Measured Radiative Properties of Thin Cirrus During TC4

Thin, often subvisible, cirrus clouds are very common in the tropics but their importance to the radiative budget, their effect on stratosphere-troposphere exchange, and their formation and persistence mechanisms are still uncertain. Here we will present direct measurements of the heating rates of tropical thin cirrus in order to gain insight into whether the absorption of radiative energy contributes to the lifting and persistence of these clouds. During TC4, measurements of the upwelling and downwelling broadband infrared irradiance and spectral solar irradiance were made from both the NASA ER-2 aircraft flying above the cirrus and the NASA DC-8 aircraft flying below the cirrus. This data will be used to derive the heating and cooling rates of the thin cirrus, while downlooking cloud lidar measurements from the ER-2 will be used to detect their presence, altitude, thickness, and estimated optical depth. We will focus on two case study days from TC4, the 25July2007 flight, where the ER-2 attempted to fly a descent profile through a thin cirrus layer, and the 6Aug2007 flight where the ER-2 flew above a thin cirrus layer while the DC-8 flew below.