Jan-Bai Nee

Lidar ratio and depolarization ratio for cirrus clouds

We report on studies of the lidar and the depolarization ratios for cirrus clouds. The optical depth and effective lidar ratio are derived from the transmission of clouds, which is determined by comparing the backscattering signals at the cloud base and cloud top. The lidar signals were fitted to a background atmospheric density profile outside the cloud region to warrant the linear response of the return signals with the scattering media. An average lidar ratio, 29 +/- 12 sr, has been found for all clouds measured in 1999 and 2000. The height and temperature dependences ofthe lidar ratio, the optical depth, and the depolarization ratio were investigated and compared with results of LITE and PROBE. Cirrus clouds detected near the tropopause are usually optically thin and mostly subvisual. Clouds with the largest optical depths were found near 12 km with a temperature of approximately -55 degrees C. The multiple-scattering effect is considered for clouds with high optical depths, and this effect lowers the lidar ratios compared with a single-scattering condition. Lidar ratios are in the 20-40 range for clouds at heights of 12.5-15 km and are smaller than approximately 30 in height above 15 km. Clouds are usually optically thin for temperatures below approximately -65 degrees C, and in this region the optical depth tends to decrease with height. The depolarization ratio is found to increase with a height at 11-15 km and smaller than 0.3 above 16 km. The variation in the depolarization ratio with the lidar ratio was also reported. The lidar and depolarization ratios were discussed in terms of the types of hexagonal ice crystals.

Middle atmospheric temperature structure over two tropical locations, Chung Li (25°N,121°E) and Gadanki (13.5°N,79.2°E)

Abstract Middle atmospheric temperature profiles were presented from the Nd:YAG Lidar measurements over Chung Li (25°N,121°E), Taiwan, and Gadanki (13.5°N,79.2°E), India. The stratopause is observed in the height range of 45–50 km with temperatures 265– 280 K over Chung Li and the stratopause width is broader at Gadanki with temperatures 260– 270 K . These data were compared with the coincidence UARS-HALOE profiles. The comparison shows fair agreement between the ground-based measurements and HALOE, but HALOE temperatures are colder by 2– 10 K in the stratosphere with maximum difference at stratopause altitudes. The stratospheric temperatures from the nearest radiosonde data show good agreement with NCU Lidar and HALOE data. The results from these two stations were compared and discussed with results available in the literature.

Recent observations of mesospheric temperature inversions over a tropical station (13.5°N, 79.2°E)

Abstract Present study mainly deals with recent observations of mesospheric temperature inversions (MTI) over Gadanki (13.5° N , 79.2° E ) , a tropical station in India using for about 40 months of Nd:YAG lidar data. Long-term measurements of halogen occultation experiment, high resolution Doppler imager on board upper atmospheric research satellite and solar mesospheric explorer have been used to compare the characteristics (amplitude, height and percentage of occurrences) of the inversions observed with ground-based lidar measurements for the first time. In general, the height and percentage occurrence of the inversions are matching well between ground and satellite based measurements most of the time and there exists a large discrepancy in the amplitudes of MTI measured by these instruments as expected. The height occurrence of these inversions is at ∼76 km and for about 60% of the time, strong inversions can be seen at this tropical latitude. The height occurrence of the inversions shows annual oscillation with peak during summer and minimum in winter. The percentage occurrences of these inversions are showing semi-annual oscillation with peak during equinoxes and minimum during solstice. The possible causative mechanism for the frequent occurrence of these inversions over this tropical latitude are explained in light of current understanding of the gravity wave activity and also with the chemical heating/cooling taking place at these heights.

Ultraviolet absorption of Cl2O

Abstract The photoabsorption spectrum of the Cl 2 O molecule has been investigated in the 150–200 nm wavelength region. This absorption spectrum consists mainly of two electronic systems: a broad continuum and a progression of bands with a vibrational wavenumber 329 cm -1 corresponding to a v 2 band excitation in the excited electronic state. Photoabsorption cross sections are reported.

Seasonal and diurnal variation of convective available potential energy (CAPE) using COSMIC/FORMOSAT-3 observations over the tropics

[1] The global pattern of convective available potential energy (CAPE) at seasonal and diurnal time scales is discussed using 1 year of COSMIC/FORMOSAT-3 satellite observations. The calculation of CAPE using temperature and humidity measurements of COSMIC is described. The estimated CAPE is grouped into 5 x 5 grid and is further classified into four seasons, namely, winter, spring, summer, and autumn. The CAPE magnitudes in general have high values over land as compared to oceanic region, which confirmed the consistency of CAPE calculations. The systematic migration of CAPE from Northern Hemisphere to Southern Hemisphere is observed during Northern Hemisphere summer to winter, coinciding with the movement of intertropical convergence zone. Once the seasonal pattern is established, the composite diurnal patterns of CAPE with 2 h resolution are obtained by combing all the observations in one season. Diurnal variation of CAPE has shown domination of semidiurnal variations at some latitudes (12 h) and diurnal variation (24 h) at some other latitudes. The mean removed CAPE is then subjected to Fourier analysis to extract the diurnal variation amplitudes. During the observational period, larger CAPE magnitudes are observed over the Indian Ocean during most of the seasons, comparable in magnitude to that of the land regions. As the CAPE and precipitation patterns have correlation, the present study demonstrated the capability of COSMIC/FORMOSAT-3 to study the diurnal patterns of CAPE, which will have implications in interpreting the tropical diurnal precipitation patterns.