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Topics to be covered

Objectives

The Top-Down Approach is a technique to assist in forecasting precipitation type

Knowledge of why a precipitation type is occuring now will increase your chances of correctly forecasting precipitation type further into the future.

Frequency of freezing precipitation in hours per year

Note two maxima over the plains and northeast, however it can occur in MANY areas of the US.

Understanding ice nucleation

Note: In a cloud saturated layer, if the temperature is warmer than -4 °C there will not be any ice forming in that layer unless it is introduced from another source (i.e. another cloud over the top seeds the clouds below with ice).

Particles that are ice nuclei

Kaolinite is the most common ice nuclei

IN's can't activate unless they are -9 °C or colder

Sounding from Jackson, Mississippi

Coldest temperature in the saturated layer is -8 °C, therefore it is most likely supercooled liquid water. Ice can't be initiated within this layer (unless it is advected in from another source). Precip type here would most likely be freezing drizzle.

Important temperatures for ice nucleation

Data collected from flying through clouds. 

Concentrate on lines (1) and (2)

At -8 °C they ran into all supercooled liquid clouds about 55% of the time while 45% of the clouds had SOME ice.

At -14 °C they ran into all supercooled liquid clouds about 28% of the time while 72% of the clouds had SOME ice.

BIG difference between -8 °C and -14 °C

These numbers fit well with activation numbers we saw earlier

Figure from Mike Schichtel

Observed precipitation type from sounding information

Minimum cloud temperature is within the saturated layer

Predominance of snow cases (red line) are on the colder side of -10 C

Predominane of freezing rain cases (green line) are on the warmer side of -10 °C

-12 °C is the key temperature. If minimum temperature in saturated layer reaches -12 °C there is a pretty good likelihood that ice will be introduced into the cloud

Ice is introduced because a good portion of the saturated layer is colder than -12 °C

Look at warm layer depth and warm layer maximum temperature. Hydrometeor size is also important but is difficult to diagnose operationally therefore will not be considered here.

Consider the blue triangle

Read off warm layer maximum temperature quite easily

Don't worry about what precipitation type was observed in the graph of warm layer depth vs warm layer maximum temperature. Concentrate on the family or population of points in this diagram. Relationship is linear, as depth of the warm layer increases, the maximum warm layer temperature increases proportionately.

Maximum warm layer temperature is represented by the light green line

Go through the sequence of soundings, as the warm layer depth increases, do does the maximum warm layer temperature. 

For the 3rd column, collision and coalesence produces the precipitation since there is no ice present

2nd column

< 1 °C, ice doesn't have a cold enough temperature to melt the snow

1 to 3 °C, is the transition area with sleet by 3 °C

Consider synoptic scale warm advection on the order of 10 °C / 12 hours at 850mb being fairly significant. The environmental cooling refered to here is an order of magnitude greater than synoptic scale warm advection changing the sounding significantly in the matter of a few hours.

Note very small warm layer (1 °C)

Wet bulb temp is quite a bit lower than that, therefore any warm layer would be removed in time.

If warm layer is unsaturated it will cool quite rapidly to the wet bulb temperature.

Ice is introduced into a supercooled liquid cloud below which glaciates supercooled liquid cloud (ice grows by the Bergeron process).

Right side is the sounding with the temperature profile (red) and dewpoint profile (orange).

Left side: Particles were dropped from around 375mb, 1's at the end of them are the lower ice concentrations, 4's are the higher ice concentrations.

Particles 1A and 1B (smaller ice concentrations) sublimed within the nearly saturated layer. Higher ice crystal concentrations made it into the unsaturated layer below 500mb then lasted about 1 km. Some of the sublimation from the ice in the higher ice concentrations took place in the saturated layer, then fell into the unsaturated layer. Lower bound of about 3500 feet where the ice crystals can seed into liquid layer below.

3500 feet is the minimum distance and 5000 feet as the maximum distance between 2 layers that you can count on introducing ice all the way through an unsaturated layer.

LAPS soundings find the cloud base, IR 10.7 um satellite imagery for cloud top temperature, upstream raobs, radar VWP (see slide 28), IR 3.9 um satellite imagery to determine ice or water clouds

At 01:04 UTC, saturated cloud layer to 7000 feet, above is cloud layer 11,000 feet and up, cloud base cannot be determined exactly, freezing drizzle was observed at MKX at this time.

With time the gates get lower and at 02:13 UTC the two gates come together meaning ice falling into a supercooled liquid layer, at this time there was a changeover to snow at MKX.

Surface cold layer deeper than 2500 feet and colder than -6 °C comes from Zerr study

Draw in -12 °C line on sounding, in this case the region colder than -12 °C is unsaturated therefore ice is not expected here.

Middle portion look for warm layer or unsaturated layer. There is a warm layer, however if ice is not introduced into the warm layer it doesn't matter what the maximum warm layer temperature is, it's going to be a freezing drizzle outcome if surface layer is supercooled.

Bottom surface cold layer. Saturated layer 0 to -6 °C meaning no ice, supercooled liquid. Freezing rain would be the result at the surface assuming no cloud is overhead seeding the lower cloud.

Assess this sounding. What kind of precipitation type do you expect?

Saturated at upper levels (in the region of -12 °C or colder) so ice falls into the warm layer. Warm layer maximum temperature is about 5 °C which would melt into liquid then fall into cold surface layer. Mostly freezing rain, possibly with ice pellets mixed in would be the result.

Draw in 1 and 3 °C lines and see how the area in between these isotherms corresponds to surface observations. It fits pretty well (mix of sleet or snow in between 1 and 3 °C with snow north of the 1 °C line and freezing rain south of the 3 °C line - but there are a FEW outliers

Choose temperature and coordinate would be all mb.

Go through the loop to see temperatures at various pressure levels, use this to find the warm layer maximum temperature and investigate it in plan view to diagnose precip type.

Saturated at upper levels (in the region of -12 °C or colder) so ice falls into the warm layer. Warm layer temperature is around 1 °C so there is a possibility of mix, surface cold layer is around -10 °C therefore expect mostly snow. 

Verification: about 80% snow, with about 20% sleet which comes from that warm layer maximum temperature around 1 °C.

Unsaturated around -12 °C therefore no ice moving into the warm layer. Warm layer is unimportant in this case. Supercooled liquid drops possibly coalescing  falling and freezing on contact at the groundand. Freezing drizzle was observed

Elevated convection casues ice to fall into the warm layer which is 1 °C. Cold air is dragged from above as well so the warm layer either stays constant or decreases in temperature. Bottom layer is below freezing therefore snow is the result.

Note occluded front from eastern Iowa to northern Illinois, warm surge of air heading towards Lake Michigan associated with higher cloud tops/more ascent. Over the LaCrosse, WI CWA we see elevated convection (colder cloud tops) move over an area of freezing rain causing the precip type to change to snow.

Cloud tops that are water in white, cloud tops containing ice are dark. White area in western Wisconsin changes to darker colors. This represents elevated convection moving over this area changing the precipitation type from freezing rain to snow.

Note stable snowbands over MN/northwest Wisconsin. Convective bands are shown in white. Area around radar becomes "red" (see radar enhancements) as ice is introduced into the environment.

Saturated at upper levels (in the region of -12 °C or colder) so ice falls into the warm layer. Warm layer maximum temperature is about 1 °C so there may be a mix of ice pellets, then the particles fall into colder layer near the surface. Result is mostly snow with perhaps some ice pellets mixed in. Also keep in mind vertical motion (determine if the vertical motion occuring in that layer - not easy to asses - will warm layer warm or cool?) being generated in "close" cases like these.

Black over most of western WI, however the observations show different types of precip type, LAPS sounding from the previous slide would suggest snow and perhaps a little sleet.

After calling around, it was indeed snowing at most locations. A little sleet did mix in briefly which caused these observations. 

Make sure you call around to observers if you have automated obs in your CWA.

At ? ob (gray shading) it had just stopped snowing and it changed to freezing drizzle. The call was made at the time of the sat image.

LAPS tries to diagnose precip type at sfc. Icons are present only where radar data is present.

In this case LAPS showed snow in that snowband.

Snow accumulation off because of bright banding, but had orientation of snowband perfectly in this case.

Draw in isotherms where you would expect your warm layer to change. 3 °C near the freezing rain area, all snow would be around 1 °C.

Fill in the boxes with the precip type expected across this region (shown on a map in slide 48)

Answer sheet can be found off the student guide web page