Search Results

You are looking at 1 - 7 of 7 items for :

  • Author or Editor: K. N. Liou x
  • Journal of Applied Meteorology and Climatology x
  • All content x
Clear All Modify Search
N. X. Rao, S. C. Ou, and K. N. Liou

Abstract

A numerical scheme has been developed to remove the solar component in the Advanced Very High Resolution Radiometer (AVHRR) 3.7-µm channel for the retrieval of cirrus parameters during daytime. This method uses a number of prescribed threshold values for AVHRR channels 1 (0.63 µm), 2 (0.8 µm), 3 (3.7 µm), 4 (10.9 µm), and 5 (12 µm) to separate clear and cloudy pixels. A look-up table relating channels 1 and 3 solar reflectances is subsequently constructed based on the prescribed mean effective ice crystal sizes and satellite geometric parameters. An adding–doubling radiative transfer program has been used to generate numerical values in the construction of the look-up table. Removal of the channel 3 solar component is accomplished by using the look-up table and the measured channel 1 reflectance. The cloud retrieval scheme described in Ou et al. has been modified in connection with the removal program. The authors have applied the removal–retrieval scheme to the AVHRR global area coverage daytime data, collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment cirrus intensive field observation (FIRE IFO) at 2100 UTC 28 October 1986 over the Wisconsin area. Distributions of the retrieved cloud heights and optical depths are comparable to those determined from Geostationary Operational Environmental Satellite visible and IR channels data reported by Minnis et al. Morwver, verifications of the retrieved cirrus temperature and height against lidar data have been carried out using results reported from three FIRE IFO nations. The retrieved cloud heights are within 0.5 km of the measured lidar values.

Full access
S. C. Ou, K. N. Liou, and J. F. King

Abstract

We have explored the applicability of the differential inversion (DI) method to temperature retrievals in both clear and cloudy atmospheres using red satellite data. The main theme of the DI is that the local Planck intensity can be exactly expressed by a linear combination of the derivatives of radiances in the logarithmic pressure coordinate. The inversion coefficients are obtained by fitting the weighting function to a generalized form. The higher-order derivatives of radiances are determined from polynomial fittings. The satellite dataset used in this work contains collocated brightness temperatures and radiosonde data that have been collected during the period of Baseline Upper Atmospheric Network (BUAN) experiments. These data include both cloudy and clear cases. A multispectral cloud-removal method using the principle of the N * method has been developed. This method uses radiances of High-Resolution Infrared Radiation Sounder channels 6, 7, and 8 to estimate clear radiances of these channels and the surface temperature simultaneously based on radiative transfer simulations. Subsequently, the quantity N * (the ratio of effective cloud cover over adjacent pixels) and the clear radiances of the rest of the channels are evaluated.

Retrieval results are presented in terms of rms temperature differences between retrieved and sounding profiles. Considering all clear and partly cloudy cases, the rms differences in temperature of approximately 2 K for retrievals using the DI are comparable to those using the minimum-variance scheme. The rms differences in temperature for retrievals using the multispectral cloud-removal scheme are slightly larger than those using the BUAN cloud-removal scheme by approximately 0.5 K. Finally, the rms temperature differences are much smaller than those for the first guess of the minimum-variance scheme. These results indicate fire that the DJ can achieve acceptable performance without first-guess or error covariance matrices; second, that the proposed multispectral cloud-removal method is also capable of generating reasonable cloud-removed clear radiances; and finally that the DI can be used as a tool to obtain first guesses in the current operational method and to perform large-volume temperature retrievals for climate studies.

Full access
S. C. Ou, K. N. Liou, and T. R. Caudill

Abstract

Surface observations show that multilayer clouds frequently occur in frontal areas where cirrus clouds overlie boundary layer convective clouds or stratus clouds. In this paper, an algorithm is presented for the retrieval of cirrus cloud optical depths and ice crystal sizes in multilayer cloud systems based on the theory of radiative transfer and parameterizations. For the validation of the retrieval program, AVHRR data is analyzed for two dates during FIRE-II-IFO in which cirrus clouds overlie a layer of low stratus cloud. It is shown that the domain-averaged retrieved cloud temperatures are within the boundaries of cirrus clouds determined from the collocated replicator, radar, and lidar data. The retrieved ice crystal mean effective sizes and optical depths are also in general agreement with the values determined from the balloon-borne replicator and 2D probe data.

Full access
T. P. Ackerman, K. N. Liou, and C. B. Leovy

Abstract

Full access
S. C. Ou, K. N. Liou, and B. A. Baum

Abstract

A numerical scheme has been developed to identify multilayer cirrus cloud systems using Advanced Very Higher Resolution Radiometer (AVHRR) data. It is based on the physical properties of the AVHRR channels 1–2 reflectance ratios, the brightness temperature differences between channels 4 and 5, and the channel 4 brightness temperatures. In this scheme, clear pixels are first separated from cloudy pixels, which are then classified into three types: cirrus, cirrus/low cloud, and low clouds. The authors have applied this scheme to the satellite data collected over the FIRE II IFO [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment II intensive field observations area during nine overseas within seven observation dates. Determination of the threshold values used in the detection scheme are based on statistical analysts of these satellite data. The authors have validated the detection results against the cloudy conditions inferred from the collocated and coincident ground-based lidar and radar images, balloonborne replicator data, and National Center for Atmospheric Research CLASS (Cross-chain Loran Atmospheric Sounding System) humidity soundings on a case-by-case basis. In every case, the satellite detection results are consistent with the cloudy conditions inferred from these independent and complementary measurement. The present scheme is well suited for the detection of midlatitude, multilayer cirrus cloud systems and tropical anvils.

Full access
Steven K. Krueger, Qiang Fu, K. N. Liou, and Hung-Neng S. Chin

Abstract

It is important to properly simulate the extent and ice water content of tropical anvil clouds in numerical models that explicitly include cloud formation because of the significant effects that these clouds have on the radiation budget. For this reason, a commonly used bulk ice-phase microphysics parameterization was modified to more realistically simulate some of the microphysical processes that occur in tropical anvil clouds. Cloud ice growth by the Bergeron process and the associated formation of snow were revised. The characteristics of graupel were also modified in accord with a previous study. Numerical simulations of a tropical squall line demonstrate that the amount of cloud ice and the extent of anvil clouds are increased to more realistic values by the first two changes.

Full access
K. N. Liou, S. C. Ou, Y. Takano, F. P. J. Valero, and T. P. Ackerman

Abstract

A dual-channel retrieval technique involving the water vapor band at 6.5 μm and the window region at 10.5 gm has been developed to infer the temperature and emissivity of tropical anvils. This technique has been applied to data obtained from the ER-2 narrow field-of-view radiometers during two flights in the field observation of the Stratosphere-Troposphere Exchange Project (STEP) near Damn, Australia, January-February 1987. The retrieved cloud temperatures are between 190 and 240 K, while the cloud emissivities derived from the retrieval algorithm range from about 0.2 to 1. Moreover, the visible optical depths have been obtained from the cloud emissivity through a theoretical parameterization with values of 0.5-10. A significant portion of tropical cirrus clouds are found to have optical depths greater than about 6. Because of the parameterization, the present technique is unable to precisely determine the optical depth values for optically thick cirrus clouds.

Full access