Weather
Gunnar Noer explains and showcases the nature of polar lows.
Polar lows are small but fairly intense low pressure systems that form in the Arctic marine regions during the winter season. They form in unstable air masses associated with cold air outbreaks from the Arctic ice cap. Polar Lows give rise to gale or storm force winds which, in combination with heavy snowfall, cause widespread traffic disruptions. In recent years, polar lows have caused several fatal incidents with snow avalanches. This lecture focuses on the key processes and the methodology for forecasting polar lows.
Thomas Krennert (ZAMG) talks about the importance of moisture gradients in analysing the possibility of development of deep moist convection.
The exact predictability of convection in the Alpine region in the absence of fronts in weak-surface-pressure-gradient-situations during the warm season remains challenging for forecasters. The development into single-cell deep moist convection SC-DMC under these conditions depends on the availability of well-known ingredients like low level moisture, steep tropospheric lapse rates and sufficient lift. Satellite studies have shown that favourable locations for the first onset of SC-DMC resulting from widespread shallow convection over mountainous terrain are water vapour gradients in the middle or upper troposphere UTMG (upper tropospheric moisture gradients, Krennert, et al., 2003, https://doi.org/10.1016/S0169-8095(03)00067-X). The contributions of the respective ingredients related to UTMG supporting the initiation of DMC are discussed. A focus is set on moist symmetric instability MSI as a possible mechanism for favouring the transition from shallow to deep moist convection.
Wilfried Jacobs (DWD) explains the power of Airmass RGB in estimating the possibility of cold front transforming into a convective line.
Especially, the airmass RGB is a powerful tool for estimating the cold front’s tendency to transform to a convective line. Convective lines are connected with strong gusts, heavy precipitation sometimes with graupel or even hail. During this presentation the indications of convective lines will be outlined by considering the airmass RGB together with other means, e.g., radiosounding. Examples of two succeeding days will be discussed in detail whereas the first case did not lead to a convective line whereas the second example did. Typical differences of patterns in the corresponding airmass RGB and additional data sources will be related to a convective line’s probability.
Andreas Wirth (ZAMG) presents the benefits of analysing the water vapour imagery to diagnose synoptic structures and weather patterns.
Water vapor (WV) imagery is very useful when it comes to visualize zonal and meridional transport of air masses, but it is also suited to get a rapid overview on vertical transport processes. This characteristic makes WV-imagery extremely helpful when it comes to diagnose fronts and cyclogenesis.
The presentation will focus on the dynamics of cyclogenesis and fronts and how they are reflected in WV-imagery. The concept of relative streams will be introduced in the context of cyclogenesis, ana- and kata fronts.
Meteorological products based on WV absorption bands such as the Total Precipitable Water product will be introduced and their usefulness will be demonstrated on case examples.
Ralph Petersen (University of Wisconsin) talks about forecasting satellite-derived moisture using all available observation data on moisture and Lagrangian methods to give forecasters more information on the possibility of storm formation.
The CIMSS Lagrangian NearCasts system 1) expands the utility of clear-air sounding and products related to the pre-convective environment (from MTG-IRS Sounder and MTG-Imager) into the 1-9 hours period before storm formation and 2) now combines Geostationary Infrared Products with less frequent microwave products from multiple Polar Orbiting systems to fill information gaps in cloudy areas. For a heavy precipitation event, quantitative measures of both retrieval and short-range forecast accuracy are provided, including a new, non-uniform bias correction approach, and explorations of predictive clear-sky RGBs.
Including “all-weather”, real-time MiRS retrievals not only provide a more visually pleasing product (improving coverage by 30-40%), but also exposes forecasters to here-to-fore underutilized POES observations over land.
Validation against hourly surface-based GPS-TPW observations testify to the ability of the Satellite-based products to capture observed small-scale moisture features properly. Results show error growth rates < 1%/hour (without initial shocks) and support applying non-uniform bias corrections derived over 5mm bands to assure realistic TPW distributions.
Because RGB presentations are more popular than quantitative retrieval product displays the parcel-based Lagrangian NearCast approach was also used to “predict” clear-air RGBs based on the projection of radiance, with quantitative values overlaid. Examples of initial tests will be shown.
Christo Georgiev (NIMH) presents influence of synoptic and mesoscale dynamical process, through water vapour imagery, on the intensification of wildfires.
Studies of large fires in Australia, Tasmania, USA and Southeastern Europe have shown a link between fire activity and dry stratospheric intrusions with downward transport of energy and momentum down to the lower troposphere. In such cases the rapid increase of fire activity was accompanied by a dry band in satellite Water Vapour (WV) imagery, which is indicative of pronounced descent of upper-tropospheric or stratospheric air which is normally much dryer than the lower tropospheric one and has much higher potential vorticity. When these descending air masses, reach the Earth’s surface they affect the wind speed, moisture and ozone concentration, which could generate a spike in fire activity.
Case-study examples of such severe events over areas of Central and Eastern Mediterranean are considered in the presentation. Combination of WV imagery, satellite soundings and satellite estimations of fire radiative energy contribute to better understanding of dry stratospheric intrusion depth and related dynamical processes, which govern the intensification of wildfire occurrence and spread.
Christo Georgiev (NIMH) discusses the effect of vorticity on tropical cyclones and development of severe thunderstorms, through the scope of water vapour imagery.
Water vapour (WV) images provide useful information to anticipate the effects of upper-level dynamic environment on the intensity of a tropical cyclone (TC) as well as on its extratropical transition (ET), a gradual process in which a TC loses tropical characteristics and becomes more extratropical in nature. Analyses of WV imagery shows that during ET stage over subtropical areas, advection of potential vorticty anomaly can influence the ET and control the process.
Large amounts of water vapour, originated by TCs may be involved in transfer of moisture by planetary waves, associated with ET developments and give rise to a narrow regions of strong meridional water vapour flux. The presentation shows WV imagery synoptic-scale analysis, which shows supply with additional large amount of moisture of an existing already favourable convective environment. The
combination lead into an environment with very high CAPE and potential to accelerate upward, acquiring kinetic energy and forming strong vertical motions in a deep tropospheric layer. The process is illustrated by a case of catastrophic hailstorm over Bulgaria, Eastern Mediterranean.
Nataša Strelec-Mahović (EUMETSAT) shows and analyses several cases of water vapour structures in the atmosphere using satellite imagery.
Features occurring in the WV6.2μm images indicate the processes going on in the atmosphere, since these dark (dry) and white (moist) structures result from a combination of vertical motion, moisture advection and horizontal deformation within the mid and upper troposphere. Frequently, water vapour images reveal structures such as dark stripes, mountain wave signatures, or circular or spiral formations known as water vapour eyes, eddies or vortices. We will look at how these features can be used as tracers of three-dimensional atmospheric motions on the meso- and synoptic scale and how they relate to e.g. jet-streams, upper-level lows or turbulence.
Christian Herold (DWD) investigates the usefulness of IASI L2 products in nowcasting, detecting and analyzing sting jets in rapid cyclogenesis.
Strong winds southwest of the center of a Shapiro-Keyser-Cyclone are often associated with a cold conveyor jet or a sting jet. The sting jet is a strong mesoscale flow with a very high damage potential. It is a massive challenge for NWP and forecasters to correctly predict a sting jet. The question is, can IASI profiles help forecasters for a better prediction of such mesoscale severe wind events connected with sting jets? In this specific context, some case studies will be presented.
Zsofia Kocsis (OMSZ) talked about the reasons behind the experiment of merging SYNOP data with the IASI L2 profiles.
Using IASI L2 profiles, different instability indices (e.g. Lifted Index, CAPE) and water vapour content in different layers can be determined, which provides information on the convective environment. These indices were studied in several convective cases which led us to try to merge the IASI profiles and Synop measurements. In this presentation we present the reasons why we choose to combine these different measurements and we also present some of our experiences with the blended IASI product.
Phil Watts and Loredana Spezzi (EUMETSAT) give an overview of the possibilities of water-vapour retrieval of the future EPS-SG satellites.
The METimage instrument on board of the future EPS-SG satellite series opens a new era for the retrieval of water wapour imaging products at EUMETSAT. Equipped with 20 channels covering the spectral range 0.4-13μm and including both Near infrared and thermal infrared water vapour absorption bands (0.9, 6.7 and 7.3μm), METimage will allow the retrieval of operational water vapour imagery products with unprecendented resolution (down to 500m) from a low-Eart orbit. These products are expected to greatly enhance the EUMETSAT service in support of numerical weather prediction, nowcasting and climate monitoring.
In this contribution, we report the retrieval approach chosen for METimage water vapour imagery products and demonstrate it using MODIS measurements. We focus in particular on the uncertainty and quality assessment planned for these products. We aim at gathering the fundamental user feedback to achieve the state-of-the-art retrieval and validation approach, and to foster cooperation and exchange for the analysis and characterization of these products once they become available. In particular, we would like to discuss key points that have an impact on the use of our products in forecasting applications (such as the use of prior forecast information in the retrieval, the product spatial resolution and content (e.g., uncertainties, quality indicators, diagnostic parameters, etc., and the advantage of combining NIR/IR measurements, not available from previous EUMETSAT missions).
Sheldon J. Kusselson (CIRA) shows us the Advected Layered Precipitable Water (ALPW) and Total Precipitable Water (TPW) products and how to apply them for forecasting hazardous events.
Water vapor availability is a key factor in forecasting heavy precipitation. Passive microwave instruments onboard several polar orbiting spacecraft measure emission from water vapor and provide a capability to sense layers of and total water vapor in the atmosphere. Via fusion of passive microwave retrievals from five or more polar orbiting spacecraft and model wind fields, the Advected Layered Precipitable Water (ALPW) and Total Precipitable Water (TPW) products are created hourly. The new ALPW product and upgraded Total Precipitable Water products, including a percent of normal TPW product, will become operational for forecasters in 2023. In the meantime, CIRA already produces the products hourly on the internet. During the past decade applications for the ALPW and TPW products have been developed for many different types of weather events, with a focus on heavy precipitation. This presentation will highlight applications for various weather hazards such as heavy rain causing flooding and heavy snow. Case studies using applications for the ALPW and TPW products in different parts of the world, including Europe, will be presented.