From the Collins WXR-2100 manual:
BOEING philosophy
Takeoff:
Boeing alert activation regions for takeoff: Warnings and Cautions are
enabled from the beginning of the take off roll (0 knots) until the aircraft
reaches 80 knots. From 80 knots until the aircraft passes 400 feet, only
Warnings are enabled. From 400 feet through 1,200 feet, Warnings and
Cautions are enabled. All alerts are disabled from the time the aircraft
passes 100 knots until it reaches 50 feet. Boeing aircraft do not display
Advisories.
Landing:
Boeing alert activation regions for landing: Warnings and Cautions are
enabled from the time the aircraft passes 1,200 feet until 400 feet.
From 400 feet until 50 feet, only Warnings are enabled. From 50 feet
until touchdown (0 feet), all alerts are disabled. Boeing aircraft do not
display Advisories
Maybe the "power up" is related with those 12 seconds??:
Windshear alerts are active in the cockpit below 1,200 feet AGL.
However, the WXR-2100 radar actually enters the windshear scanning
mode at 2,300 feet AGL to provide time for the system to power up
(if necessary) and update the displays before the aircraft reaches the
1,200 feet AGL level.