Darren L. Jackson

Trends in Global Cloud Cover in Two Decades of HIRS Observations

The frequency of cloud detection and the frequency with which these clouds are found in the upper troposphere have been extracted from NOAA High Resolution Infrared Radiometer Sounder (HIRS) polar-orbiting satellite data from 1979 to 2001. The HIRS/2 sensor was flown on nine satellites from the Television Infrared Observation Satellite-Next Generation (TIROS-N) through NOAA-14, forming a 22-yr record. Carbon dioxide slicing was used to infer cloud amount and height. Trends in cloud cover and high-cloud frequency were found to be small in these data. High clouds show a small but statistically significant increase in the Tropics and the Northern Hemisphere. The HIRS analysis contrasts with the International Satellite Cloud Climatology Project (ISCCP), which shows a decrease in both total cloud cover and high clouds during most of this period.

A physical retrieval of cloud liquid water over the global oceans using special sensor microwave/imager (SSM/I) observations

A method of remotely sensing integrated cloud liquid water over the oceans using spaceborne passive measurements from the special sensor microwave/imager (SSM/I) is described. The technique is comprised of a simple physical model that uses the 19.35- and 37-GHz channels of the SSM/I. The most comprehensive validation to date of cloud liquid water estimated from satellites is presented. This is accomplished through a comparison to independent ground-based microwave radiometer measurements of liquid water on San Nicolas Island, over the North Sea, and on Kwajalein and Saipan Islands in the western Pacific. In areas of marine stratocumulus clouds off the coast of California a further comparison is made to liquid water inferred from advanced very high resolution radiometer (AVHRR) visible reflectance measurements. The results are also compared qualitatively with near-coincident satellite imagery and with other existing microwave methods in selected regions. These comparisons indicate that the liquid water amounts derived from the simple scheme are consistent with the ground-based measurements for nonprecipitating cloud systems in the subtropics and middle to high latitudes. The comparison in the tropics, however, was less conclusive. Nevertheless, the retrieval method appears to have general applicability over most areas of the global oceans. An observational measure of the minimum uncertainty in the retrievals is determined in a limited number of known cloud-free areas, where the liquid water amounts are found to have a low variability of 0.016 kg m−2. A simple sensitivity and error analysis suggests that the liquid water estimates have a theoretical relative error typically ranging from about 25% to near 40% depending on the atmospheric/surface conditions and on the amount of liquid water present in the cloud. For the global oceans as a whole the average cloud liquid water is determined to be about 0.08 kg m−2. The major conclusion of this paper is that reasonably accurate amounts of cloud liquid water can be retrieved from SSM/I observations for nonprecipitating cloud systems, particularly in areas of persistent stratocumulus clouds, with less accurate retrievals in tropical regions.

Interannual Variability of Upper-Troposphere Water Vapor Band Brightness Temperature

Abstract A method for the intercalibration of the high-resolution infrared sounder (HIRS) upper-tropospheric water vapor band brightness temperature data is developed and applied to data from 1981 to 1993. Analysis of the adjusted anomaly time series show the location and strength of both the large-scale ascending and descending circulations in the Tropics as well as water vapor anomalies. Comparison of these HIRS data with outgoing longwave radiation and sea surface temperature anomalies reveals that both convection and increased upper-tropospheric moisture occur over anomalously warm water in the deep Tropics. The development and movement of deep convection and increased upper-tropospheric moisture can clearly he traced during the El Nino/Southern Oscillation warm events. These HIRS data are particularly useful in monitoring upper-tropospheric water vapor variability between the Tropics and subtropics.

Trends in upper‐tropospheric humidity

Water vapor is the most radiatively active greenhouse gas, and the process of water-vapor feedback may significantly amplify global warming induced by anthropogenic greenhouse gasses. Satellite radiance observations from the past 20 years, which are sensitive to the water vapor and temperature of the upper troposphere, provide the first global observations of trends in upper-tropospheric humidity. These decadal trends are strongly positive in the deep tropics, negative in the Southern Hemisphere subtropics and midlatitudes, and of mixed sign in the Northern Hemisphere subtropics and midlatitudes. The trends are shown to be consistent with atmospheric circulation changes observed in the past 20 years, including a tendency toward more El Nino-Southern Oscillation warm events and changes in transient eddy activity in the subtropics.

Global Observations of Upper-Tropospheric Water Vapor Derived from TOVS Radiance Data

Abstract This paper describes a physically based method for the retrieval of upper-tropospheric humidity (UTH) and upper-tropospheric column water vapor (UTCWV) based an the use of radiance data collected by the TIROS Operational Vertical Sounder (TOVS), principally channels 4 (14.2 μm), 6 (13.7 μm), and 12 (6.7 μm) of High-Resolution Infrared Radiation Sounder. This paper demonstrates how TOVS radiance data, particularly that of the upper-tropospheric water vapor channel 12, can be modeled usefully using a single band Malkmus model with parameters tuned to a particular sensor on a particular satellite. A significant uncertainty arises from the treatment of continuum absorption, even in regions where line absorption is dominant. This uncertainty can introduce a bias as large as 2 K, which in turn leads to an uncertainty of approximately 15%–20% in the retrieved UTH and UTCWV. The research described in this paper points to the critical need for high-accuracy measurements of upper-tropospheric water vapor t...

A Study of SSM/I-Derived Columnar Water Vapor over the Global Oceans

Abstract Four years of columnar water vapor (CWV) data from July 1987 through June 1991 derived from a satellite-based physical retrieval method are analysed using microwave observations from the Special Sensor Microwave/Imager. This retrieval along with three statistically based retrievals were compared to radiosonde data compiled for the GEWEX Water Vapor Project. It is shown that the root-mean-square (rms) difference (R) between the radiosonde data and these satellite retrievals ranges from 4.66 kg m−2 to 5.08 kg m−2. The rms difference was found to have a seasonal variability of up to 1.0 kg m−2 with the highest R in JJA. It was also found to significantly depend on the specified time and spatial coincidence of the satellite pixels with the radiosonde observation. The rms difference decreased by about 0.65 kg m−2 when the time constraint was reduced from 2 h to 0.5 h, but R increased by about 0.3 kg m−2 when the spatial coincidence was reduced from 50 to 20 km. The relationship between 4 yr of CWV and...

Variability of tropical upper tropospheric humidity 1979–1998

We update the time series of upper tropospheric humidity observations, begun in 1979 through May 1998, which includes the major El Nino event of 1997–1998. The intercalibration of different satellites is updated and compared to a physically based intercalibration. The results show excellent agreement and indicate that the main source of intersatellite bias is the known filter response function of the different instruments. Interannual variability of spatial fields is dominated by the major El Nino events in the 1979–1998 time period. Tropical average anomaly time series of upper tropospheric humidity, however, are dominated by a pronounced seasonal preference in extremes. Large deviations from the anomaly time series are twice as likely to occur in boreal winter and spring as in boreal summer and fall. During boreal winter and spring, the tropical basic state circulation permits the opening of a westerly wave duct such that midlatitude Rossby waves can propagate into the subtropics and tropics. We hypothesize that variation in the Rossby wave activity modulates the vertical water vapor flux and is responsible for large variations in the tropical upper tropospheric humidity time series.

Spaceborne observation of columnar water vapor : SSMI observations and algorithm

An analysis of the transfer of microwave radiation with a frequency between 10 and 40 GHz through a cloud-free atmosphere is given, and the limitations of employing observations at 22.235 GHz for sensing the columnar water vapor (CWV) in the moist atmosphere (CWV > 40 kg m−2) are shown. It is because of this limitation that a new method for the retrieval of columnar water vapor is described. This method uses both the horizontally and vertically polarized brightness temperatures measured at 19.35 GHz by the special sensor microwave imager (SSMI) which was launched as part of the Defense Military Satellite Program. The water vapor derived from the SSMI observations is validated against radiosonde observations that are near coincident in time and space with an rms error of 6 kg m−2. This large rms is in part due to sensitivity of the retrieval method to the surface emission. Distributions of CWV over the oceans were obtained for a 3-month period starting September 1987. These results are compared to other water vapor retrievals for the same period using the same microwave radiance data; they show close agreement except over the warmest water in the equatorial tropics.

Upper tropospheric humidity algorithm assessment

This study assesses a common analytic expression used to compute upper tropospheric humidity (UTH) from satellite brightness temperatures at 6.7 μm. Vertical averaging methods for UTH were found to have significant impact on the algorithm. The temperature lapse parameter β had more latitudinal variance than previously found; however, large vertical gradients in midlatitude profiles degraded UTH retrievals. The scaled reference pressure parameter P 0 did sufficiently improve the radiance to humidity relationship with exception to midlatitude profiles where P 0 exceeds 1.7. A new method for computing the reference pressure parameter is presented using HIRS channel 6 observations. Error analysis of radiance to humidity relationship indicates that absolute UTH errors increase with increasing humidity with largest errors of 15% per I K bias between simulated and observed brightness temperatures.

Detection and Correction of Diurnal Sampling Bias in HIRS/2 Brightness Temperatures

Abstract Diurnal sampling biases arise in the High-Resolution Infrared Radiation Sounder (HIRS) satellite observations because some of the NOAA polar-orbiting satellites drift significantly from their original local observation time. Such bias adversely affects interpretation of these data for climate studies. Twenty-six years of HIRS/2 radiance satellite data (1979–2004) were examined by creating monthly mean gridded data that categorize the observations by local observing time through averaging ascending and descending orbits separately. Corresponding HIRS/2 simulated radiance data from the Geophysical Fluid Dynamics Laboratory (GFDL) climate model were constructed using HIRS/2 satellite sampling and were found to accurately represent the diurnal sampling bias. Correction of the HIRS/2 observations from the observed diurnal sampling bias was using the model simulations of HIRS brightness temperatures to adjust the observed brightness temperatures to the model daily mean. The diurnal bias was found to va...