The traditional Earth Observation landscape is changing dramatically in meteorology by the introduction of new satellite technology to measure winds. At the same time atmospheric circulation models are improving to describe phenomena in the 3D turbulence and moist convection regimes, potentially providing improved descriptions of vertical exchange processes, atmospheric dynamics, circulation and air-surface interaction processes. These developments in observation and modelling capabilities are timely, as cloud dynamics and processes are currently one of the World Climate Research Programme’s Grand Challenges.

Concerning progress in technology, the launch of the ESA core explorer Aeolus is the most unique and which brings long-awaited upper air weather information over the oceans, tropics, southern hemisphere and poles, including winds in upper air clouds and at cloud boundaries. In addition, dedicated new space technologies are being proposed using high-resolution Doppler cloud radar and tandem multi-angle imagers to capture cloud dynamics and processes. Moreover, cloud-resolving models (CRM) are being developed to simulate the satellite system measurements and perform trade-offs in their envisaged measurement capabilities.

Besides new explorers, also operational meteorological missions provide enhanced capability for studying atmospheric and cloud dynamics. The launch of MetOp-C will result in almost complete ocean wind coverage at 9:30 local time in the morning and evening, while Chinese and Indian wind scatterometers, planned for launch this year, will provide coverage at around 0:00, 6:00, 12:00 and 18:00 too, resulting in unprecedented ocean vector wind coverage in space and time. Scatterometers depict ocean wind shear, divergence and convergence associated with moist convection events and their temporal evolution. Finally, the resolution of geostationary imagers is being improved in space and time, providing enhanced views on cloud dynamics, gravity waves and motion in the cloudy atmosphere, both in nominal and in extreme weather conditions.

The presentation serves as introduction and overview of the session and briefly addresses or examplifies:
- Requirements for the study of cloud processes and atmospheric dynamics;
- Satellite wind instruments that support cloud and mesoscale dynamics studies;
- End-to-end simulation of cloud-measuring systems, using Cloud-Resolving Models;
- Science results on cloud processes and dynamics with satellite winds;
- Calibration of extreme satellite winds at the sea surface.


The figure shows extreme ASCAT and ECMWF (green) winds for typhoon Mangkhut (http://projects.knmi.nl/scatterometer/tile_prod/tile_app.cgi)

Atmospheric Motion Vectors (AMVs commonly extracted from imagers provide information of cloud motions at a single level in the troposphere. However, the vertical wind shear impacts the dynamic meteorology of the troposphere, and the assimilation of horizontal wind at different levels in NWP models is still a critical gap in user requirements. Such wind profiles may theoretically be estimated from temperature and humidity profiles retrieved from hyperspectral sounding, for which the height information is already available together with the retrievals. Time sequences of retrieved water vapour fields become in this approach the “imagery” for tracking winds and the corresponding AMV heights can be directly inferred from the profile heights information.

Recent studies involving 3D optical flow technics (Heas and Mémin, 2008) to extract AMVs from humidity fields have been realised by EUMETSAT and have clearly shown the potential benefits of these methods. The latest version of the algorithm has been adapted to ingest both Metop/IASI level 2 humidity, temperature and ozone fields, and can extract dense wind fields from a pair of IASI data taken consecutively by Metop A and Metop B satellites over high latitudes areas (poleward 45 deg). The use of ozone field allows the extraction of wind fields in the lower stratosphere when the humidity fields are the main tracer used for the troposphere. Although the results are still in preliminary stage, and that several rooms of improvement are under planning, a one month demonstration period has been produced (June 2017) to check the potential of these wind profile retrievals. Comparison of 3D IASI winds against forecast fields, and against winds retrieved by the algorithm from ECWMF forecast fields have been done to better understand the current performances and limitations of this technic, and to propose a way forward for future improvement.

This paper will present the method and the 3D IASI winds demonstrational product, highlighting the feasibility and the current limitations of wind profile extraction from IASI level 2 data, and discussing the short-term perspective at EUMETSAT.

Air-sea fluxes are greatly enhanced by the winds and wind structures generated by Mesoscale Convective Systems (MCSs). Rain-induced downbursts are extensive in the tropics and, with deep convection, result in near-surface turbulent winds that drive intense vertical exchanges of momentum, heat and moisture, in both the ocean and the atmosphere. Moreover, gust fronts produced by downbursts in MCSs promote cloud development, and triggering and self-aggregation of convection. This has an important impact on convective organization and ocean circulation, and affects the net Hadley circulation.
These important rain-induced dynamics are poorly represented by current parameterizations in global Numerical Weather Prediction models. However, they are resolved by space-borne scatterometers, while the heavy rain events themselves appear in the rain fields derived from the Meteosat Second Generation measurements of cloud physical properties.
In this study, the wind fields from the Advanced Scatterometers ASCAT-A and ASCAT-B onboard Metop-A and Metop-B, respectively, are collocated with short time series of Meteosat Second Generation rain fields, to identify and quantify correlations between wind and rain. The analysis shows that when there is extreme rain, there is extreme convergence/divergence in the vicinity. Cross-correlations calculated for each time step in the collocation yield response curves with a well-defined peak, showing a time lag of 30 minutes for extreme convergence, implying that extreme rain generally appears after (lags) extreme convergence. The overall conclusion is that the temporal scale of moist convection is determined by the slower updraft process. Results for wind divergence and wind stress are qualitatively similar, wind stress divergence showing the stronger response. This is probably due to its focus on high winds. It is also found that the extreme convergence/divergence occurs in well-defined spatial clusters. In on-going work we are extending our analysis to extremes in wind/wind stress curl, and identifying and tracking spatial clusters of extreme rain and the change in their correlation with wind events between the ASCAT-A and ASCAT-B passes.

Flow past a circular cylinder is a classic problem in fluid dynamics. In an unstratified fluid and non-rotating reference frame, the flow pattern in the wake is solely determined by the Reynolds number (Re). For Re > ~49, vortices with alternate senses of rotation are shed into the flow from alternate sides of the cylinder. As these contrarotating vortices are advected downstream, they form an upper row of negative vortices and a lower row of positive vortices in a staggered (anti-symmetric) configuration; a pattern universally referred to as the Kármán vortex street. Below a critical Reynolds number of ~190, vortex shedding is purely periodic and two dimensional. For ~190 < Re < ~260, however, vortex shedding transitions into a 3D regime as streamwise and transverse vorticity structures develop in addition to the primary spanwise Kármán vortices. At higher Re, the fine-scale three dimensionality of the wake becomes increasingly disordered, but periodic coherent vortex shedding is observed even at very large Re of O(10^6).

Vortex streets in laboratory flows were first studied by Bènard and Kármán a century ago. The earliest photographs were taken by Bènard during his investigations of vortex spacing. Kármán, in turn, was the first to apply linear stability theory to an infinite array of point vortices embedded in an inviscid fluid and showed that all configurations were unstable, except for one specific staggered arrangement with an aspect ratio (inter-vortex spacing in one row divided by the distance between vortex rows) of 0.28. Since Bènard and Kármán’s pioneering work, more than a thousand papers, and counting, have been published on the subject. Significant advances in the understanding of laboratory wake dynamics have been made thanks to sophisticated experiments, direct numerical simulations, and the introduction of new theoretical concepts such as absolute or convective instability and Hopf bifurcation.

In contrast, the atmospheric analogs of the classic Kármán vortex street developing in the wake of mountainous islands have attracted considerably less attention. Such vortex streets consist of mesoscale eddies in a well-mixed boundary layer capped by a temperature inversion and are often made visible by stratocumulus clouds acting as tracer. The observation and numerical modeling of these mesoscale structures is still challenging though. The few existing observational studies have all been limited to simply estimating the aspect ratio or the vortex shedding frequency from satellite imagery. The quantitative description of the wind field and dynamics of atmospheric vortex streets has been out of reach due to the lack of satellite wind retrievals of sufficiently high spatial resolution.

In the current work, we take advantage of newly developed high-resolution wind retrievals from MISR and ASCAT to investigate, for the first time, the flow field of three dozen atmospheric vortex streets in the lee of Jan Mayen, Madeira, Gran Canaria, Fogo, and Guadalupe islands. MISR and ASCAT horizontal winds are computed on a 4.40-km and 5.60-6.25-km grid, respectively, which is sufficient to capture the fine structure of the island wake and resolve individual vortices. Cloud-motion winds obtained at cloud-top level from multiangle MISR imagery are well complemented by ocean surface winds from ASCAT, allowing the description of the flow both at the top and the bottom of the vortex tubes spanning the boundary layer. The large-scale temperature and wind profiles of the unperturbed upstream flow are characterized by ERA-Interim reanalysis data.

We report on the magnitude and downstream evolution of spanwise vorticity and that of cross-street and along-street wind anomaly. We also discuss the effects of stratification (expressed by the Froude number) and rotation (expressed by the Rossby number) on atmospheric vortex streets, such as an observed asymmetry in the strength of cyclonic and anti-cyclonic vorticity. In addition to the unique high-resolution MISR and ASCAT wind data, we also use traditional MODIS and METEOSAT/GOES geostationary imagery to determine the geometric and temporal properties of vortex streets, such as aspect ratio, along-street and cross-street spacing, vortex shedding frequency and vortex advection speed. We show that these properties are not constant but systematically vary with downstream distance due to viscous diffusion and also depend on the structure of the boundary layer. We then contrast the satellite-observed properties of atmospheric vortex streets with theoretical and laboratory results obtained for the classic Kármán vortex street.

We have also performed WRF and ICON-LEM simulations at 1.0-2.5 km resolution of three particular atmospheric vortex streets, one each in the lee of Madeira, Gran Canaria, and Fogo. Validation of the simulated wind fields with satellite retrievals finds the models accurately capturing coherent vortex shedding with the right frequency, but producing vortices that dissipate faster and thus form vortex streets of reduced vorticity, width, and length compared to their natural counterparts.

ASCAT wind products at 25 km grid resolution are assimilated into the Integrated Forecasting System (IFS) at the European Centre for Medium-Range Weather Forecasts (ECMWF). An error of 1.5 m/s is given to the zonal (U) and meridional (V) wind components and the observations are thinned every 100 km in both the along and the across track direction. An alternative way to thinning is to reduce the spatial representativeness error by aggregating (averaging) wind observations up to those scales that are representative of those resolved by the NWP model. This process is called superobbing in data assimilation.

ASCAT superobbing research products were generated by averaging the nominal EUMETSAT ASCAT 25-km or 12.5-km grid product winds to lower resolution in an aggregation process. A 2-month test dataset of superobbing 50 km, 62.5 km and 100 km grid ASCAT products was generated and provided together with related bias correction estimation, quality control flags and situation-dependent observation/background errors estimation.

Assimilation experiments were run at ECMWF using the nominal and the new superobbing products with a final resolution that ranges from 12.5 km to 100 km. Results showed a reduced standard deviation for analysis and background departures, compared to the ones for the corresponding 12.5 km and 50 km thinned products. The averaging of small-scale variance in the superobbing product will explain (some of) the reduction in standard deviations of the departures. To account for this effect in the observations and for the effect of sample variation due to quality control differences, all experiments were also validated against a fixed resolution and sample set of ASCAT winds in order to obtain a metric for the optimum weight and aggregation of the ASCAT winds for achieving NWP impact.

The situation-dependent observation errors provided with the superobbing products were also tested, replacing the constant error value of 1.5 m/s as used in operations.

We describe a conically scanning spaceborne Dopplerized 94-GHz radar Earth science mission concept: Wind Velocity Radar Nephoscope (WIVERN) that has been proposed to ESA in response to the recent call for Earth Explorer missions. WIVERN aims to provide global measurements of in-cloud winds using the Doppler-shifted radar returns from hydrometeors. The conically scanning radar could provide wind data with daily revisits poleward of 50°, 50-km horizontal resolution, and approximately 1-km vertical resolution. The measured winds, when assimilated into weather forecasts and provided they are representative of the larger-scale mean flow, should lead to further improvements in the accuracy and effectiveness of forecasts of severe weather and better focusing of activities to limit damage and loss of life. It should also be possible to characterize the more variable winds associated with local convection. Polarization diversity would be used to enable high wind speeds to be unambiguously observed; analysis indicates that artifacts associated with polarization diversity are rare and can be identified. Winds should be measurable down to 1 km above the ocean surface and 2 km over land. The potential impact of the WIVERN winds on reducing forecast errors is estimated by comparison with the known positive impact of cloud motion and aircraft winds. The main thrust of WIVERN is observing in-cloud winds, but WIVERN should also provide global estimates of ice water content, cloud cover, and vertical distribution, continuing the data series started by CloudSat with the conical scan giving increased coverage. As with CloudSat, estimates of rainfall and snowfall rates should be possible. These nonwind products may also have a positive impact when assimilated into weather forecasts.