A LIFEGUARD MEDICAL FLT INITIATED AN IFR TKOF WITHOUT TWR CLRNC SHORTLY AFTER THE CLOSED TWR OPENED.

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PF (CAPT-MYSELF) WAS CLRED FOR POS AND HOLD; BUT WAS PREDETERMINED THAT ARPT WOULD BE CLOSED. I FORGOT THAT FACT; TWR ADVISED RWY CONDITIONS; COPLT (PNF) ADVISED RECEIPT AND SAID WE WERE ROLLING. I RELEASED BRAKES; ADDED PWR AND TOOK OFF. TWR ADVISED NEGATIVE; ABORT TKOF; NO IFR RELEASE. TOO LATE TO ABORT ON SNOW/ICE COVERED RWY. TWR ADVISED TO REMAIN VFR AND ENTER R DOWNWIND FOR RWY. AT THAT POINT; TWR GAVE US IFR RELEASE TO FLY DIRECT TO DEST. CAUSE OF PLTDEV ERROR: (TKOF WITHOUT CLRNC) WAS THAT I HAD PLANNED ON DEPARTING PRIOR TO TWR OPENING. HOWEVER; DURING TAXI FOR TKOF; TWR ANNOUNCED IT WAS NOW OPENING AS CLASS D AIRSPACE AND THEN ISSUED OUR CLRNC AS FILED. THIS FLT BEING A 'LIFEGUARD' MEDICAL FLT TRANSPORTING LONG PATIENT TO CLINIC; TIME WAS CRITICAL AND I GOT FOCUSED ON TKOF WITH PROPER TWR TKOF CLRNC. AFTERWARDS; CREW BRIEFED AND I INSTRUCTED COPLT TO BE MORE ASSERTIVE AND BE AGGRESSIVE AND TELL ME 'HOLD IN POS' UNTIL WE GET CLRNC FOR TKOF! IN SUMMARY: WHEN A PLT IS CLRED FOR 'TAXI INTO POS AND HOLD;' HE'S COCKED LIKE A 45 CALIBER REVOLVER READY TO RELEASE WITH THE TWR'S NEXT WORDS. WHEN TWR SAID RWY CONDITIONS; AND COPLT RESPONDED; I AUTOMATICALLY RELEASED BRAKES; ADDED PWR AND 'ROLLED;' JUST LIKE ALWAYS. I WAS FIXED ON THE TKOF AND PREDETERMINED TO GO WITH THE NEXT XMISSION FROM TWR!