D328 DEPARTING FROM DCA AT 400 FT CLBING HAS NMAC WITH ACFT ON MISSED APCH PROC.

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OUR ATC CLRNC WAS AS FOLLOWS: 'DEPART NW ON THE DCA 328 DEG RADIAL; RADAR VECTORS TO AML; AS FILED. MAINTAIN 5000 FT....' THIS IS A STANDARD IFR DEP CLRNC AT DCA. WE (FLT CREW) AGREED THAT RWY 1 WOULD BE OUR DEP RWY BASED ON WX CONDITIONS. DCA TWR ISSUED US A CLRNC TO TAXI INTO POS AND HOLD RWY 1. TWR CTLR STATED; 'TFC ON A 4 MI FINAL; BE READY FOR AN IMMEDIATE TKOF.' WE RESPONDED THAT WE WERE READY. AS SOON AS WE TAXIED INTO POS; WE WERE GIVEN CLRNC FOR TKOF. AFTER THE FINAL BEFORE TKOF ITEMS WERE ACCOMPLISHED (APPROX 2-3 SECONDS); THE FO ASSUMED DUTY AS PF AND CAPT AS PNF. DURING ROTATION AND LIFTOFF; WE HEARD TFC ANNOUNCE A MISSED APCH. WE LIFTED OFF AND WERE IMC NO LATER THAN 300 FT MSL. PF TURNED TO INTERCEPT THE DCA 328 DEG RADIAL AND CONTINUED NORMAL DEP PROCS. THE TWR CTLR QUICKLY ISSUED US AMENDED DEP INSTRUCTIONS OF A L TURN TO 270 DEGS AND MAINTAIN 1500 FT. WHILE COMMENCING THE L TURN; PNF WAS SETTING ALT SELECTOR TO NEW ALT WHEN 'TFC' WAS ANNOUNCED BY TCASII. WE THEN SAW AN AMBER TARGET APPEAR ON THE BOTTOM L CORNER OF THE MFD SCREEN. TCASII RANGE WAS SET TO 2.5 NM AND WE ONLY RECEIVED A TA. THE TFC WAS OBSERVED ON THE MFD AT +400 FT VERT SEPARATION AND MOVING TOWARDS US VERY RAPIDLY. WE COMPLIED WITH ATC'S INSTRUCTIONS AND TRIED TO MAINTAIN SEPARATION. THE OTHER TFC'S FLT PATH WAS UNKNOWN; BUT SEEMED TO PROCEED FROM BEHIND AND ABOVE US TO BELOW AND TO THE R OF OUR ACFT. WE WERE CLBING SO THE OTHER ACFT WAS NOT NECESSARILY DSNDING; BUT RATHER IN A SHALLOW CLB OR LEVEL FLT. THE ACFT WAS A SMALL JET; EITHER AN EMB-135 OR 145 BASED ON THE ACR OPERATING THE FLT. DURING INCIDENT; PNF TRIED TO ACQUIRE A VISUAL ON THE TFC BUT WAS UNSUCCESSFUL. THE INTRUDER ACFT PASSED OUR ACFT (THE WHITE SILHOUETTE ON MFD) AT -100 FT BELOW OUR ALT WITHIN AN ESTIMATED 1/4 MI HORIZ DISTANCE OR LESS. FLT CONTINUED ON WITHOUT INCIDENT. AFTER WRITING THE ABOVE NARRATIVE; I SPOKE WITH THE DCA TWR SUPVR. SHE INFORMED ME OF A FEW THINGS THAT I THINK CLRED UP THE INCIDENT AND WOULD QUALIFY AS CONTRIBUTING FACTORS. ADDITIONALLY; I HAD SEVERAL OTHER THOUGHTS ON THE INCIDENT: 1) SHE SAID THE OTHER ACFT STARTED AT AN ALT BELOW OURS (OR; IN RELATION TO) AND THEN CLBED ABOVE US AND THEN ENDED UP PASSING BELOW US. THIS SOUNDS CORRECT BUT I AM NOT SURE AT WHICH POINT OUR TCASII ACTIVATED (IE; OTHER ACFT'S PHYSICAL LOCATION) SO I BELIEVE MY INITIAL PERCEPTION ABOVE IS CORRECT. THIS INCIDENT DID; HOWEVER; TAKE PLACE VERY QUICKLY SO I AM WILLING TO ACCEPT THAT THE TFC TOOK A SLIGHTLY DIFFERENT VERT PROFILE BEFORE PASSING UNDER US. 2) SHE SAID THE ACFT ON ILS RWY 1 PERFORMED MISSED APCH PRIOR TO REACHING MINIMUMS; ACCELERATING THEIR CLOSURE RATE. 3) SHE SAID THE ACFT ON ILS RWY 1 TURNED PRIOR TO MISSED APCH POINT AND EFFECTIVELY CUT-THE-CORNER ON THEIR PROC AND OUR SEPARATION. 4) SHE SAID ACFT ON FINAL WERE REGISTERING GND SPDS OVER 50 KTS FASTER THAN NORMAL; IMPLYING A VERY LARGE TAILWIND ALOFT. 5) SHE STATED THAT WHEN OUR ACFT WAS GIVEN CLRNC TO DEPART; THE TFC WAS 2.5 MI OUT. WE WERE TOLD 4 MI WHEN GIVEN POS-AND-HOLD AND THEN 3 MI AS WERE BEGINNING TKOF ROLL. 6) OUR ACFT WAS LOADED TO WITHIN 100 LBS OF THE MAX STRUCTURAL TKOF WT AND IN THE INITIAL CLB PHASE OF FLT. THE AIRSPD WAS LOW BUT ACCELERATING; THE ALT WAS LOW AND INCREASING SO OUR CHOICES FOR EVADING TFC ARE LIMITED. WE RELY ON NOT ONLY TCASII BUT ATC TO ISSUE US INSTRUCTIONS IN IMC TO KEEP CLR OF OTHERS. 7) OUR OPS PROHIBIT US FROM RESPONDING TO ONLY A TA WHILE IMC WITHOUT TFC IN SIGHT. THE IDEA IS NOT TO CAUSE OTHER TFC CONFLICTS OR HAZARDOUS SITS WITH MINIMAL DATA. EXAMPLE: ACFT CLBING RAPIDLY TOWARDS YOU AND BELOW MAY CAUSE A TA; BUT NOT AN RA IF IT DOES NOT INTRUDE ON OUR TCASII-PROTECTED AIRSPACE. THE LOW ALT OF THIS INCIDENT CAUSED TCASII TO ONLY ISSUE A TA. WE RESPONDED THE BEST WE COULD GIVEN THIS LIMITED INFO; BEING IMC; AND NOT WANTING TO GET IN DANGEROUS PROX OF ANY OF THE FOLLOWING: OTHER ACFT; PROHIBITED AREAS; GND OR NEARBY BUILDINGS. IN MY EXPERIENCE WITH THE DORNIER 328; I HAVE WITNESSED FALSE TA'S AND TARGETS APPEARING QUICKLY AND WITHOUT WARNING SO TCASII IS USEFUL BUT NOT A SOLE MEANS TO AVOID TFC. IN THIS PARTICULAR INCIDENT; TCASII WAS ESSENTIAL TO THE SAFETY OF FLT WHEN PAIRED WITH THE QUICK-THINKING INSTRUCTIONS OF THE TWR CTLR. IT'S HARD TO SAY THAT IF THE OTHER PLT FIRST PROCEEDED TO THE MISSED APCH POINT BEFORE TURNING AND IF OUR SEPARATION AT THE TIME OF TKOF ACTUALLY WAS 3-4 MI INSTEAD OF 2.5 MI; THIS INCIDENT WOULD NOT HAVE OCCURRED. SEPARATION AT DCA IS TYPICALLY LESS THAN OTHER HIGH VOLUME ARPTS AND A TKOF CLRNC WITH THE STATED SEPARATION IS NOT UNUSUAL. ATC SHOULD; HOWEVER; USE THE GND SPD READOUT OF ACFT ON FINAL BEFORE ISSUING A TKOF CLRNC TAKING INTO CONSIDERATION A POTENTIALLY-HVY TAILWIND AND THEY SHOULD STATE SEPARATION ACCURATELY TO FLT CREWS.