Effect of Dust Load on the Cloud Top Ice-Water Partitioning Over Northern Middle to High Latitudes With CALIPSO Products

We quantified effects of dust load on the cloud top ice cloud fraction (ICF) in terms of the dust extinction coefficient (σ_ext). We analyzed 3-year data sets obtained from an active satellite sensor over middle to high latitudes in the northern hemisphere for temperatures (T) between 230 and 273 K and σ_ext values between 0.005 and 0.145 km⁻¹. At about 250 K, ICF changed by about 30% in response to the above range of σ_ext, whereas at extreme T values, ICF was relatively insensitive to σ_ext. Thus, we concluded that ICF was primarily determined by T, with substantial influence of σ_ext at about 250 K, likely due to increased opportunities for freezing as σ_ext increases. Sensitivity of ICF was the lowest both at the largest σ_ext and lowest T and at the smallest σ_ext and highest T, while it was the highest at about 0.03 km⁻¹ of σ_ext and about 250 K. If there are any physical parameters that influence the ICF except temperature (T), how much does this parameter influence ICF in a given T? Dust particles have been long known as efficient ice nucleating particles. Although previous studies suggested that more dust particles increased ICF, they did not use quantitative parameters of the dust amount, but less-quantitative indicators such as relative dust frequency. Therefore, we used the dust extinction coefficient (σ_ext) as a quantitative parameter of dust amount and examined the relationship between the dust amount and ICF for T between 230 and 273 K. We observed the following phenomena from satellite data. At about 250 K, ICF substantially depended on σ_ext likely due to increased opportunities for freezing as σ_ext increases. However, at extreme T values, ICF was relatively insensitive to σ_ext. Moreover, we found that sensitivity of ICF was the lowest both at the largest σ_ext and lowest T and at the smallest σ_ext and highest T, while it was the highest at about 0.03 km⁻¹ of σ_ext and about 250 K. These behaviors of the ICF sensitivity could be understood from characteristics of T (the lower, the easier for freezing) and σ_ext (the larger, the easier for freezing).