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Long S. Chiu and Alfred T. C. Chang

Abstract

The climatology of oceanic rain column height derived from 12 years (July 1987–June 1999) of Special Sensor Microwave Imager (SSM/I) data is presented. The estimation procedure is based on a technique developed by Wilheit et al. In the annual mean, the SSM/I-derived oceanic rain height shows a maximum of about 4.7 km in the Tropics and decreases toward the high latitudes to less than 3.5 km at 50°. Interannual variations exhibit seasonal dependency and show maxima of about 200–300 m in the oceanic dry zones and in the midlatitude storm track regions. The rain heights estimated from the morning passes of the SSM/I are lower than those computed from the afternoon passes by about 60 m in the Tropics but are higher north of 40°N. This small difference cannot change the conclusion about the morning maximum in rain rate. The nonsystematic error increases with decreasing rain column height and is estimated to be about 120 m for rain heights of 4–5 km and 200 m at 3.5 km. Comparison with the height of the 0°C isotherm derived from the Goddard Laboratory for Atmospheres general circulation model (GCM) results shows a mean zonal low bias (SSM/I lower than GCM freezing height) of about 200 m in the Tropics. Outside the Tropics, the SSM/I rain column heights are much higher, reaching a difference of 2 km at 50°N. The small bias in the Tropics is consistent with the notion that the melting layer extends over hundreds of meters below the freezing level. Outside the Tropics, the sampling of the SSM/I rain height and the inclusion of nonraining observations in GCM calculations may contribute to the large discrepancy. The freezing height is interpreted as the columnar water content and found to be consistent with columnar water vapor maps retrieved from SSM/I data.

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Alfred T. C. Chang and Long S. Chiu

Abstract

About 10 yr (July 1987–December 1997 with December 1987 missing) of oceanic monthly rainfall based on data taken by the Special Sensor Microwave/Imager (SSM/I) on board the Defense Meteorological Satellite Program satellites have been computed. The technique, based on the work of Wilheit et al., includes improved parameterization of the beam-filling correction, a refined land mask and sea ice filter. Monthly means are calculated for both 5° and 2.5° latitude–longitude boxes.

Monthly means over the latitude band of 50°N–50°S and error statistics are presented. The time-averaged rain rate is 3.09 mm day−1 (std dev of 0.15 mm day−1) with an error of 38.0% (std dev of 3.0%) for the 5° monthly means over the 10-yr period. These statistics compare favorably with 3.00 mm day−1 (std dev of 0.19 mm day−1) and 46.7% (std dev of 3.4%) computed from the 2.5° monthly means for the period January 1992–December 1994. Examination of the different rain rate categories shows no distinct discontinuity, except for months with a large number of missing SSM/I data.

An independent estimate of the error using observations from two satellites shows an error of 31% (std dev of 2.7%), consistent with the 38% estimated using (a.m. and p.m.) data from one satellite alone. Error estimates (31%) based on the 5° means by averaging four neighboring 2.5° boxes are larger than those (23%) estimated by assuming the means for these neighboring boxes are independent, thus suggesting spatial dependence of the 2.5° means.

Multiple regression analyses show that the error varies inversely as the square root of the number of samples but exhibits a somewhat weaker dependence on the mean rain rate. Regression analyses show a power law dependence of −0.255 to −0.265 on the rain rate for the 5° monthly means using data from a single satellite and a dependence of −0.366 for the 5° monthly means and −0.337 for the 2.5° monthly means based on two satellite measurements. The latter estimate is consistent with that obtained by Bell et al. using a different rainfall retrieval technique.

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James C. Barnard and Charles N. Long

Abstract

In this paper, an empirical equation is presented that can be used to estimate shortwave cloud optical thickness from measurements and analysis of shortwave broadband irradiances. When applied to a time series of broadband observations, this method can predict cloud optical thickness distributions that are very similar to those obtained using the algorithm of Min and Harrison (henceforth the Min algorithm). For a given site, medians of the Min algorithm–derived and empirically derived distributions differ by less than 10%. This level of agreement holds over a wide geographical range of sites. The equation is designed for fully overcast skies, surface albedos less than 0.3, and a cosine of the solar zenith angle greater than 0.15.

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Alfred T. C. Chang, Long S. Chiu, and Thomas T. Wilheit

Abstract

Global averages and random errors associated with the monthly oceanic rain rates derived from the Special Sensor Microwave/Imager (SSM/I) data using the technique developed by Wilheit et al. are computed. Accounting for the beam-filling bias, a global annual average rain rate of 1.26 m is computed. The error estimation scheme is based on the existence of independent (morning and afternoon) estimates of the monthly mean. Calculations show overall random errors of about 50%–60% for each 5° × 5° box. The results are insensitive to different sampling strategy (odd and even days of the month). Comparison of the SSM/I estimates with raingage data collected at the Pacific atoll stations showed a low bias of about 8%, a correlation of 0.7, and an rms difference of 55%.

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Kingtse C. Mo, Lindsey N. Long, and Jae-Kyung E. Schemm

Abstract

Atmosphere–land–ocean coupled model simulations are examined to diagnose the ability of models to simulate drought and persistent wet spells over the United States. A total of seven models are selected for this study. They are three versions of the NCEP Climate Forecast System (CFS) coupled general circulation model (CGCM) with a T382, T126, and T62 horizontal resolution; GFDL Climate Model version 2.0 (CM2.0); GFDL CM2.1; Max Planck Institute (MPI) ECHAM5; and third climate configuration of the Met Office Unified Model (HadCM3) simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) experiments.

Over the United States, drought and persistent wet spells are more likely to occur over the western interior region, while extreme events are less likely to persist over the eastern United States and the West Coast. For meteorological drought, which is defined by precipitation (P) deficit, the east–west contrast is well simulated by the CFS T382 and the T126 models. The HadCM3 captures the pattern but not the magnitudes of the frequency of occurrence of persistent extreme events. For agricultural drought, which is defined by soil moisture (SM) deficit, the CFS T382, CFS T126, MPI ECHAM5, and HadCM3 models capture the east–west contrast.

The models that capture the west–east contrast also have a realistic P climatology and seasonal cycle. ENSO is the dominant mode that modulates P over the United States. A model needs to have the ENSO mode and capture the mean P responses to ENSO in order to simulate realistic drought. To simulate realistic agricultural drought, the model needs to capture the persistence of SM anomalies over the western region.

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Thomas T. Wilheit, Alfred T. C. Chang, and Long S. Chiu

Abstract

An algorithm for the estimation of monthly rain totals for 5° cells over the oceans from histograms of SSM/I brightness temperatures has been developed. Them are three novel features to this algorithm. First, it uses knowledge of the form of the rainfall intensity probability density function to augment the measurements. Second, a linear combination of the 19.35 and 22.235 GHz channels has been employed to reduce the impact of variability of water vapor. Third, an objective technique has been developed to estimate the rain layer thickness from the 19.35- and 22.235-GHz brightness temperature histograms. Comparison with climatologies and the GATE radar observations suggest that the estimates are reasonable in spite of not having a beam-filling correction. By-products of the retrievals indicate that the SSM/I instrument noise level and calibration stability am quite good.

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C. N. Long, J. M. Sabburg, J. Calbó, and D. Pagès

Abstract

A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of sky-imaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.

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C. N. Long, J. H. Mather, and T. P. Ackerman
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Long S. Chiu, Alfred T. C. Chang, and John Janowiak

Abstract

Three years of monthly rain rates over 5° × 5° latitude–longitude boxes have been calculated for oceanic regions 50°N–50°S from measurements taken by the Special Sensor Microwave/Imager on board the Defense Meteorological Satellite Program satellites using the technique developed by Wilheit et al. The annual and seasonal zonal-mean rain rates are larger than Jaeger's climatological estimates but are smaller than those estimated from the GOES precipitation index (GPI) for the same period. Regional comparison with the GPI showed that these rain rates are smaller in the north Indian Ocean and in the southern extratropics where the GPI is known to overestimate. The differences are also dominated by a jump at 170°W in the GPI rain rates across the mid Pacific Ocean. This jump is attributed to the fusion of different satellite measurements in producing the GPI.

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Matthew C. Long, Keith Lindsay, Synte Peacock, J. Keith Moore, and Scott C. Doney

Abstract

Ocean carbon uptake and storage simulated by the Community Earth System Model, version 1–Biogeochemistry [CESM1(BGC)], is described and compared to observations. Fully coupled and ocean-ice configurations are examined; both capture many aspects of the spatial structure and seasonality of surface carbon fields. Nearly ubiquitous negative biases in surface alkalinity result from the prescribed carbonate dissolution profile. The modeled sea–air CO2 fluxes match observationally based estimates over much of the ocean; significant deviations appear in the Southern Ocean. Surface ocean pCO2 is biased high in the subantarctic and low in the sea ice zone. Formation of the water masses dominating anthropogenic CO2 (Cant) uptake in the Southern Hemisphere is weak in the model, leading to significant negative biases in Cant and chlorofluorocarbon (CFC) storage at intermediate depths. Column inventories of Cant appear too high, by contrast, in the North Atlantic. In spite of the positive bias, this marks an improvement over prior versions of the model, which underestimated North Atlantic uptake. The change in behavior is attributable to a new parameterization of density-driven overflows. CESM1(BGC) provides a relatively robust representation of the ocean–carbon cycle response to climate variability. Statistical metrics of modeled interannual variability in sea–air CO2 fluxes compare reasonably well to observationally based estimates. The carbon cycle response to key modes of climate variability is basically similar in the coupled and forced ocean-ice models; however, the two differ in regional detail and in the strength of teleconnections.

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