Warm Fronts Mixed Phase Case

Credits "Image/Text/Data from the University of Illinois WW2010 Project http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml MetED : Freezing and Melting, Precipitation Type and NWP Gary Lackmann The Radar Palette Phil Chadwick

Outline Warm fronts and the Warm Conveyor Belt Typical precipitation phase transitions Temperature profiles associated with these precipitation types. Latent heat effects on profiles and phase changes. Approaching warm fronts as seen on Doppler radial velocity displays. Case study of a mixed phase event.

The Conveyor Belt Conceptual Model

R IP S ZR

Virga unlikely except along the leading edge of the WCB Warm Frontal Cross-section along Trailing Branch of the Warm Conveyor Belt (WCB) A Virga unlikely except along the leading edge of the WCB WCB WCB oriented for maximum frontal lift Virga Precipitation Increasing CCB Moistening Lower Hydrometeor Density Mixing Zone Surface Warm Front Precipitation At Surface CCB A B Cold air in Cold Conveyor Belt (CCB) even more shallow and more moist Notes: All descriptive terms are intended to be comparative between the various conveyor belts in the Conveyor Belt Conceptual Model. All quantities are intended to be the average or typical values Virga may be the result of melting snow or evapourating rain to cause the reduced hydrometeor density and thus increased visibility or reduced obstruction to visibility These comments will need validation – many are just my simple operational observations Compared to the previous slide and the central branch of the WCB cross-section: The depth of the WCB with a component of flow normal to the warm front is even deeper. The cold air mass is increasingly moist from the precipitation. The area of precipitation at the ground will continue to show rapid increase as a result of the precipitation extending further downward into the moistened, modified CCB. This expansion of the precipitation area is a result of the moistened CCB and not any increases in the precipitation processes. The frontal slope is likely to be greater than the average of 1:200. The warm front is more likely to be anabatic or active. Just poleward of the warm front, the cloud type will certainly be nimbostratus Moist portion of Warm Conveyor Belt (WCB) is thicker, higher and backed from frontal perpendicular – anabatic tendency Lower levels of WCB have the same origin as the upper level of the WCB WCB probably backs slightly with height in spite of the warm air advection. A greater WCB depth is frontal perpendicular Frontal slope likely steeper than the typical 1:200 Precipitation extends further into the moistened, modified CCB. Horizontal rain area expands rapidly as CCB moistened.

R IP S ZR Let’s look at a series of temperature profiles as we head towards the warm front from the cold side. Lets look at temperature profiles as we move towards the warm front in the cold air.

Ice Pellets

The Wonders of Latent Heat Melting Snow Cools the Above Freezing level aloft The Freezing of Rain warms the Above freezing Level near the surface

This effect opposes the warm air advection going on in the warm conveyor belt. That’s why when you have warm fronts where the warm air advection is not too strong we often get periods where the rain will go back to snow.

One of the reasons ice pellets don’t usually last very long is that the air is warmed by the forming of the ice pellets aloft eventually enough that it is not cold enough to freeze the rain before it hits the ground. Both freezing rain and ice pellets require a cold advection input from the cold conveyor belt to counteract the latent heat effect to keep going.

Doppler Radar and Winter Warm Fronts Conceptual model of the warm and cold conveyor belts implies certain patterns on a Doppler radial velocity display

Y A Slope of the front, cold air advection B X

The Conveyor Belt Conceptual Model

Cold Conveyor Belt A to B Warm Conveyor Belt X to Y Slope of the front, cold air advection B X

Warm fronts and Precipitation Phase From radial velocity patterns Depth of cold air Nowcasting of Temperature Advections Changes in Strength of low level flow. From logz and cross sections bright band Freezing level, lowest extent of melting snow Possibility ZR IP at the surface Classic Pattern February 1990 freezing rain Bright band case from B.C.

Classic cases- big ZR events

00z Feb15

Cold Conveyor Belt A to B Warm Conveyor Belt X to Y Slope of the front, cold air advection B X

Watch what happens as the warm front approaches

13Z Feb15

16Z Feb15

22Z Feb15

Two Different Warm Fronts It’s “What Lies Beneath” That Counts.

Intensifying Front

Weakening Front

Combining Doppler Pattern and Bright Band A Warm Frontal Wave

X

X

X 18Z Feb 13

Doppler Monitors Low level as low and warm front Approach

Big Changes in 7 hours as this wave approaches. Can you identify the warm conveyor belt at 13z… at 20z What changes occur? The Cold Conveyor Belt? What does this mean for the track of the wave approaching?

Y B A A X

Y B A X

Monitoring the Bright Band Using the 3.5 degree PPI

What happens between 1800z and 1900z What do you expect from surface observations near the radar. Remember YFC (just west of radar) was Minus 6 C at 1800z?

YFC/YQM obs Feb 13 2008 YFC YQM

Polarimetric Radar and Mixed Phase

Precipitation Type Boundary In a uniform precip type (all rain, all snow) RHOHV equals close to one Melting snow lower values of rhohv. Rain snow lines show up well on rhohv. With our 0.2 degree ppi for polarimetric radar we can view a near horizontal projection of the boundary.

Rain snow boundary moves through radar range Nov 15 2008 Rain snow boundary moves through radar range

K

K

Conclusions Doppler Wind Patterns allow us to monitor the cold and warm conveyor belts associated with warm fronts Bright bands can confirm the existence of above an above freezing layer aloft. Polarimetric radar can provide a semi horizontal depiction of rain/snow lines.

THE END