Lecture Panel 7 Flashcards
A problem with gauge network that radar attempts to solve
gauge networks are sparse and can miss storms
What happened in Waltonia that highlights the necessity for rainfall knowledge?
- 143 deaths due to thunderstorm and resulting flood
- people downstream had no idea it had rained at top of watershed, were not prepared for influx of 6m wall of water at night
RADAR - what is it
RAdio Detection and Ranging
-amount of reflectance is proportional to intensity of precipitation (as related to drop size)
RADAR advantages
spatial coverage - 200km effective range, total of 10,000km
temporal coverage - 1h, 3h, and storm duration are typical products
-accuracy increases if gauges are used, most useful for areal estimation
What has better resolution - ground based radar or passive microwave sensors?
ground based radar
What areas of Canada and the US are covered by ground based radar?
basically all of 50 states + hawaii, southwestern alaska, and southern border of canada
Radar reflectivity scale
radar reflectivity = Z in mm^6/m^3
Z = SUM 1 to n D^6
Z=reflectivity factor
D=drop diameter
n= number of drops of a given diameter/m^3
-measured in logarithmic scale dBZ=10logZ as measurements vary from 10^-5 to 10^7
Relationship between radar reflectivity and rain rates
power law relationship Z=a(R)^b Z=reflectivity factor R=rain rate [mm/h] a and b are regional constants determined by gauges. Related to intercept and slope on log-log plot
Why must we know typical rainfall patterns to asses Z-R?
because different precipitation events can have the same reflectivity but vastly different rain rates due to drop characteristics
What does Z-R vary with?
- time
- location
- season
- type of storm
- phase or shape of precipitation
- distance from RADAR
- —might need more than one Z-R relationship for one storm if precipitation character changes based onlocation, etc.
What must we know to effectively use RADAR?
- hydrometeor size
- density/m^3
- liquid/ice
- are there insects/birds around?
- is the RADAR sampling a region that is representative of the precipitation hitting the ground?
What is radar more sensitive too in terms of drop characteristics?
- more sensitive to small numbers of large drops vs large numbers of small drops, even if smaller drops are contributing more precipitation
- more sensitive to liquid precipitation
What hinders radar effectiveness?
- obstructions from terrain/buildings
- bad density of radars
- earth’s curvature, eg. lake effect snow can be very low in atmosphere, so radar can miss it
What is better sampled by radar - convection or strataform stroms?
convection - dep precipitating system, often well sampled to 150 km
strataform is bad, must be sampled in lowest 1km zone, must be closer to radar and limited in rugged terrain. sample range <50km
How does radar reliability change with season
better in warm months
