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 SST data was found to vary significantly with season and hemisphere. The correlation between 4 yr of monthly mean, globally averaged SST and CWV was approximately 0.95 for the hemispheric means and 0.65 for the global means. The 4-yr correlation of SST and CWV was found to be strongest in the subtropical regions and weakest in several tropical regions.

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 vary with channel, surface type, latitude, satellite, and cloud cover, but showed little dependence on satellite scan angle. Diurnal bias is most pronounced for ascending orbit observations of the afternoon [1400 local solar time (LST)] satellites with 60°N to 60°S domain averaged brightness temperatures variations up to 0.78 K yr⁻¹. Lower tropospheric temperature and water vapor channels contained the largest bias, and biases over land were more than twice as large as those over the ocean. Brightness temperature adjustments of up to 10 K were needed in the most extreme situations.

Abstract

A 10-m air temperature (Ta) retrieval using Advanced Microwave Sounding Unit A (AMSU-A) and satellite-derived sea surface temperature (Ts) observations is presented. The multivariable linear regression retrieval uses AMSU-A brightness temperatures from the 52.8- and 53.6-GHz channels and satellite-derived daily sea surface temperatures to determine Ta. A regression error of 0.83°C using 841 matched satellite and ship observations demonstrates a high-quality fit of the satellite observations with in situ Ta. Validation of the retrieval using independent International Comprehensive Ocean–Atmosphere Dataset (ICOADS) ship and buoy observations results in a bias of −0.21°C and root-mean-square (RMS) differences of 1.55°C. A comparison with previous satellite-based Ta retrievals indicates less bias and significantly smaller RMS differences for the new retrieval. Regional biases inherent to previous retrievals are reduced in several oceanic regions using the new Ta retrieval. Satellite-derived Ts–Ta data were found to agree well with ICOADS buoy data and were significantly improved from previous retrievals.

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 to test retrievals such as the one described herein. The results suggest that the relative humidity of the upper troposphere, especially over the domain of the Hadley circulation taken to be between 30°N and 30°S, undergoes a significant seasonal change. This is contrary to the usual assumption of fixed relative humidity adopted in simple climate feedback studies. Large seasonal changes in the region from 30°N to 30°S are possibly associated with the seasonal swings in the Hadley circulation. Similar seasonal changes in the 350-hPa overburden indicate that these swings in relative humidity occur as a result of significant seasonal shifts in the upper-tropospheric water vapor content. In the region equatorward of 30° latitude, the Southern Hemisphere winter is shown to be significantly drier than the Northern Hemisphere winter. This enhanced drying is consistent with the existence of more extensive regions of subsidence producing larger regions of dry upper-tropospheric air in the SH during winter than in the corresponding NH during winter, especially in the subtropical Eastern Hemisphere. Analyses of the data show the clear effects of moistening in the NH subtropics through the monsoonal circulations over Asia and North America and little effect of monsoon circulation in the Southern Hemisphere.

Abstract

Satellite microwave and infrared instruments sensitive to upper-tropospheric water vapor (UTWV) are compared using both simulated and observed cloud-cleared brightness temperatures (Tb’s). To filter out cloudy scenes, a cloud detection algorithm is developed for the Special Sensor Microwave/Temperature-2 (SSM/T2 or T2) data using the 92- and 150-GHz window channels. An analysis of the effect of clouds on the T2 183-GHz channels shows sensitivity primarily to high clouds containing ice, resulting in significantly better sampling of UTWV Tb’s over the convective zones and regions of persistent cloudiness. This is in contrast to the infrared sensors, which are extremely sensitive to any cloud contamination in the satellite field of view. A comparison of simulated UTWV Tb’s from T2, the High-resolution Infrared Sounder (HIRS), and the Visible Infrared Spin Scan Radiometer (VISSR) indicates a higher overall sensitivity to changes in UTWV in the T2 channel. HIRS and VISSR, however, are more sensitive to moisture at higher levels. Cloud-cleared Tb’s from T2 and HIRS were found to be highly correlated in the tropical dry zones and in regions of strong seasonal variability but less correlated at higher latitudes. The advantages of the microwave T2 sensor for monitoring UTWV are demonstrated by its greater sensitivity to changes in upper-tropospheric moisture and superior coverage over cloudy regions.

Abstract

A near-surface specific humidity (Qa) and air temperature (Ta) climatology on daily and 0.25° grids was constructed by the objectively analyzed air–sea fluxes (OAFlux) project by objectively merging two recent satellite-derived high-resolution analyses, the OAFlux existing 1° analysis, and atmospheric reanalyses. The two satellite products include the multi-instrument microwave regression (MIMR) Qa and Ta analysis and the Goddard Satellite-Based Surface Turbulent Fluxes, version 3 (GSSTF3), Qa analysis. This study assesses the degree of improvement made by OAFlux using buoy time series measurements at 137 locations and a global empirical orthogonal function (EOF) analysis. There are a total of 130 855 collocated daily values for Qa and 283 012 collocated daily values for Ta in the buoy evaluation. It is found that OAFlux Qa has a mean difference close to 0 and a root-mean-square (RMS) difference of 0.73 g kg⁻¹, and Ta has a mean difference of −0.03°C and an RMS difference of 0.45°C. OAFlux shows no major systematic bias with respect to buoy measurements over all buoy locations except for the vicinity of the Gulf Stream boundary current, where the RMS difference exceeds 1.8°C in Ta and 1.2 g kg⁻¹ in Qa. The buoy evaluation indicates that OAFlux represents an improvement over MIMR and GSSTF3. The global EOF-based intercomparison analysis indicates that OAFlux has a similar spatial–temporal variability pattern with that of three atmospheric reanalyses including MERRA, NCEP-1, and ERA-Interim, but that it differs from GSSTF3 and the Climate Forecast System Reanalysis (CFSR).

Abstract

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.

Abstract

The changes of the outgoing longwave radiation (OLR) in clear-sky conditions have been calculated using High Resolution Infrared Radiation Sounder (HIRS) observations from 1979 to 2004. After applying corrections for satellite orbital drift and intercalibration of the HIRS/2 data from the NOAA satellites, the OLR is calculated from a multivariate regression over the tropical ocean region. The clear-sky OLR retrievals compare well with the observed top-of-atmosphere radiation measurements, although the precision and stability uncertainties are larger. While the tropical ocean surface temperature has risen by roughly 0.2 K from 1982 to 2004, the reconstructed OLR remains stable over the ocean. Consequently, there is an increase in the clear-sky greenhouse effect (GHE) of 0.80 W m⁻² decade⁻¹. This trend is shown to be larger than the uncertainty in the stability of the HIRS retrievals.

The observations are compared with two phase 3 of the Coupled Model Intercomparison Project model ensembles: one ensemble includes both natural and anthropogenic forcings [the twentieth-century (20C) ensemble] and the other ensemble only contains natural climate variability (the control ensemble). The OLR trend in the 20C simulations tends to be more negative than observed, although a majority is found to be within the observational uncertainty. Conversely, the response of the clear-sky OLR to SST is shown to be very similar in observations and models. Therefore, the trend differences between the 20C simulations and observations are likely because of internal climate variability or uncertainties in the external forcings. The observed increase in GHE is shown to be inconsistent with the control ensemble, indicating that anthropogenic forcings are required to reproduce the observed changes in GHE.

Abstract

An analysis of atmospheric rivers (ARs) as defined by an automated AR detection tool based on integrated water vapor transport (IVT) and the connection to heavy precipitation in the southeast United States (SEUS) is performed. Climatological water vapor and water vapor transport fields are compared between the U.S. West Coast (WCUS) and the SEUS, highlighting stronger seasonal variation in integrated water vapor in the SEUS and stronger seasonal variation in IVT in the WCUS. The climatological analysis suggests that IVT values above ~500 kg m⁻¹ s⁻¹ (as incorporated into an objective identification tool such as the AR detection tool used here) may serve as a sensible threshold for defining ARs in the SEUS.

Atmospheric river impacts on heavy precipitation in the SEUS are shown to vary on an annual cycle, and a connection between ARs and heavy precipitation during the nonsummer months is demonstrated. When identified ARs are matched to heavy precipitation days (>100 mm day⁻¹), an average match rate of ~41% is found.

Results suggest that some aspects of an AR identification framework in the SEUS may offer benefit in forecasting heavy precipitation, particularly at medium- to longer-range forecast lead times. However, the relatively high frequency of SEUS heavy precipitation cases in which an AR is not identified necessitates additional careful consideration and incorporation of other critical aspects of heavy precipitation environments such that significant predictive skill might eventually result.

Abstract

The wettest period during the CalWater-2014 winter field campaign occurred with a long-lived, intense atmospheric river (AR) that impacted California on 7–10 February. The AR was maintained in conjunction with the development and propagation of three successive mesoscale frontal waves. Based on Lagrangian trajectory analysis, moist air of tropical origin was tapped by the AR and was subsequently transported into California. Widespread heavy precipitation (200–400 mm) fell across the coastal mountain ranges northwest of San Francisco and across the northern Sierra Nevada, although only modest flooding ensued due to anomalously dry antecedent conditions. A NOAA G-IV aircraft flew through two of the frontal waves in the AR environment offshore during a ~24-h period. Parallel dropsonde curtains documented key three-dimensional thermodynamic and kinematic characteristics across the AR and the frontal waves prior to landfall. The AR characteristics varied, depending on the location of the cross section through the frontal waves. A newly implemented tail-mounted Doppler radar on the G-IV simultaneously captured coherent precipitation features. Along the coast, a 449-MHz wind profiler and collocated global positioning system (GPS) receiver documented prolonged AR conditions linked to the propagation of the three frontal waves and highlighted the orographic character of the coastal-mountain rainfall with the waves’ landfall. A vertically pointing S-PROF radar in the coastal mountains provided detailed information on the bulk microphysical characteristics of the rainfall. Farther inland, a pair of 915-MHz wind profilers and GPS receivers quantified the orographic precipitation forcing as the AR ascended the Sierra Nevada, and as the terrain-induced Sierra barrier jet ascended the northern terminus of California’s Central Valley.