The Epic 2001 Stratocumulus Study

Bretherton, Christopher S.; Uttal, Taneil; Fairall, C. W.; Yuter, Sandra E.; Weller, Robert A.; Baumgardner, Darrel; Comstock, K.; Wood, Robert; Raga, Graciela B.

doi:10.1175/bams-85-7-967

Cited by 326 publications

(301 citation statements)

References 29 publications

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“…3b, yields a low bias in the ECMWF analysis. The correlation is lower (∼0.54) and the 1-sigma uncertainty is ∼70 m. The systematic low bias in ECMWF ABL height is consistent with previous weather and climate model studies over the region (e.g., Bretherton et al, 2004;Rahn and Garreaud, 2010;Wyant et al, 2010;von Engeln and Teixeira, 2011;Wang et al, 2011).…”

Section: Abl Height Comparisonssupporting

confidence: 88%

Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean

Xie

et al. 2012

Atmos. Chem. Phys.

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Abstract. The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1-2 km) as observed from the radiosondes. However, the ECMWF analysis systematically underestimates the ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., −157 N-unit km −1 ) and becomes the so-called ducting condition, which results in a systematic RO refractivity bias (or called N -bias) inside the ABL. Simulation study based on radiosonde profiles reveals the magnitudes of the N-biases are vertical resolution dependent. The N -bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding −110 N-unit km −1 ) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very large RO bending angle and the sharp refractivity gradient due to ducting allow reliable detection of the ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show an increase of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows an overall deeper ABL and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. At low latitudes, despite the decreasing number of COSMIC RO soundings and the lower percentage of soundings that penetrate into the lowest 500-m above the mean-sea-level, there are small sampling errors in the mean ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies.

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Section: Abl Height Comparisonssupporting

confidence: 88%

Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean

Xie

et al. 2012

Atmos. Chem. Phys.

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“…Dai (2001b) found that drizzle is most frequent at 0600 LST over land areas, similar to the results from the present study, and from midnight to 0400 LST over the oceans. Dai's (2001b) results for the ocean are consistent with those of Bretherton et al (2004) for a region west of South America over the subtropical and tropical South Pacific Ocean. Dai (2001b) suggested that diurnal variations in relative humidity (primarily due to the cooler temperatures in the morning) contribute to this morning maximum over land, although he did not elaborate on the mechanism.…”

Section: Diurnal Cyclesupporting

confidence: 58%

Spatial and Temporal Variability of Nonfreezing Drizzle in the United States and Canada

Sears-Collins

Schultz

Johns³

2006

Journal of Climate

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A climatology of nonfreezing drizzle is created using surface observations from 584 stations across the United States and Canada over the 15-yr period 1976-90. Drizzle falls 50-200 h a year in most locations in the eastern United States and Canada, whereas drizzle falls less than 50 h a year in the west, except for coastal Alaska and several western basins. The eastern and western halves of North America are separated by a strong gradient in drizzle frequency along roughly 100°W, as large as about an hour a year over 2 km. Forty percent of the stations have a drizzle maximum from November to January, whereas only 13% of stations have a drizzle maximum from June to August. Drizzle occurrence exhibits a seasonal migration from eastern Canada and the central portion of the Northwest Territories in summer, equatorward to most of the eastern United States and southeast Canada in early winter, to southeastern Texas and the eastern United States in late winter, and back north to eastern Canada in the spring. The diurnal hourly frequency of drizzle across the United States and Canada increases sharply from 0900 to 1200 UTC, followed by a steady decline from 1300 to 2300 UTC. Diurnal drizzle frequency is at a maximum in the early morning, in agreement with other studies.Drizzle occurs during a wide range of atmospheric conditions at the surface. Drizzle has occurred at sea level pressures below 960 hPa and above 1040 hPa. Most drizzle, however, occurs at higher than normal sea level pressure, with more than 64% occurring at a sea level pressure of 1015 hPa or higher. A third of all drizzle falls when the winds are from the northeast quadrant (360°-89°), suggesting that continental drizzle events tend to be found poleward of surface warm fronts and equatorward of cold-sector surface anticyclones. Two-thirds of all drizzle occurs with wind speeds of 2.0-6.9 m s Ϫ1 , with 7.6% in calm wind and 5% at wind speeds у 10 m s Ϫ1. Most drizzle (61%) occurs with visibilities between 1.5 and 5.0 km, with only about 20% occurring at visibilities less than 1.5 km.

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“…By reflecting sunlight back to space, stratiform clouds are ''the vast climate refrigerator of the tropics and subtropics'' (ref. 25, see also ref. 26).…”

mentioning

confidence: 87%

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

Kaufman

Koren

Remer

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

600

480

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Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30°S-20°S), smoke (20°S-5°N), mineral dust (5°N-25°N), and pollution aerosols (30°N-60°N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2-0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is ؊11 ؎ 3 W͞m 2 for the 3 months studied; 2͞3 of it is due to the aerosolinduced cloud changes, and 1͞3 is due to aerosol direct radiative effect.cloud cover ͉ cloud height ͉ indirect effect ͉ radiative forcing ͉ air quality D uring June through August, the Atlantic Ocean is covered by varying concentrations of several aerosol types, each covering a separate latitude belt (see Fig. 1). The Southern Tropical Atlantic (30°S-20°S) is dominated by clean maritime air. The region between 20°S and 5°N is a relatively well defined region covered by smoke from biomass burning in Africa (1, 2). The Northern Tropical Atlantic (5°N-30°N) is under heavy influx of dust from Africa (3), and the Northern Atlantic (30°N-60°N) is impacted by anthropogenic pollution aerosol from North America and Europe. These aerosols absorb and reflect solar radiation to space (4), thereby affecting the regional atmospheric energy balance. Clouds that form in air laden with high aerosol concentrations tend to contain more numerous but smaller droplets that reflect sunlight and cool the Earth (5). The smaller cloud droplets reduce the efficiency of droplet growth by collision coalescence, which at least under some conditions (6) reduce precipitation formation and increase cloud lifetime (7,8). However, there is a second pathway for aerosols to affect clouds: Smoke, pollution, and dust aerosols absorb solar radiation, heat the atmosphere, and reduce evaporation from the surface (9-11). As a result, smoke over the Amazon or pollution aerosol over the Indian Ocean can inhibit cloud...

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The Epic 2001 Stratocumulus Study

Cited by 326 publications

References 29 publications

Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean

Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean

Spatial and Temporal Variability of Nonfreezing Drizzle in the United States and Canada

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

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