1. Warm Fronts
Mixed Phase Case

2. Credits
"Image/Text/Data from the University of Illinois WW2010 Project
MetED : Freezing and Melting, Precipitation Type and NWP
Gary Lackmann
The Radar Palette
Phil Chadwick

3. 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.

4. The Conveyor Belt Conceptual Model

6. R IP S ZR

7. 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.

8. 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.

10. Ice Pellets

14. 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

16. 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.

19. 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.

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

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

22. The Conveyor Belt Conceptual Model

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

24. 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.

25. Classic cases- big ZR events

26. 00z Feb15

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

28. Watch what happens as the warm front approaches

29. 13Z Feb15

30. 16Z Feb15

31. 22Z Feb15

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

33. Intensifying Front

34. Weakening Front

35. Combining Doppler Pattern and Bright Band
A Warm Frontal Wave

36. X

37. X

38. X
18Z Feb 13

39. Doppler Monitors Low level as low and warm front Approach

42. 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?

43. Y B A A X

44. Y B A X

45. Monitoring the Bright Band Using the 3.5 degree PPI

49. 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?

50. YFC/YQM obs Feb
YFC
YQM

53. Polarimetric Radar and Mixed Phase

54. 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.

55. Rain snow boundary moves through radar range
Nov
Rain snow boundary moves through radar range

56. K

57. K

62. 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.

63. THE END