Determining Precipitation Type. Rain (R, RA)- Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters

DeterminingPrecipitation Type

Determining Precipitation Type• Rain (R, RA)- Rain is liquid precipitation

that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter.

• The intensity of rain is determined by the accumulation over a given time.

• Categories of rain are light, moderate and heavy.

Cold Rain Sounding

Cold Rain Sounding1. Check for moisture in the snow

dendritic snow growth region.• -10C to -20C = dendritic snow

growth region.• Temp – Dwpt = Dwpt

Depression• Sufficient Moisture: DD < 5C• Moisture in the snow growth

region = YES!

Temp

Dewpoint

-10C Isotherm

-20C Isotherm 0C Isotherm





region = YES!

Env Temp

Dewpoint

2. Any warm layers? (warmer than 0C)• Env Temp crosses 0C isotherm• YES! = the Env Temp crosses

the 0C isotherm at ~875mb.

-10C Isotherm






region = YES!

Env Temp

Dewpoint

-10C Isotherm

-20C Isotherm

2. Any warm layers? (warmer than 0C)• Env Temp crosses 0C isotherm• YES! = the Env Temp crosses


0C Isotherm

3. Any dry layers? (Dwpt Dep > 10C)• Dwpt Depression > 10C or

greater considered dry.• No significant dry layers.

Sfc T ~ +4.5CTd ~ +0.5C

Cold Rain SoundingEnv Temp

Dewpoint


greater considered dry.• No significant dry layers.

4.Any secondary sub-freezing layers?• NO! Warm layers extends to sfc.

1. Check for moisture in the snow dendritic snow growth region.• -10C to -20C = dendritic snow



region = YES!2. Any warm layers? (warmer than 0C)• Env Temp crosses 0C isotherm• YES! = the Env Temp crosses


-10C Isotherm


Cold Rain SoundingEnv Temp

Dewpoint

Precip starts out as snow, falls through a warm layer from 875mb to SFC (1000mb) and completely melts into rain.


greater considered dry.• No significant dry layers.• Driest layer near SFC (DD ~3C)


5.Check the sfc temperatures• Sfc temps near +4.5C• Not conducive for snow accum.




region = YES!2. Any warm layers? (warmer than 0C)• Env Temp crosses 0C isotherm• YES! = the Env Temp crosses


Sfc T ~ 4.5C

-10C Isotherm


Cold Rain Sounding

Determining Precipitation Type• Snow (SN, SNW, S)- Snow is an aggregate of

ice crystals that form into flakes. • Snow forms at temperatures below freezing. For

snow to reach the earth's surface the entire temperature profile in the troposphere needs to be at or below freezing.

• It can be slightly above freezing in some layers if the layer is not warm or deep enough the melt the snow flakes much.

• The intensity of snow is determined by the accumulation over a given time. Categories of snow are light, moderate and heavy.

Snow Sounding

Snow SoundingTemp

Dewpoint

-10C Isotherm





region = YES!

Snow SoundingTemp

Dewpoint

-10C Isotherm





region = YES!2. Any warm layers? (warmer than 0C)• NO! Env Temp. does NOT

cross the 0C isotherm, therefore entire sounding below freezing!

Snow SoundingTemp

Dewpoint

-10C Isotherm



greater considered dry.• No significant dry layers.• Driest layer only DD ~ 1C






Snow SoundingTemp

Dewpoint

-10C Isotherm

-20C Isotherm



4.Any secondary sub-freezing layers?• No, because the entire column is

below freezing.






0C Isotherm

Snow SoundingTemp

Dewpoint

-10C Isotherm





5.Check the sfc temperatures• Sfc temps around 0C• Accumulating snow likely.






Precip starts out as snow, falls through a column of air entirely below freezing, so no melting occurs. No dry layers for evap. and sfc temps below freezing, so snow at the sfc.

Sfc T ~ 0C

Snow Sounding

Determining Precipitation Type• Snow Pellets (GS)- A snow pellet is precipitation that

grows by supercooled water accreting on ice crystals or snow flakes.

• Snow pellets can also occur when a snowflake melts about half way then refreezes as it falls. Snow pellets have characteristics of hail, sleet and snow.

• Snow pellets will crush and break apart when pressed. • They can bounce off objects like sleet does. Snow

pellets have a whiter appearance than sleet. • Snow pellets have small air pockets embedded within

their structure and have visual remnants of ice crystals unlike sleet. Snow pellets are typically a couple to several millimeters in size.

Determining Precipitation Type• Snow grains are small grains of ice. They

do not produce much accumulation and are the solid equivalent to drizzle.

• Ice Crystals (IC)- Also called diamond dust. They are small ice crystals that float with the wind.

Determining Precipitation Type• Sleet / Ice Pellets (PE, PL, IP, SLT)- Sleet (Ice Pellets)

are frozen raindrops that strike the earth's surface. • In a sleet situation the precipitation aloft when it is first

generated will be snow. • The snow falls through a layer that is a little above

freezing and the snow partially melts. • If the snow completely melts it will be more likely to

reach the earth's surface as supercooled water instead of sleet.

• If the snow partially melts there will still be ice within the falling drop for water to freeze on when the drop falls into a subfreezing layer. The lowest layer of the troposphere will be below freezing in a sleet situation and deep enough to freeze drops completely.

Sleet Sounding

Sleet (Ice Pellets) SoundingTemp

Dewpoint

-10C Isotherm


1. Check for moisture in the snow dendritic snow growth region.• Sufficient Moisture: DD < 5C• Moisture in the snow growth

region = YES!


Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!2. Any warm layers? (warmer than 0C)

• Yes! Env Temp. increases to ~+2.5C near 850mb.

• Thus, partial melting likely• Liquid drop w/ ice nucleus

0C Isotherm

Warm Layer ~ +2.5C


Dewpoint

-10C Isotherm



region = YES!

3. Any dry layers? (Dwpt Dep > 10C)• No significant dry layers.• Driest layer only DD ~ 1.5C

2. Any warm layers? (warmer than 0C)• Yes! Env Temp. increases to

~+2.5C near 850mb.• Thus, partial melting likely• Liquid drop w/ ice nucleus

Temp

Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!


4. Any secondary sub-freezing layers?• YES! Env temp. goes below

freezing again around 875mb.• Secondary sub-freezing layer

extends down to sfc.• Re-freezing as a solid ice pellet.



Env Tempgoes below

freezing again

0C Isotherm

Sleet (Ice Pellets) Sounding


Dewpoint



region = YES!





5. Check surface temperatures.• Sfc temps ~ -7.5C• Accumulating ice pellets likely.



Sfc T ~ -7.5C

Precip starts out as snow, falls through a +2.5C warm layer and partial melting occurs. Liquid drop w/ ice nucleus falls through secondary sub-freezing layer and re-freezes as ice pellets (sleet) before striking the sfc.

Sleet Sounding

Determining Precipitation Type• Hail (GR, A)- Hail is dense precipitation ice that is that

least 5 millimeters in diameter. • It forms due to ice crystals and supercooled water that

freeze or stick to the embryo hail stone. • Soft hail is more white and less dense since it has air

bubbles. Soft hail occurs when hail grows at a temperature below freezing by ice crystals and small supercooled water and cloud droplets merging onto the hail.

• Hard hail occurs when liquid water drops freeze on the outer edges of the hailstone after the outer edge is above freezing.

• The freezing of supercooled water releases latent heat and this can result in the outer edge of the hail stone warming above freezing. Then the water refreezes creating solid ice. Hail will commonly have soft ice and hard ice layers when it is sliced open.

Determining Precipitation Type• Graupel (GS)- Graupel forms in the same

way as hail except the diameter is less than 5 millimeters. It usually grows by soft hail processes.

• Drizzle (DZ, L)- Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.

Determining Precipitation Type• Freezing Drizzle (FZDZ, ZL)- Freezing Drizzle is

liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter. The drops then freeze on the earth's surface.

• Freezing Rain (FZRA, ZR)- Freezing Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter. The drops then freeze on the earth's surface.

Freezing Rain Sounding

Freezing Rain SoundingTemp

Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!


Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!

0C Isotherm

Warm Layer ~ +7C


~+7C near 850mb.• Thus, complete melting likely.• All liquid drops.


Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!





Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!




extends down to sfc.• Supercooled liquid water drops.



0C Isotherm


Dewpoint

-10C Isotherm

-20C Isotherm


region = YES!





5. Check surface temperatures.• Sfc temps ~ -4.5C• Freezing rain on contact w/ sfc.



Sfc T ~ -4.5C

Precip starts as snow, falls through a +7C warm layer and complete melting occurs. Liquid drops fall through secondary sub-freezing layer & become supercooled. Sfc temps < 0C, so SC drops freeze on contact.

Freezing Drizzle Sounding

Freezing Drizzle SoundingTemp

Dewpoint

-10C Isotherm

-20C Isotherm

1. Check for moisture in the snow dendritic snow growth region.• Dwpt Depression ~ 18C• NO sufficient moisture in the

snow growth region, thus very few snowflakes form.

• Moisture present in 0C to -10C layer, thus mostly small supcooled liquid drops form.

Dewpoint DepressionDD ~ 18C


Dewpoint

-10C Isotherm

-20C Isotherm



• Moisture present in 0C to -10C layer, thus mostly small supercooled liquid drops form.


2. Any warm layers (T > 0C)?• Entire sounding below freezing.

0C Isotherm


Dewpoint

-10C Isotherm

-20C Isotherm






3. Any dry layers? (Dwpt Dep > 10C)• Only aloft. Once precip forms,

saturated down to the sfc.


Dewpoint

-10C Isotherm

-20C Isotherm







saturated down to the sfc.4. Any secondary sub-freezing layers?

• NO! Entire sounding below 0C.0C Isotherm


Dewpoint

-10C Isotherm

-20C Isotherm







saturated down to the sfc.4. Any secondary sub-freezing layers?

• NO! Entire sounding below 0C.5. Check surface temperatures.

• Sfc temps ~ -4.5C• Freezing drizzle will freeze on

contact w/ the surface.

Sfc T ~ -4.5C

Too dry aloft for snow growth, thus precip forms as small supercooled drizzle drops. These small supercooled drops fall through an entirely sub-freezing column and freeze on contact as freezing drizzle drops.

Freezing Drizzle / Freezing Rain Sounding

Freezing Rain Sounding

Determining Precipitation Type• Freezing Fog (FZFG)- Freezing fog is a fog

composed of supercooled water drops. These drops freeze just after they wet the earth's surface.

• Mixed Precipitation (MXD PCPN)- The combination of two or more winter precipitation types occurring at the same time or over a period of time at the same place.

Ice Crystal Formation• Three processes can cause ice crystal

formation in a cloud

– Heterogeneous nucleation

– Deposition

– Ice Multiplication

Ice Crystal Formation• Heterogeneous nucleation is the process

by which ice crystals form from liquid water molecules as the molecules collect and freeze onto foreign particles, such as dust, clay, and aerosols – Heterogeneous nucleation is most likely at

temperatures less than -10 C (especially around -15 C), but can occur as warm as -5 C; heterogeneous nucleation accelerates the freezing of drops, i.e., if no foreign particles are present, the air must be much colder for freezing to occur

Ice Crystal Formation• Heterogeneous nucleation cont.

– However, ice embryos also can pre-exist in clouds, which can freeze drops at temperatures warmer (around -5 C) than those associated with heterogeneous nucleation freezing.

– If freezing occurs by this process, larger drop diameters tend to have higher/warmer freezing temperatures (e.g., a 1.0 mm drop freezes at about -12 C) than smaller drops (e.g., a 0.2 mm drop freezes at about -20 C), since large drops have more freezing nuclei/particles in them

Ice Crystal Formation• Deposition is the process by which ice crystals

form directly from the water vapor stage in a saturated, subfreezing cloud, although deposition nuclei (centers upon which ice crystals form) must be present – The number of crystals forming in a cloud via

deposition is a function of temperature. At temperatures greater than -10 C, not enough deposition nuclei are present to form enough crystals for an efficient precipitation process to occur, although crystals can still form.

– Temperatures colder than -10 C, i.e., -10 to -20 C, are necessary for efficient crystal formation via the deposition process

Ice Crystal Formation• Deposition Cont.

– The deposition process is maximized at temperatures of -12 to -15 C when coincident with saturated ascent. At these temperatures, ice crystals grow at the expense of water droplets, as water vapor molecules migrate to the crystals.

– Also, these temperatures typically are found in the 700 to 500 mb layer in winter, i.e., where ascent usually is maximized near the level of non-divergence in winter storms.

– Deposition can produce cloud size crystals around 1 mm but this generally is not large enough to produce much surface precipitation

Ice Crystal Formation• Ice multiplication results in many more

crystals in a cloud than that caused by heterogeneous nucleation or deposition alone.

• Ice multiplication results in fragmented or splintered ice crystals; ice (rime) splintering occurs at temperatures warmer than -10 C while mechanical fracture of ice occurs at colder than -10 C

Ice Crystal Formation• Ice (rime) splintering is the primary ice

multiplication mechanism and is common at temperatures from 0 to -10 C with a peak at about -5 C.

• Splintering occurs when crystals originating in saturated layers aloft that are colder than -10 C (from either heterogeneous nucleation or deposition) fall into a 0 to -10 C layer where many supercooled droplets exist.

• The droplets freeze as they contact the crystals, then tend to break up (fragment/splinter) if large enough as the crystal/frozen droplet particles travel through the cloud

Ice Crystal Formation• Ice multiplication via splintering is common

where low and middle-level saturated ascent is occurring with mean temperatures about -5 C (e.g., in a comma head of a mature cyclone or where strong isentropic ascent and frontal forcing are occurring). – In these areas, a lot of crystals and nuclei are present

due to ice multiplication which, due to strong ascent, rise into layers with temperatures less than -10 C.

– This in turn leads to additional crystal formation and deposition which can then result in even more ice multiplication and so on. The result is that bursts of heavy snow can occur

Ice Crystal Growth• Ice Crystal Types:• Ice crystals come in different types and shapes

which are dependent on the temperatures in which they form and grow. The shapes of crystals and their associated temperature range include:

Ice Crystal Growth• Dendritic crystals are the dominant type in many

snow events as they form in a range of temperatures that is most conducive to deposition and maximum ascent in the 700 to 500 mb layer

• Ice crystal formation and growth by deposition alone typically cannot produce much precipitation sized particles. Ice crystal growth to precipitation sized particles (about 10 mm) is achieved via riming and aggregation. Growth rate increases as the size of ice particles increases

Ice Crystal Growth• Riming

– Growth by riming occurs and is most efficient when crystals fall from a cold layer aloft (less than -10 C) and contact supercooled water droplets in a saturated 0 to -10 C layer below, causing freezing of the droplets. Many of these particles then break apart via ice multiplication and splintering resulting in many different sized crystals. Excessive riming can produce snow graupel and sleet

Ice Crystal Growth• Aggregation

– Aggregation is a very important method for crystal growth, and occurs when the terminal fall speeds of varying crystal shapes are different resulting in crystal collisions and subsequent growth

– Fall speeds generally are strongly dependent on the shapes and sizes of crystals, which is a function of the temperatures in which they are growing (which affects shape) and the amount of riming (which affects size) crystals have undergone. For dendrites, fall speed is independent of crystal diameter; for columns and graupel, fall speed is dependent on diameter

Acknowledgements

• Portions of this lecture were adapted from material found at the following locations:– Meteorology Today Textbook– Severe and Hazardous Weather Textbook– Atmospheric Science, An Intro Survey Textbook– NWS Louisville Science and Technology web site

• http://www.crh.noaa.gov/lmk/soo/docu/precip_type.htm• http://www.crh.noaa.gov/lmk/soo/

– The Weather Prediction web site• http://www.theweatherprediction.com• http://theweatherprediction.com/preciptypes/

http://www.crh.noaa.gov/lmk/soo/docu/precip_type.htm

http://www.crh.noaa.gov/lmk/soo/

http://www.theweatherprediction.com/

http://theweatherprediction.com/preciptypes/

Documents

Determining Precipitation Type. Rain (R, RA)- Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters