Convective cloud base, freezing level and icing on Lagos arrival
At 1400 UTC in Lagos (DNMM, elevation 135 ft), surface observations are 32°C, dewpoint 25°C, QNH 1009 hPa, wind 210/10 kt. A line of cumulus congestus is growing west of the field; radar echo tops are 38,000 ft. The 700 hPa temperature is +8°C and the 500 hPa temperature is -6°C; the tropopause is at 48,000 ft. You are operating a Dash 8 Q400 at FL180 on arrival and will descend via the ILS to runway 18R. The aerodrome minima are 500 ft ceiling and 1,500 m visibility. The destination TAF has TEMPO 1414/1418 3000 TSRA BKN012 FEW025CB. Dispatch asks whether the cloud base will be above minima, what freezing level to expect, and whether icing is a threat during the descent.
1. Estimate the convective cloud base in feet AGL and AMSL. Is it above the 500 ft minima? What does the TAF TEMPO imply for the approach?
Read the worked answer
- Surface spread is 32 - 25 = 7°C. The convective condensation level is roughly 125 m per °C of spread: 7 x 125 = 875 m AGL.
- Convert: 875 m x 3.281 = about 2,870 ft AGL. Add aerodrome elevation 135 ft, giving about 3,000 ft AMSL.
- This is well above the 500 ft ceiling minimum. The estimate assumes a well-mixed boundary layer and is only a guide; in a humid tropical boundary layer the actual base can be 2,500-3,500 ft.
- The TEMPO 1414/1418 3000 TSRA BKN012 FEW025CB means temporarily, from 1400 to 1800, visibility 3,000 m in thunderstorm rain, broken cloud at 1,200 ft, and cumulonimbus at 2,500 ft. The 1,200 ft ceiling and 3,000 m visibility are still above the 500 ft and 1,500 m minima, but a rain-cooled downdraft can lower the base quickly. The approach is legal but not stable; plan for a go-around and avoid the CB core.
2. Determine the approximate freezing level and assess icing risk at FL180 and during descent. Would you request a deviation?
Read the worked answer
- Use the 700 hPa and 500 hPa temperatures. The 0°C level lies between them: from +8°C to -6°C is a 14°C change. The 0°C point is 8/14 of the way from 700 hPa to 500 hPa.
- Pressure heights are roughly 10,000 ft at 700 hPa and 18,000 ft at 500 hPa. The 0°C level is therefore about 10,000 + (8/14 x 8,000) = about 14,500 ft. A standard lapse-rate check from 32°C at the surface also gives about 15,000 ft, so 14,500-15,000 ft is reasonable.
- At FL180 the temperature is about -6°C, above the freezing level. In ordinary stratiform cloud that would mean little supercooled water. In a cumulonimbus, however, strong updraughts carry supercooled water to -20°C or lower, and hail is possible. Icing in the CB core can be severe, and turbulence is a greater immediate threat.
- During descent from FL180 to the surface, the aircraft crosses the freezing level near 14,500 ft. In stratiform cloud the icing risk is light to moderate; in the CB it can be severe. The safe action is lateral avoidance of the CB, not merely flying above the freezing level. Request a 20 nm deviation and radar vectors, and do not descend through the core.