Modeling Cirrus Clouds. Part I: Treatment of Bimodal Size Spectra and Case Study Analysis

Mitchell, David L.; Chai, Steven K.; Liu, Yangang; Heymsfield, Andrew J.; Dong, Yayi

doi:10.1175/1520-0469(1996)053<2952:mccpit>2.0.co;2

Cited by 86 publications

(73 citation statements)

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“…Their estimates for E agg tend to be around 0.3, but values of 0.1 also gave reasonable agreement with the observations; important to note was that there was no strong dependence of the aggregation efficiency on temperature. Mitchell et al (1996) found from a case study in cirrus clouds that the cloud-averaged E agg was ∼ 0.5. More recently, Mitchell et al (2006) applied a new snow growth model to calculate that in a case dominated by dendrites at cloud top E agg ∼ = 0.55 whereas in other cases E agg ∼ = 0.07.…”

Section: P J Connolly Et Al: Aggregation Of Ice Crystalsmentioning

confidence: 99%

A laboratory investigation into the aggregation efficiency of small ice crystals

2012

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Abstract. The aggregation of ice crystals and its temperature dependence is studied in the laboratory using a large ice cloud chamber. This process is important to the evolution of ice clouds in earth's atmosphere, yet there have been relatively few laboratory studies quantifying this parameter and its dependence on temperature. A detailed microphysical model is used to interpret the results from the experiments and derive best estimates for the aggregation efficiency as well as error bars. Our best estimates for the aggregation efficiency, at temperatures other than −15 • C, (in the interval −30 ≤ T ≤ 5 • C) are mostly in agreement with previous findings, which were derived using a very different approach to that described here. While the errors associated with such experiments are reasonably large, statistically, at temperatures other than −15, we are able to rule out aggregation efficiencies larger than 0.5 at the 75th percentile and rule out nonzero values at −15 • C, whereas at −15 • C we can rule out values higher than 0.85 and values lower than 0.35. The values of the aggregation efficiency shown here may be used in model studies of aggregation, but care must be taken that they only apply for the initial stages of aggregate growth, with humidities at or close to water saturation, and for particles up to a maximum size of ∼500 µm. They may therefore find useful application for modelling supercooled mid-level layer clouds that contain ice crystals, which are known to be important radiatively.

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Section: P J Connolly Et Al: Aggregation Of Ice Crystalsmentioning

confidence: 99%

A laboratory investigation into the aggregation efficiency of small ice crystals

2012

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“…A bimodal ice crystal size distribution, as observed by, e.g., McFarquhar and Heymsfield [1996] and Mitchell et al [1996b], is assumed with an exponential behavior for small crystals and a À-distribution for larger ice crystals. The size distribution used is the same as given by Stubenrauch et al [1999c], with the difference that the mean maximum dimension from the contribution of small particles is now fixed to 20 mm instead of 40 mm.…”

Section: Randomly Oriented Planar Polycrystalsmentioning

confidence: 99%

Retrieval of effective ice crystal size in the infrared: Sensitivity study and global measurements from TIROS‐N Operational Vertical Sounder

et al. 2003

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[1] The difference between cirrus emissivities at 8 and 11 mm is sensitive to the mean effective ice crystal size of the cirrus cloud, D e . By using single scattering properties of ice crystals shaped as planar polycrystals, diameters of up to about 70 mm can be retrieved, instead of up to 45 mm assuming spheres or hexagonal columns. The method described in this article is used for a global determination of mean effective ice crystal sizes of cirrus clouds from TOVS satellite observations. A sensitivity study of the D e retrieval to uncertainties in hypotheses on ice crystal shape, size distributions, and temperature profiles, as well as in vertical and horizontal cloud heterogeneities shows that uncertainties can be as large as 30%. However, the TOVS data set is one of few data sets which provides global and long-term coverage. Having analyzed the years 1987-1991, it was found that measured effective ice crystal diameters D e are stable from year to year. For 1990 a global median D e of 53.5 mm was determined. Averages distinguishing ocean/land, season, and latitude lie between 23 mm in winter over Northern Hemisphere midlatitude land and 64 mm in the tropics. In general, larger D e s are found in regions with higher atmospheric water vapor and for cirrus with a smaller effective emissivity.

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“…Aggregation is a process where ice particles collide with each other and combine to form larger "aggregates" comprised of individual ice crystals. It has been demonstrated that aggregation is an important growth process for ice particles in cirrus clouds (Mitchell et al 1996). Aggregation will act on all PSD, depleting the concentrations of small ice crystals to form larger ones and possibly producing a secondary maximum in concentration ~ 200 µm particle size when ice crystals are produced only through heterogeneous processes.…”

Section: Tropical Cirrus Clouds: Tc4 Resultsmentioning

confidence: 99%

“…The PSD associated with in situ cirrus do not show a transition from bimodal to mono-modal size-spectra near -40°C, which appears consistent with their lower updrafts. The mono-modal PSD at the coldest level for in situ cirrus may reflect a reduced impact from aggregation near cloud top (aggregation influences on PSDs increase with distance from cloud top (Mitchell 1988, Mitchell et al 1996). The above information can be presented in terms of the bulk properties of the PSD; total number concentration N and mean ice particle size (maximum dimension),D.…”

Section: Tropical Cirrus Clouds: Tc4 Resultsmentioning

confidence: 99%

Cirrus Clouds and Climate Engineering: New Findings on Ice Nucleation and Theoretical Basis

Mitchell¹,

Mishra²,

Lawson³

2011

Planet Earth 2011 - Global Warming Challenges and Opportunities for Policy and Practice

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Modeling Cirrus Clouds. Part I: Treatment of Bimodal Size Spectra and Case Study Analysis

Cited by 86 publications

References 0 publications

A laboratory investigation into the aggregation efficiency of small ice crystals

A laboratory investigation into the aggregation efficiency of small ice crystals

Retrieval of effective ice crystal size in the infrared: Sensitivity study and global measurements from TIROS‐N Operational Vertical Sounder

Cirrus Clouds and Climate Engineering: New Findings on Ice Nucleation and Theoretical Basis

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