GPWS ON RADAR VECTORING CAUSES ACR FLC RESPONSE IN ALTDEV ALT EXCURSION.

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GPWS TERRAIN WARNING ON APCH TO RWY 24R; LAX. FLT ORIGIN: SAN DIEGO. IN LAX TERMINAL AREA; SOCAL APCH VECTORED US N ACROSS RWY 24R LOC W OF ROMAN AT 7000 FT. WE WERE TURNED DOWNWIND HDG E AND DSNDED TO 5000 FT; BASE TURN WAS ABOUT 5-6 MI E OF MERCE AT 3500 FT. WE WERE GIVEN A 230 DEG INTERCEPT HDG AND TOLD TO DSND TO 2500 FT. WE LEVELED AND INTERCEPTED RWY 24R LOC ABOUT 4-5 MI OUTSIDE OF MERCE. JUST AFTER LEVELOFF; ON THE LOC; OUR GPWS CALLED 'TERRAIN; TERRAIN.' BY COMPANY PROC IN IMC; WE CLBED WITH MAX PWR. THE WARNING STOPPED ABOUT 3000 FT IN THE CLB. WE LEVELED AT 3500 FT STILL ON THE LOC INBOUND. APCH CTL WAS ADVISED OF THE MANEUVER AND OUR ALT. HIS REPLY; 'THERE ISN'T ANY TERRAIN NEAR YOUR POS' AND HE THEN CLRED US FOR APCH. WE CONTINUED UNEVENTFULLY TO LNDG. WE WERE IMC AND COULD NOT SEE THE GND; EVEN THOUGH WE WERE CERTAIN OF OUR POS WHEN THE GPWS CALLED 'TERRAIN; TERRAIN; TERRAIN;' WE CLBED PER SOP. EVEN THOUGH NUMEROUS TARGETS WERE IN THE VICINITY; WE DID NOT GET A TCASII WARNING. 2 ITEMS STAND OUT: 1) WE WERE CLRED TO AN ALT 1500 FT BELOW THE SEGMENT ALT FOR THAT APCH. WE ACCEPTED IT AFTER A QUICK RECHK OF THE APCH PLATE SHOWED NO TERRAIN INBOUND ON THE LOC. HOWEVER; THERE IS A SPOT ELEVATION 1388 FT JUST BELOW THE LOC COURSE; ABOUT 6 MI E OF MERCE. THE GPWS POSSIBLY COULD HAVE INTERPRETED A STEEP OR ABRUPT INCREASE IN TERRAIN AS RISING TERRAIN WITHIN THE PARAMETERS OF THE GPWS. 2) MAKING A GAR CLB; IN IMC; IN THE LOS ANGELES BASIN TERMINAL AREA; WITH A LOAD OF PAX; IS SCARY. AVOIDING OTHER TFC AND GETTING ANOTHER ATC CLRNC BECAME THE PRIORITY. IN A 2-MAN COCKPIT IT GOT BUSY FOR A FEW MOMENTS. THIS WAS MY FIRST GPWS ALERT. I REACTED THE WAY I WAS TRAINED.