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11th Warnemunde Turbulence Days on Linking Ocean Mixing and Circulation at Various Scales What: Forty-five participants from 10 countries met to discuss contemporary issues of marine turbulence with a focus on the linkage between mixing and overturning circulation on all scales (https://www.io-warnemuende.de/wtd-2023.html). When: 17-20 September 2023 Where: Rostock, Germany KEYWORDS: Eddies; Inertia-gravity waves; Meridional overturning circulation; Mixing; Ocean circulation; Turbulence, 1. Introduction Walter Munk, in his famous abyssal recipes, showed more than half a century ago that the strength of the global overturning circulation is closely linked to diapycnal mixing. [...]

QUESTION: Coral reefs throughout the world are threatened by increasing sea temperatures, with recent studies indicating as much as a 14% decline in the global reef population. How can gravity [...]

It is known that gravity waves in the troposphere, which are often excited by preexisting convection, can favor or suppress the formation of new convection. Here it is shown that in the presence of wind shear or barotropic wind, the gravity waves can create a more favorable environment on one side of preexisting convection than the other side. Both the nonlinear and linear analytic models developed here show that the greatest difference in favorability between the two sides is created by jet shears, and little or no difference in favorability is created by wind profiles with shear at low levels and no shear in the upper troposphere. A nonzero barotropic wind (or, equivalently, a propagating heat source) is shown to also affect the favorability on each side of the preexisting convection. It is shown that these main features are captured by linear theory, and advection by the background wind is the main physical mechanism at work. These processes should play an important role in the organization of wave trains of convective systems (i.e., convectively coupled waves); if one side of preexisting convection is repeatedly more favorable in a particular background wind shear, then this should determine the preferred propagation direction of convectively coupled waves in this wind shear. In addition, these processes are also relevant to individual convective systems: it is shown that a barotropic wind can lead to near-resonant forcing that amplifies the strength of upstream gravity waves, which are known to trigger new convective cells within a single convective system. The barotropic wind is also important in confining the upstream waves to the vicinity of the source, which can help ensure that any new convective cells triggered by the upstream waves are able to merge with the convective system. Ali of these effects are captured in a two-dimensional model that is further simplified by including only the first two vertical baroclinic modes. Numerical results are shown with a nonlinear model, and linear theory results are in good agreement with the nonlinear model for most cases.

Observations made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board NASA's Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite have been processed using Salby's fast Fourier synoptic mapping (FFSM) algorithm. The mapped data provide a first synoptic look at the mean structure and traveling waves of the mesosphere and lower thermosphere (MLT) since the launch of the TIMED satellite in December 2001. The results show the presence of various wave modes in the MLT, which reach largest amplitude above the mesopause and include Kelvin and Rossby-gravity waves, eastward-propagating diurnal oscillations ('non-sun-synchronous tides'), and a set of quasi-normal modes associated with the so-called 2-day wave. The latter exhibits marked seasonal variability, attaining large amplitudes during the solstices and all but disappearing at the equinoxes. SABER data also show a strong quasi-stationary Rossby wave signal throughout the middle atmosphere of the winter hemisphere; the signal extends into the Tropics and even into the summer hemisphere in the MLT, suggesting ducting by westerly background zonal winds. At certain times of the year, the 5-day Rossby normal mode and the 4-day wave associated with instability of the polar night jet are also prominent in SABER data.

This study examines the effect of combining equatorial planetary wave drag and gravity wave drag in a one-dimensional zonal mean model of the quasi-biennial oscillation (QBO). Several different combinations of planetary wave and gravity wave drag schemes are considered in the investigations, with the aim being to assess which aspects of the different schemes affect the nature of the modeled QBO. Results show that it is possible to generate a realistic-looking QBO with various combinations of drag from the two types of waves, but there are some constraints on the wave input spectra and amplitudes. For example, if the phase speeds of the gravity waves in the input spectrum are large relative to those of the equatorial planetary waves, critical level absorption of the equatorial planetary waves may occur. The resulting mean-wind oscillation, in that case, is driven almost exclusively by the gravity wave drag, with only a small contribution from the planetary waves at low levels. With an appropriate choice of wave input parameters, it is possible to obtain a QBO with a realistic period and to which both types of waves contribute. This is the regime in which the terrestrial QBO appears to reside. There may also be constraints on the initial strength of the wind shear, and these are similar to the constraints that apply when gravity wave drag is used without any planetary wave drag. In recent years, it has been observed that, in order to simulate the QBO accurately, general circulation models require parameterized gravity wave drag, in addition to the drag from resolved planetary-scale waves, and that even if the planetary wave amplitudes are incorrect, the gravity wave drag can be adjusted to compensate. This study provides a basis for knowing that such a compensation is possible.

The effects of surface gravity waves on pore-water release from permeable sediment (k = 1.3-1.8 x [10.sup.-11] [m.sup.2]) in shallow water were studied in a wave tank. Our tracer experiments demonstrated that shallow-water waves can increase fluid exchange between sandy sediment and overlying water 50-fold, relative to the exchange by molecular diffusion. The main driving force for this increased exchange are the pressure gradients generated by the interaction of oscillating boundary flows and sediment wave ripples. These gradients produce a pore-water flow field, with a regular pattern of intrusion and release zones, that migrates with ripple propagation. The ensuing topography-related filtering rates in the wave tank ranged from 60 to 590 L [m.sup.-2] [d.sup.-1] and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L [m.sup.-2] [d.sup.-1]) and initial molecular diffusion (corresponding to 10-12 L [m.sup.-2] [d.sup.-1]). Wave-induced filtration is ecologically relevant because permeable sandy sediments are very abundant on the continental margins and can be converted into effective filter systems, which suggests that these sediments are sites for rapid mineralization and recycling. We propose that the wave influenced continental shelf may be subdivided into two zones: a shallow zone (water depth < wavelength/2), where wave orbital motion at the sea floor creates ripples and causes topography related advective filtering; and a deeper zone (wavelength/2 < water depth < wavelength), where wave pumping enhances interfacial exchange by hydrostatic pressure oscillations.

A simple ray-tracing model for the equatorward propagation of inertia--gravity waves in the lower stratosphere is investigated. The model is based on a zonally symmetric wave source and incorporates radiative wave damping. It is shown that steady extratropical wave sources are able to produce spectra of potential energy that exhibit a conspicuous peak at the equator, which resembles the spectral peaks that are often observed in field data. This reinforces the recent suggestion by other authors that such peaks are caused by the latitudinal variation of the local Coriolis parameter, though these authors did not consider horizontal wave propagation. Notably, horizontal wave propagation can produce equatorial peaks even with strictly extratropical wave sources. Wave source intermittency is then investigated by allowing for a time-dependent wave source. The source is treated statistically as a stationary random process and a number of general comments are made with respect to the impact of intermittency on gravity wave parameterizations in general circulation models and on the expected variance of observational estimates of energy spectra. Finally, the possibility of an observational bias toward waves with lower group velocities (as recently suggested by other authors) is examined using a simple example of topographic wave generation by time-varying surface winds.

An investigation of the generation of turbulence above deep convection is presented. This investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near Dickinson, North Dakota, on 10 July 1997. Very high-resolution two- and three-dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter. These simulations explicitly resolve the convection and the turbulence-causing instabilities. The configurations of the models are consistent with the meteorological conditions surrounding the event. The turbulence generated in the numerical simulations can be placed into two general categories. The first category includes turbulence that remains local to the cloud top, and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves. In both the two- and three-dimensional calculations, the local turbulence develops rapidly and occupies a layer about 1 km deep above the top of convective updrafts after their initial overshoot into the stratosphere. This local turbulence is generated by the highly nonlinear interactions between the overshooting convective updrafts and the tropopause. Gravity wave breakdown is only present in the two-dimensional calculation and occurs in a layer about 3 km deep and 30 km long. This gravity wave breakdown is attributed to an interaction between the gravity waves and a critical level induced by the background wind shear and cloud-induced wind perturbations above cloud top.

Three idealized simulations of thunderstorm outflows are presented. Each outflow is a response to an instantaneous low-level cooling. The vertical structures of the coolings differ as do the environments in which the outflows form, and consequently the dynamics of the outflows differ. One outflow is a gravity current, another is a gravity wave, and the third comprises both a gravity current and a gravity wave. The horizontal transport of mass is important for the advance of the gravity-current outflow, but not for the gravity wave outflow, and it is suggested that this is the defining dynamical distinction between the two outflows. The simulations are compared to observations and it is suggested that some outflows previously characterized as gravity currents may better fit the gravity wave or gravity current/wave archetypes. It is also noted that the gravity wave component of an outflow may be generated directly by low-level cooling in addition to the commonly suggested mechanism of the interaction of a gravity current with a stable layer.

A parameterization scheme of gravity wave drag induced by cumulus convection (GWDC) is implemented in the Yonsei University atmospheric general circulation model (GCM) and the effects of GWDC on the zonalmean flow and planetary waves are investigated through perpetual July simulations. The GWDC parameterization scheme used in this study includes a momentum gain in the cloud region to conserve the momentum. The gravity wave stress at the cloud top is concentrated in the intertropical convergence zone (ITCZ) with its maximum value of 0.14 N [m.sup.-2] near the tropopause due to deep cumulus clouds. The wave breaking occurs mainly in the upper troposphere and lower stratosphere. The maximum westerly acceleration in the ITCZ is 0.6 m [s.sup.-1] [day.sup.-1], which is close to that observed. It is surprising to observe that the zonal wind difference between the simulations with and without the GWDC parameterization is largest in the Southern Hemisphere (SH) midlatitude stratosphere, where a westerly jet exists, rather than in the major drag forcing region and that there is an associated warming in the SH polar stratosphere. The excessive westerly jet in the SH that appears in the simulation without the GWDC parameterization is alleviated significantly (7 m [s.sup.-1]) by its inclusion. This result implies that the nonlinear process through planetary waves rather than by direct drag forcing might play an important role in changing the zonal-mean flow. The analysis of the geopotential height perturbation reveals that the amplification of the waves of zonal wavenumbers 1 and 2 in the SH stratosphere is responsible for the change in the zonal-mean flow there. In particular, the wave amplitude of zonal wavenumber 2 significantly increases (more than three times) by the GWDC process in the SH midlatitude upper stratosphere. It is suggested that understanding interactions between the gravity wave drag, zonal-mean flow, and planetary waves is necessary to better parameterize the gravity wave drag. This study is particularly encouraging in that including the GWDC parameterization can alleviate the excessive westerly bias in the SH midlatitude and its associated cold temperature bias in the SH polar region reported for many GCMs.

This study illustrates opportunities for much improved orographic quantitative precipitation forecasting, determination of orographic cloud seedability, and flash flood prediction through state-of-the-art remote sensing and numerical modeling of gravity wave clouds. Wintertime field observations with multiple remote sensors, corroborated in this and related papers with a mesoscale--cloud scale numerical simulation, confirm that storm-embedded gravity waves can have a strong and persistent influence on orographic cloud liquid water (CLW) and precipitation. Where parallel mountain ridges dominate the landscape, an upwind ridge can force the wave action, and a downwind ridge can receive the precipitation. The 1995 Arizona Program was conducted in such terrain. In the scenario examined, traveling waves cyclically caused prefrontal cross-barrier winds that produced gravity waves. Significant cloud bands associated with the waves carried substantial moisture to the area. With the passage and waning of the cloud bands, vapor influxes (precipitable water [P.sub.w]) cycled through large changes in magnitude, and prefrontal peaks in [P.sub.w] coincided with the gravity waves in a succession of episodes during a five-day period. Thus, the cyclic trend in [P.sub.w] and the magnitudes of peak [P.sub.w] were simple indicators of wave cloud development. The first two cycles, with minor peak [P.sub.w], were precursors. Significant wave clouds first appeared during the second episode. During the final two episodes with large vapor influxes, very deep, precipitating wave clouds were coupled with underlying clouds formed in flow up the mountain slopes to create the prefrontal storms. Rain fell on an existing snowpack on the main recipient ridge and, in the end, produced rapid runoff and flash flooding. The gravity waves persistently condensed CLW that averaged 0.5 mm and reached 1.0 mm in the first of the main storm episodes, and averaged 1.0 mm and reached 2.0 mm and more in the second (column-integrated values). These values equaled or exceeded the larger of those represented in liquid water climate datasets for orographic cloud systems in other locations in the West, where only the upslope and not the wave component had been examined. The effect of shifts between cross-barrier and barrier-parallel flows was reflected in abrupt buildups and declines in wave CLW, but the gravity wave clouds persisted for a total of 22 h during the two storm periods. In the wave updrafts, the condensation rate regularly exceeded the consumption rate by ice, even though ice was usually present. Conversion to ice consumed and precipitated wave CLW. Pulses of available [P.sub.w] and wave CLW on a 2- to 4-h timescale, cyclically followed by partial glaciation, produced the precipitation from the wave clouds. Their seeder effect on the upslope feeder clouds was to enhance the total precipitation from the coupled system. Estimates of the liquid water fluxes in comparison with the precipitation rates suggest precipitation efficiencies in the 11%-33% range from the seeder-feeder couplets. The periods of gravity wave forcing contributed some 80% or more of the total precipitation, and trailing fronts produced the remainder. Several factors derived from the observed availability of CLW determine the potential for precipitation enhancement by seeding wave clouds; these are enumerated. Given demands for improved water supply, the challenge often presented in mountain watersheds of separating seeding opportunities from potential flash flood situations is examined. The results here show that storms that could threaten flash floods can be readily identified by continuous monitoring with polarization radar and in real-time simulations as those with the altitude of the melting level above the elevation of the highest terrain with existing snowpack. In the sense that orographically generated gravity waves will significantly influence cloud water and precipitation, geographic transferability of the results is indicated by the existence of wave-generating and precipitation-generating parallel ridges in many places throughout the world. The quantitative effects will, of course, depend on particulars of the locale such as nature of the prevalent forcing, available moisture, and physical stature of the ridges.