Russel P. Morison

Investigation of the physical scaling of sea spray spume droplet production

[1] In this paper we report on a laboratory study, the Spray Production and Dynamics Experiment (SPANDEX), conducted at the University of New South Wales Water Research Laboratory in Australia. The goals of SPANDEX were to illuminate physical aspects of spume droplet production and dispersion; verify theoretical simplifications used to estimate the source function from ambient droplet concentration measurements; and examine the relationship between the implied source strength and forcing parameters such as wind speed, surface turbulent stress, and wave properties. Observations of droplet profiles give reasonable confirmation of the basic power law profile relationship that is commonly used to relate droplet concentrations to the surface source strength. This essentially confirms that, even in a wind tunnel, there is a near balance between droplet production and removal by gravitational settling. The observations also indicate considerable droplet mass may be present for sizes larger than 1.5 mm diameter. Phase Doppler Anemometry observations revealed significant mean horizontal and vertical slip velocities that were larger closer to the surface. The magnitude seems too large to be an acceleration time scale effect. Scaling of the droplet production surface source strength proved to be difficult. The wind speed forcing varied only 23% and the stress increased a factor of 2.2. Yet, the source strength increased by about a factor of 7. We related this to an estimate of surface wave energy flux through calculations of the standard deviation of small-scale water surface disturbance, a wave-stress parameterization, and numerical wave model simulations. This energy index only increased by a factor of 2.3 with the wind forcing. Nonetheless, a graph of spray mass surface flux versus surface disturbance energy is quasi-linear with a substantial threshold.

Improved Hurricane Track Forecasting from the Continuous Assimilation of High Quality Satellite Wind Data

Abstract Despite recent improvements in the accuracy of hurricane track forecasts, mean position errors still remain unacceptably large. For example, recurvature is captured poorly by forecast models and can produce excessively large position errors. This study addresses the problem of hurricane track forecasting in three ways. First, the initial conditions for the forecast model are augmented by a dense coverage of high spatial and temporal resolution satellite-derived wind vectors. Second, to gauge the extent to which this additional four-dimensional detail of the atmospheric structure can be exploited, three distinct types of data assimilation methods are examined. These are 1) conventional (intermittent, cycled) 6-h assimilation, 2) nudging over a 12- or 24-h period up to the initial time, and 3) recently developed barotropic and four-dimensional variational assimilation schemes, also over a 12- or 24-h period. The nudging and variational methods are continuous assimilation procedures and incorporate ...

An overview of sea state conditions and air-sea fluxes during RaDyO

[1] Refining radiative-transfer modeling capabilities for light transmission through the sea surface requires a more detailed prescription of the sea surface roughness beyond the probability density function of the sea surface slope field. To meet this need, exciting new measurement methodologies now provide the opportunity to enhance present knowledge of sea surface roughness, especially at the microscale. In this context, two intensive field experiments using R/P Floating Instrument Platform were staged within the Office of Naval Research’s Radiance in a Dynamic Ocean (RaDyO) field program in the Santa Barbara Channel and in the central Pacific Ocean south of Hawaii. As part of this program, our team gathered and analyzed a comprehensive suite of sea surface roughness measurements designed to provide optimal coverage of fundamental optical distortion processes associated with the air-sea interface. This contribution describes the ensemble of instrumentation deployed. It provides a detailed documentation of the ambient environmental conditions that prevailed during the RaDyO field experiments. It also highlights exciting new sea surface roughness measurement capabilities that underpin a number of the scientific advances resulting from the RaDyO program. For instance, a new polarimetric imaging camera highlights the complex interplay of wind and surface currents in shaping the roughness of the sea surface that suggests the traditional Cox-Munk framework is not sufficient. In addition, the breaking crest length spectral density derived from visible and infrared imagery is shown to be modulated by the development of the wavefield (wave age) and alignment of wind and surface currents at the intermediate (dominant) scale of wave breaking.

Recent developments in the continuous assimilation of satellite wind data for tropical cyclone track forecasting

Abstract Tropical cyclone track forecasting remains a vexing problem. Here, we have examined nine forecasts in the Australian Region which were generally deemed difficult. The forecast problem has been addressed in three ways. First, modelling and data assimilation have been performed at very high (15 km) resolution. Second, a newly developed data source, namely, high spatial and temporal resolution cloud drift winds, has been used to augment the often quite poor data base upon which forecasts are based and, in one case, these winds have been augmented with scatterometer-based surface winds. Finally, a range of continuous assimilation techniques, including recently developed 4-D variational assimilation and hourly nudging have been tested. In these cases, where often, conventional forecast guidance and CLIPER (Morison, 1993) were poor, continuous assimilation procedures, i.e. 1-hourly nudging and 4-D variational assimilation, took best advantage of the high resolution winds and provided much improved forecasts. Worthy of note was, for the case examined, scatterometer winds were able to reduce the initial position error of the cyclone. Overall, it would appear continuous assimilation, combined with a substantial high resolution data base and high resolution modelling have the potential to greatly improve the accuracy of tropical cyclone track forecasts.

On the Variation of the Effective Breaking Strength in Oceanic Sea Statesa

AbstractA spectral framework for quantifying the geometric/kinematic and dynamic/energetic properties of breaking ocean waves was proposed by Phillips in 1985. Phillips assumed a constant breaking strength coefficient to link the kinematic/geometric breaking crest properties to the associated excess energy and momentum fluxes from the waves to the upper ocean. However, a scale-dependent (spectral) breaking strength coefficient is needed, but is unavailable from measurements. In this paper, the feasibility of a parametric mean effective breaking strength coefficient valid for a wide range of sea states is investigated. All available ocean breaking wave datasets were analyzed and complemented with wave model behavior. Robust evidence is found supporting a single linear parameter relationship between the effective breaking strength and wave age or significant wave steepness. Envisaged applications for the effective breaking strength are described.

Turbulent Flow over Steep Steady and Unsteady Waves under Strong Wind Forcing

AbstractTurbulent flow over strongly forced steep steady and unsteady waves is simulated using large-eddy simulation (LES) with time t and space x varying wave height h(x, t) imposed as a lower boundary condition. With steady waves, h(x, t) is based on measurements of incipient and active breaking waves collected in a wind-wave flume, while a numerical wave code is used to generate an unsteady evolving wave packet (group). Highly intermittent airflow separation is found in the simulations, and the results suggest separation near a wave crest occurs prior to the onset of wave breaking. The form (pressure) drag is most sensitive to the wave slope, and the form drag can contribute as much as 74% to the total stress. Wind and scalar profiles from the LES display log-linear variations above the wave surface; the LES wind profiles are in good agreement with the measurements. The momentum roughness increases as the water surface changes from wind ripples to incipient breaking to active breaking. However, the sca...

Recent Progress on Understanding and Modelling Ocean Wave Breaking

This presentation highlights recent scientific progress towards a deeper understanding of wave breaking. For this familiar sea surface process, a basic understanding of the mechanisms determining the onset and strength of breaking events has remained elusive. Recent numerical studies of the role of nonlinear wave group dynamics in breaking onset of unidirectional wave groups found that breaking onset was a threshold process, where the threshold reflected the convergence rate of wave energy flowing to the tallest wave from the other waves in the group. It was proposed that this convergence rate should also underpin the strength of breaking events.

Atmospheric Modelling and Prediction

This chapter is intended to provide the reader with an introduction to the development and application of models of the Earth’s atmospheric flow. It describes a range of past and present methods for solving the equations governing the behaviour of the Earth’s atmosphere. These equations can be cast in predictive form and their solution therefore provides forecasts of the future state of the atmosphere. The governing equations are mathematical expressions of the various conservation laws governing the atmosphere, as a fluid in motion. The equations also allow for the effects of a wide range of forcing mechanisms. Here we mention but a few, such as the differential heating of the sun’s radiation between the equator and the poles, dissipation of momentum, heat and moisture at the Earth’s surface and boundary layer, turbulence in the free atmosphere, the drag on the atmosphere from steep, high or rough terrain, and the profound influence of Earth’s rotation. The complexity of the interaction between the Earth’s atmosphere and the other components of the Earth’s environmental system are illustrated schematically in Fig. 3.1, which shows the coupled atmosphere-ocean-landsurface model, which is the focus of much of this book. Each of the components represented in Fig. 3.1 is a model in its own right and interacts with the atmospheric model in two main ways.