RPTR ENCOUNTERS ICE AND TURB UNABLE TO MAINTAIN ALT AND AIRSPD.

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COLD FRONT CAUSED UNSTABLE RISING AIR IN CUMULUS CLOUDS. TSTMS IN AREA HAD LOTS OF LIGHTNING BUT TOPS TO ONLY 25000 FT. THIS IS TYPICAL OF A WINTER TSTMS. TAILWINDS OF 65-75 KTS GAVE OUR 210 KT TACTICAL AIRSPD QUITE A BOOST. ATC WAS KEEPING US HIGH WE WERE FLT PLANNED FOR MEMPHIS' WALET ARR WHERE WE WOULD BE XING WALET AT 6000 FT 77 NM N OF GLH VORTAC. WE WERE 35 NM N AT 17000 FT WITH NO DSCNT CLRNC. WE ASKED FOR LOWER: CTLR GAVE US 15000 FT. THE ACFT'S RADAR WAS INEFFECTIVE AT SEEING ANY PRECIPITATION IN FRONT OF US SINCE THE PRECIPITATION WAS PROBABLY FROZEN. OUTSIDE AIR TEMP WAS ABOUT MINIMUM 10 DEG CENTIGRADE. THERE WAS NO MOON. THEREFORE; ANY SMALL BUILDUPS WERE VERY HARD TO SEE. WE STARTED OUT OF 17000 FT FOR 15000 FT. AS I LEVELED OFF AT 15000 FT THE TOP OF A BABY CUMULUS BUILDUP LOOMED AHEAD IN THE LNDG LIGHTS. WITH SPLIT-SECOND WARNING; WE ENTERED THE CLOUD AND WERE DOUSED WITH ICE. WE ALSO HIT A SHEAR WITH MODERATE TURB. WE ENTERED THE CLOUD AT OUR ROUGH AIR SPD OF 180 KTS AND POPPED OUT THE OTHER SIDE ABOUT 10 SECONDS LATER AT 140 KTS WITH AN ALT LOSS OF ABOUT 400 FT. IT WAS ALL I COULD DO TO FLY ATTITUDE WITH SPD AND ALT BE DAMNED. AS WE RECOVERED OUR LOST ALT; ATC QUERIED OUR DEV WITH A 'MEMPHIS ALTIMETER SETTING 13 XXXX AND MAINT 15000 FT.) A FEW MINS LATER; THE CTLR GAVE US 13000 FT AND A SIMILAR EXPERIENCE OCCURRED AGAIN BUT WE DID NOT HAVE AN ALTDEV. IT WAS AFTER THE SECOND EXPERIENCE THAT WE RPTED MODERATE TURB AND MODERATE ICING IN THE BUILDING CUMULUS.