Microburst Forecasting - Question 3

Sounding plotted on skew-T diagram

To answer this question, you may wish to
Open the WINDEX calculator in a new window.

Now, make a forecast of the maximum wind gusts associated with microburst activity. First, determine the values of the variable in the WINDEX equation:
Enter your answers in the text boxes below, then click Done.

1. Height of the melting level (HM) km-AGL

The height of the melting level is 3.2 km AGL. A good estimate for melting level is the wet-bulb zero level in the sounding which is 691 hPa. Given the surface pressure is 1000 hPa, the 691 hPa is about 3.2 km above the surface level.

2. Mixing ratio in lowest 1 km (QL) g/kg

The mean mixing ratio in the lowest 1 km is 7.5 g/kg. To a reasonable approximation, the 1 kilometer surface layer has a thickness of 100 hPa, so the layer is defined as the 1000-900 hPa layer. The mixing ratios at 1000, 950, and 900 hPa are 12.4, 6.2, and 4.0 g/kg, respectively. The dewpoint curve is essentially linear in that layer, so the mean mixing ratio is average of the 3 values: (12.4 + 6.2 + 4.0) / 3 = 7.5 g/kg = QL.

3. Mixing ratio at the melting level (QM) g/kg

The mixing ratio at the melting level (691 hPa) is 5.5 g/kg

4. Lapse rate from the melting level to the surface °C/km

The lapse rate from the melting level to the surface is 7.9 °C/km. The surface temperature is 25°C and the melting level temperature is 0.5°C. We previously established that the melting level is at 3.2 km, so the lapse rate is (25°C - 0.5°C)/3.1 km or 7.9°C/km

Use the WINDEX Calculator to compute the maximum wind gust (WI).
WI = kt

The maximum wind gust is 37 knots. This is found by plugging the computed variables above into the WINDEX calculator.

Note that while the 37 knot value is below the severe gust threshold of 50 knots, it could have a potentially devastating impact on aviation. If an aircraft encounters a microburst, there could be an initial 37 knot increase in headwind, followed by a 37 knot tailwind, resulting in a net airspeed loss of 74 knots!

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