Abundance of Biological Ice Nucleating Particles in the Mississippi and Its Major Tributaries

Ice nucleating particles (INPs) are rare among atmospheric aerosols. However, through their ability to induce freezing of cloud droplets in cold clouds, they affect cloud lifetime, cloud albedo, and the efficiency and distribution of precipitation. While terrestrial sources of INPs are the focus of much research, the potential of rivers and lakes to be significant INP reservoirs has been neglected. In the first survey of a major river system, surface waters from the Mississippi, Missouri, Platte, and Sweetwater Rivers, all draining east and south from the Great Divide in the United States of America (USA), were tested for their INP concentrations. The survey comprised 49 samples, taken approximately every 150–250 km along 90% of the Mississippi (from Natchez, MS to the source at Bemidji, MN), the full length of the Missouri, 90% of the North Platte, and all of the Sweetwater. Samples were analysed using the immersion freezing method. The highest freezing temperature varied between −4 and −6 °C, and the concentration of INPs active at −10 °C or warmer ranged from 87 to 47,000 mL−1. The average INP concentration at −10 °C was 4950 mL−1, almost four orders of magnitude greater than the numbers of INPs typically found active at this temperature in seawater. The majority of INPs (69 to >99%) were heat labile (deactivated by heating to 95 °C) and therefore likely to be biological. Although the surface area of rivers is limited, their significant concentrations of INPs suggest that freshwater emissions should be investigated for their potential impact on regional cloud processes.

The effect of wind-generated bubbles on sea-surface backscattering at 940 Hz

Reliable predictions of sea-surface backscattering strength are required for sonar performance modeling. These are, however, difficult to obtain as measurements of sea-surface backscattering are not available at small grazing angles relevant to low-frequency active sonar (1-3 kHz). Accurate theoretical predictions of scattering strength require a good understanding of physical mechanisms giving rise to the scattering and the relative importance of these. In this paper, scattering from individual resonant bubbles is introduced as a potential mechanism and a scattering model is derived that incorporates the contribution from these together with that of rough surface scattering. The model results are fitted to Critical Sea Test (CST) measurements at a frequency of 940 Hz, treating the number of large bubbles, parameterized through the spectral slope of the size spectrum for bubbles whose radii exceed 1 mm, as a free parameter. This procedure illustrates that the CST data can be explained by scattering from a small number of large resonant bubbles, indicating that these provide an alternative mechanism to that of scattering from bubble clouds.

Model calculations of the underwater noise of breaking waves and comparison with experiment

A model for the underwater noise of whitecaps is presented and compared with the noise measured beneath plunging seawater laboratory waves. The noise from a few hundred hertz up to at least 80 kHz is assumed to be due to the pulses of sound radiated by bubbles formed within a breaking wave crest. The total noise level and its dependence on frequency are a function of bubble creation rate, bubble damping factor and an 'acoustical skin depth' associated with scattering and absorption by the bubble plume formed within the crest. Calculation of breaking wave noise is made using estimates of these factors, which are made independently of the noise itself. The results are in good agreement with wave noise measured in a laboratory flume when compensated for reverberation. A closed-form, analytical expression for the wave noise is presented, which shows a -11/6 power-law dependence of noise level on frequency, in good agreement with the -10/6 scaling law commonly observed in the open ocean.