: water vapor diffuses directly to ice nuclei

* Freezing mode: ice phase is initiated from within a supercooled water droplet because of the presence of ice nuclei within the droplet


Snow Crystal Habits

simple plate

dendrite

crystal with broadbranches

sheath

hollow column

solid column

combinationof bullets

bullet

combinationof needles

(Pruppacher and Klett, p. 33)


Snow Crystal Habits

Habit is a functionof temperature andhumidity.

(Puppacher and Klett, p 32)


Ice Particle Multiplication Processes

* In the presence of air saturated with respect to ice, ice multiplication processes increase the number of ice crystals and ultimately create more or larger snowflakes.

* Two processes create additional ice nuclei from a single ice crystal:

mechanical fracturing

ice splintering

fragmentation of individual cloud drops during freezing (controversial)


Mechanical Fracturing

Occurs when fragile ice crystals, such as dendrites and other plates, collide with other crystals, graupel particles, or cloud drops.

The process requires turbulence or differential crystal fall speeds.


Ice Splintering

* Occurs during riming conditions

* Splintering is caused by collisions with relatively large cloud droplets

* Depends on drop size distribution, supercooled water content, velocity of the drops impacting the riming particle

* Pronounced maximum occurs at a temperature of -5 ˚C and drop impact velocity of 2.5 m/s


Snowflakes

Snowflakes are anaggregation of ice(snow) crystals.

(Rodgers 1974)

Avg. diameter of componentcrystals: (1) < 1.5 mm, (2) 1.5to 2.5 mm, (3) 2.5 to 3.5 mm,(4) > 3.5 mm


Crystal Growth by Vapor Diffusion


Growth Rate

Growth rate depends on temperature (T), pressure (p), and size (r) of ice crystal

(Ryan et al. 1976)P=constant

Plot shows growth rate dependence on size and temperature


Growth Rate

Growth rate is temperatureand pressure dependent.

(Rogers, p. 126)

Plot shows growth rate dependence on pressure and temperature


Crystal Growth Among Supercooled Water Droplets

Ice crystals grow at the expense of supercooled water droplets because thesaturation vapor pressure is lower over ice than it is over liquid water.

© Wadsworth Publishing Company 1998


Saturation Vapor Pressure

Difference betweensaturation vapor pressureover water and over iceis greatest near -12˚C.

0

0.05

0.1

0.15

0.2

0.25

0.3

-40 -35 -30 -25 -20 -15 -10 -5 0

Water - Ice (mb)

T (˚C)

Excess Saturation Vapor Pressure


Aggregation

Aggregation increases the mass of a snowflake and is afunction of temperature,maximizing near 0˚C because of sticky dendrites.

Aggregates are composed mostly of dendrites andsome thick plates. Secondary maximumoccurs near -12˚C, thetemperature at which the growth rate fordendrites is largest.

(Rogers 1974)


dMdt

=4πrD(ρ∞−ρr)

sublimation - air coolsdeposition - air warmsevaporation - air coolscondensation - air warmsmelting - air coolsfreezing - air warmsconduction - air cools or warms

Ice Crystal Decay


Sublimation

* Occurs when ice crystals or snowflakes fall through ice-subsaturated environments

(Hall and Pruppacher 1977)

(1) 0.3 g/cm3

(2) 0.5 g/cm3

(3) 0.75 g/cm3

(4) 0.9 g/cm3

(A) column, 800X164 mm(B) sphere, r=160 mm

Comparison between survival distance of cirrus ice particles

theoretical

observed

SLE Radiosonde

Air craft


Snowflake Melting

* The melting rate is a function of: air temperature near the hydrometeor surface relative humidity size of hydrometeor amount of liquid water present

* Temperature of air near snowflake surface is determined primarily by latent heating and conduction.

* Begins when temperature at surface of snowflake is > 0˚C stage 1: small drops of tens of microns in diameter appear at the tips of crystal

branches stage 2: capillary forces and surface tension draws liquid water to center stage 3: small branches of interior melt stage 4: main ice frame collapses, pulls itself together into drop


Melting Model

(I) Cooling from sublimation exceeds warming fromconduction, therefore snowflake temperature remains at freezing and no melting occurs.

(II) Warming from conduction exceeds cooling fromsublimation and snowflakes begin to melt. Melting rateis slow because of additional cooling from evaporation.

(III) Air is supersaturated with respect to ice anddeposition occurs. Heating from conduction and deposition exceed the cooling from evaporation, andthe snowflake melts completely.

air temperature

Vapor density

Theoretical and observational studies show snowflakes can descend 800 m below the melting level before

complete melting in a subsaturated layer.

(Matsuo and Sasyo 1981)

Dewpoint temperature

RH = 90%


Melting Experiments (Numerical)

density = 0.005 g/cm3

density = 0.02 g/cm3

density = 0.04 g/cm3RH = 100%, no sublimation, some evap.RH = 90%RH = 80% (Matsuo and Sasyo 1981)


Melting Experiment (Laboratory)

(Mitra et. al 1990)


Dependence of Melting on Air Temperature and Relative Humidity

(Matsuo and Sasyo 1981)


Cooling by Melting

* the latent heat used to melt precipitation cools the atmosphere

* significant and continuous melting can cool an entire vertical column below freezing, causing the melting level to descend to the surface

(Stewart and McFarquhar 1987)


Forecasting Rain/Snow Lines* Examine forecast soundings when possible:

wet-bulb profile melting level amount of precipitation expected general type and size of snowflakes presence of adiabatic cooling caused by upward motion ground temperature surface air temperature and relative humidity

* Given the existence of ice crystals, snow usually occurs when: the wet-bulb temperature throughout the entire troposphere is expected to be

equal to or less than 0 ˚C and the melting layer is within 0-800 m of the ground

» high melting levels require low humidity (< 70%) surface layers or large snowflakes for snow to reach the surface

» melting level can descend to surface with significant precipitation (in the absence of other thermal processes) : accum. Rain (in.) = [dT (˚C)*dP (mb)]/500 (Kain and Goss 2000), e.g. (2 * 200)/500 = 0.8”


Partial Thickness

* Thickness

* Advantages: only requires 4 data values more accurate than 1000-500 thickness easy to compute some skill with rain/snow decisions

* Disadvantages: does not identify shallow warm layers does not work for high terrain most accurate when “tuned” for specific

regions (no national uses)

700 mb

850 mb

dz=-(R/g p) [Tv dp]

1000 mb

}}

500 mb

}dz=-(R/g p) [Tv dp]

Heppner (1992)


Partial Thickness (Heppner 1992)

* Determine the thermal profile of the troposphere and its static stability.

* Examine the 850-700-mb thickness. (thickness > 1550 m, snow unlikely).

* Use care when evaluating 1000-500-mb thickness (snow can occur with thickness < 5400 m).

* Always consider effect of diabatic processes (evaporation or melting) on changing thermal profile.

* At 850 mb, 0 deg C isotherm doesn’t always work well to delineate rain and snow, especially with unstable lapse rates.


Freezing Rain

* Freezing rain occurs when snowflakes melt completely in an elevated warm layer and continue to fall into subfreezing surface layer with a temperature greater than -5 ˚C

* The lack of ice nuclei at -5 ˚C inhibits the liquid water droplets from freezing in the cold surface layer (become supercooled)

* Contact of supercooled droplets with subfreezing surface causes nearly instant freezing upon the cold surface

3 mm Maximum Diameter

(Stewart and King 1987)

FZRA

S


Sounding Analysis (Freezing Rain)

Height of MaximumTemperature

Depth of Cold Layer

Depth of Warm Layer

Tw


Sounding Analysis (Freezing Rain)

Reports of FZRA at 00 and/or 12 UTC (1976-1990)

* Albany, New York - 18

* Bufffalo, New York - 16

* Greensboro, North Carolina - 18

* Peoria, Illinois - 10

* Spokane, Washington - 6


Warm Layer Depth (Tw)

0

500

1000

1500

2000

2500

3000

ALB BUF GEG GSO PIA All

Depth (m)


Rawinsonde Data

* Data distributions at most stations are similar

* Median values: Depth of warm layer = 1500 m Depth of cold layer = 500 m Height of maximum temperature = 1500 m Maximum inversion temperature = 4˚C “Warm” area of sounding = 2000 deg - m “Cold” area of sounding = 125 deg - m

* Some variability exists between stations and events

* Important: Examine all sounding data to determine freezing rain potential


Ice Pellets

* ice pellets form when: snowflakes melt partially in an

elevated warm layer and fall into a subfreezing surface layer

fully-melted particles fall into a deep (.5 - 1 km), cold (T < -10 ˚C) surface layer.

* surface layer must be sufficiently cold (T < -10˚C) and deep enough to activate ice nuclei and cause liquid drop to freeze as it descends to the ground

3 mm Maximum Diameter

(Stewart 1987)


November 8, 2000 12 UTC


Microphysics Summary

* The amount of snowflake melting, and corresponding cooling, is dependent upon air temperature, relative humidity, snowflake size and density.

* Numerical and laboratory experiments show that complete melting occurs between 0 and 800 m below the melting level in saturated or unsaturated conditions.

* Rain/snow lines can exist on the warm side of the 0˚C isotherm when the RH is less than 100% and the melting level is within several hundred meters of the ground.

* Freezing rain conditions include a elevated deep warm layer and a subfreezing surface layer that is usually no colder than ~ -5 ˚C.

* Ice pellets conditions include a elevated, shallow warm layer and a cold (T < -5 ˚C) deep surface layer.


Precipitation Type Forecasts Using Numerical Model Data

* MOS

* Bulk microphysics scheme in numerical models Precipitation type through AWIPS browser (maybe) and NTRANS

* Experimental precipitation type algorithms <www.spc.noaa.gov/exper/ptax>


IAD E NGM MOS GUIDANCE 1/01/98 0000 UTC DAY /JAN 1 /JAN 2 HOUR 06 09 12 15 18 21 00 03 06 09 12 15 MX/MN 37 22 TEMP 19 17 16 26 33 37 32 30 27 26 25 36 DEWPT 6 6 7 10 11 12 14 14 14 14 16 22 CLDS SC SCSCSCSC SCCLCLCLCLSCCL WDIR 30 29 24 24 21 21 24 22 23 22 23 22 WSPD 08 06 04 08 10 11 12 13 13 12 09 11 POP06 0 0 1 0 0 POP12 1 0 QPF 0/ 0/ 0/0 0/ 0/0 0/ 0/0 0/ 0/0 PTYPE S S S S S S S S S R Z Z POZP 0 0 5 4 3 13 21 19 17 12 38 25 POSN 89 97 95 96 97 87 64 55 42 21 0 0

MOS

* Precipitation-type determined by relationship between model variables and observed precip. type

* Advantages: Automated system 3 hour output available Provides initial guess

* Disadvantages: Equations must be derived

for each location Only available every three

hours Accuracy uncertain


ETA: Cloud/Precipitation PredictionWater VaporCloudsLiquid WaterIce ParticlesRainSnowCondensation

of CloudsEvaporationof Clouds

AccretionAutoconversionAggregationAutoconversionMeltingEvaporation/Sublimation of Precipitation


Experimental Algorithms

* Precipitation-type algorithms evaluate the entire thermodynamic profile to determine the most probable type of precipitation based upon precipitation microphysics.

* Algorithm currently run with ETA data does not use cloud and precipitation data.

* Algorithm used with RUC data uses cloud and precipitation data, as well as thermodynamic data.

* Current algorithms are superior to thickness rules since they use all thermodynamic data and they incorporate physical processes.


Precipitation Type Algorithms


WINTER WEATHER MESOSCALE DISCUSSIONS

GOAL IS TO PROVIDE SHORT-TERM (0-6 HOUR) GUIDANCEON HAZARDOUS WINTER WEATHER FOR LOCAL NWSOFFICES AND OTHER USERS BOTH BEFORE AND DURINGTHE EVENT

- FIRST PARAGRAPH PROVIDES THE WHAT, WHEN AND

- SECOND PARAGRAPH PROVIDES THE MESOSCALEREASONING (THE WHY) OF THE FORECAST

- ISSUED FOR BLIZZARDS, HEAVY SNOW AND FREEZING RAIN

SPC Mesoscale Discussion


ZCZC MKCSWOMCD ALL;345,0867 385,0773 365,0773 325,0867;ACUS3 KMKC 240455 MKC MCD 240455NCZ000-SCZ000-TNZ000-240800-

SPC MESOSCALE DISCUSSION #1124 FOR WRN NC...ERN TN...AND NWRN SCCONCERNING...FREEZINGRAIN...AREAS OF LIGHT FREEZING RAIN/DRIZZLE ARE EXPECTEDTO CONTINUEACROSS PARTS OF WRN NC AND NWRN SC. LIGHT TO MODERATEFREEZINGRAIN IS LIKELY OVER PARTS OF ERN TN. ICEACCUMULATIONS AREEXPECTED TO VARY BETWEEN 0.10 AND 0.50 AN INCHTHROUGH 24/08Z.24/00Z RUC2 AND ETA MODEL FORECAST SOLUTIONS SUGGEST AN AREA OFENHANCED VERTICAL MOTION ACROSS NRN AL/SRN MIDDLE TN WILL MOVEENEWD ACROSS ERN TN INTO WRN NC. LATEST IR SATELLITE IMAGERYINDICATES COOLING CLOUD TOPS NEAR TCL TO HSV. ALTHOUGH BOTH MODELS SUGGEST A WELL FOCUSED AREA OF UVV OVER ERN TN... SATELLITE/RADAR AND SOUNDING DATA SEEM TO INDICATE A GENERAL AREA OF UVV FROM NRN AL INTO SRN/SERN VA.

EXPECT AREAS OF LIGHT TO BRIEFLY MODERATE FREEZING RAIN WILLGRADUALLY MOVE ENEWD AS UPPER LEVEL DIVERGENCE MOVES ACROSSERN TN AND WRN NC. FSL/AIRCRAFT SOUNDING DATA CONTINUES TO INDICATETHERMODYNAMIC PROFILES REMAIN FAVORABLE FORFREEZING/MIXEDPRECIPITATION.

..VAN SPEYBROECK..12/24/98...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHICPRODUCT...NNNN

SAMPLE WINTER WEATHER MESOSCALE DISCUSSION

Previous Mesoscale Discussion


References

* Baldwin, M., R. Treadon, and S. Contorno, 1994: Precipitation type prediction using a decision tree approach with NMCs mesoscale eta model. Preprints, 10th Conf. on Numerical Weather Prediction, Portland, OR, AMS, 30-31.

* Czys, R., et al., 1996: A physically based, nondimensional parameter for discriminating between locations of freezing rain and ice pellets. Wea. Forecasting, 11, 591-598.

* Erickson, M., 1995: Evaluation NWS precipitation type forecasts. Preprints, Sixth Conf. On Aviation Weather Systems, AMS, Dallas, TX, 219-224.

* Hall, W.D., H.R. Pruppacher, 1976: The survival of ice particles falling from cirrus clouds in subsaturated air. J. Atmos. Sci., 33, 1995–2006.

* Heppner, P., 1992: Snow versus rain: Looking beyond the “magic” numbers. Wea. Forecasting, 7, 683-691.

* Keeter, K. and J. Cline, 1991: The objective use of observed and forecast thickness values to predict precipitation type in North Carolina, Wea Forecasting, 6, 456-469.


References

* Matsuo, T., and Y. Sasyo, 1981: Melting of snowflakes below the freezing level in the atmosphere. J. Met. Soc. Japan, 59, 10-25.

* Nakaya, U., 1954: Snow crystals. Harvard Univ. Press, 521 pp.* Pruppacher H., and J. Klett, 1980: Microphysics of clouds and

precipitation. D. Reidel, 714 pp.* Roger, R., 1979: A short course in cloud physics. Pergamon Press,

New York, 235 pp.* Stewart, R., and P. King, 1987: Freezing precipitation in winter

storms. Mon. Wea. Rev., 115, 1270-1279.* Stewart, R., and G. McFarquhar, 1987: On the width and motion of

the rain/snow boundary. Water Res. Res., 23, 343-350.* Zerr, R., 1997: Freezing rain: An observational and theoretical

study. Wea. Forecasting, 36, 1647-1661.* Zhao, Q., T. Black, and M. Baldwin, 1997: Implementation of the

cloud prediction scheme in the Eta model at NCEP. Wea. Forecasting, 12, 697-712.

comet rfc/hpc hydrometeorology course 02-1 12/3/01 john cortinas, jr., ph.d. [email protected]...

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