Temperature dependence of tropical cirrus properties and radiative effects

The temperature dependencies of cirrus properties are studied using a dual polarization lidar and Mesosphere Stratosphere Troposphere (MST) radar at the tropical station Gadanki (13.5 degrees N, 79.2 degrees E). Cirrus clouds are generally observed in the altitude region 10 to 18 km, with midcloud temperature in the range -85 degrees to -40 degrees C. The cloud temperature decreases with increase in cloud altitude as expected. The mean cloud thickness is generally in the range 0.7 to 1.7 km. For temperatures in the range -75 degrees to -50 degrees C the cloud thickness is similar to 1.7 km and shows a tendency to decrease at lower temperatures. The linear depolarization ratio (LDR) within the cloud shows a small increase with decrease in temperature. The cloud extinction and optical depth increases with increase in temperature. The temperature dependence of cirrus extinction/optical depth has been parameterized using different analytical forms such as exponential, linear, and polynomial, which shows that a second-order polynomial function is well suited for describing the temperature dependence of extinction coefficient/optical depth of tropical cirrus. The climate sensitivity factor derived based on the empirical relations shows an increase with decrease in cloud optical depth. The present study, however, indicates that the cirrus becomes radiatively significant when its optical depth exceeds a threshold value of 0.03.