Typesof precipitation
1. Rain (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.
2. Snow (SN)- 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.
3. 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. With sleet (ice pellets), the snowflake almost completely melts before refreezing thus sleet has a hard ice appearance. Soft hail grows in the same way snow pellets can grow and that is ice crystals and supercooled water accreting on the surface. 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.
4. Snow Grains (SG)- Snow grains are small grains of ice. They do not produce much accumulation and are the solid equivalent to drizzle.
5. Ice Crystals (IC)- Also called diamond dust. They are small ice crystals that float with the wind.
6. Sleet/Ice Pellets (PL): 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. The lower boundary layer can be above freezing and sleet occur if the sleet does not have time to melt before reaching the surface.
7. Hail (GR) - 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.
8. 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.
9. Drizzle (DZ): Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.
10. Freezing Drizzle (FZDZ): 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.
11. Freezing Rain (FZRA): 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.
12. Freezing Fog (FZFG): Freezing fog is a fog composed of supercooled water drops. These drops freeze just after they wet the earth's surface.
13. 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.
Physical processes:
1.Microphysical processes
Warm cloud (T > 0ºC) in tropical grows by collision-coalescence (合并)
Cold cloud (T<0ºC) in mid-latitude. The upper and midlevels (-15 to -20°C) supply ice crystal via depositional growth (maximized at -15°C). Aggregation (聚合), rime(结晶)splintering(分裂), and accretion(增长) occur most rapidly at the lower levels of the cloud between 0°C and -10°C. The lowest levelis the splintering and accretion process.
Seeder-Feeder Mechanism.
This occurs when a cloud with ice crystals (seeder) at a given T moves over the top of a lower supercooled liquid cloud and precipitates ice down into that cloud (feeder).
the seeder (ice) cloud must not be more than 5000 feet above the supercooled liquid cloud (feeder)
2.The vertical thermal structure of the atmosphere.
LIQUID: Deep surface layer with temperature above zero.
FREEZING:Surface temperature below zero with an above freezing layer.
FROZEN: Deep surface base layer with temperatures below zero
PCPN Forecasting Strategies
- NWP (vertical motion ?): 250 mb jet streak; 500 mb PVA; 700 mb vertical velocity progs and RH panel; 850 mb PTA
- Tephi: convective precipitation and precipitation type. Pay attention to moist/dry layers, wind profile. Modify Tephi as necessary.
- Examine MOS precipitation data
- Look at satellite,radar (bright band), how they will move through time?
- Correlating current and/or past observations of precipitation type boundaries with thermal fields such as thickness and by predicting the short term evolution of the correlated thermal field
- Decide on characteristics of PCPN: Convective or stratiform?Heavy or light? widespread or numerous? Severe or non-severe? Long lasting or brief?
RAIN(radii 0.5 mm)
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warm rain produced by warm cloud several km indepth
the wet-bulb temperature is above 0°C
Rain from ice crystals: snowflakes or graupel in the sub-freezing layers of the cloud melt and fall to ground.
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DRIZZLE (diameter ~0.1 mm)
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Falls from stratus cloud 1km or less in depth.
Cloud-top T < -10 C
strong cooling by the underlying surface,
gradual upslope flow providing sufficient lift.
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SNOW
Large snowflakes at T near freezing while small flakes at colder T
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Is the surface temperature < 1.7°C ?
Is the freezing level < 1200ft (366m)? Note: If the freezing level is > 1200ft, mixed precipitation is likely.
Is the 850 mb temp < 0°C?
Is the 700 mb temp <-4°C?
Is the 1000-500mh thickness < 5400?
Is the temperature <0°C from 1200ft to 700 mb?
Is there a moist layer (T-Td depression 5°C) from surface to 700 mb?
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FREEZING RAIN
Super cooled rain droplets that freeze upon contact with the ground or objects
An above freezing layer exists which is at least 30 mb.
A below freezing layer is present (about 1500 feet thick).
Surface T around 0°C.
There is a low concentration of ice nuclei in the sub-freezing layer.
Alert area between zero line in 850mb and surface warm front
ICE PELLET (Diameter 2-7 mm)
form in a similar manner to freezing rain.
If the below-freezing layer is sufficiently deep the supercooled raindrops will freeze into ice pellets before they reach the ground.
Ice pellets often precede or accompany freezing rain and with ice pellets there is freezing rain aloft
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HAIL (Diameter 5-30 mm)
- The surface temperature of graupel particles is less than 0C while for hailstones it is approximately 0C.
- High liquid water content in the cloud and strong updrafts.
- Hail usually falls from CB clouds, small hail may develop in TCU.
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FREEZING DRIZZLE
“non-classical” (and most prevalent) mechanism for formation of freezing drizzle is through the collision-coalescence process
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Entire sounding below freezing -15 to 0°C.
An inversion with a deep dry layer above the inversion and a shallow moist layer below the inversion.
Significant wind shear near the inversion level
The wind speeds 5 knots or more.
The cloud bases under 4000 feet, with the best cases under 1000 feet.
Cloud thicknesses are 2000 to 6000 feet
The air is often saturated though 850 or 800 mbs.
At 700 mb the flow tends to be light to moderate cyclonic, and the relative humidity is 30-50% or less.
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SNOW GRAIN (SG, diameter < 1 mm)
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Solid equivalent of drizzle
Very small white & opaque grains of ice
occur at colder T than freezing drizzle
Fall from low, thin stratus clouds;
grow by diffusion and aggregation.
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ICE CRYSTALS (Diameter < 1 mm)
- Occurs at the surface falling from either clear or cloudy skies, usually when the temperatures are very cold.
- The crystals grow by diffusion of water vapour primarily onto the sharp corners and edges.
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