R&D to support volcanic ash dispersion forecsting for aviation
The HYSPLIT transport and dispersion model is run operationally by the NOAA National Weather Service (NWS) to provide forecast guidance to the NOAA-operated U.S. Volcanic Ash Advisory Centers (VAACs). For relatively large eruptions, the NWS issues a HYSPLIT output graphical product showing the location of ash in three altitude-layers with time, in addition to the forecaster-developed Volcanic Ash Advisory in text and graphical format (VAA/VAG). Operational NWS HYSPLIT runs use a unit source because mass eruption rates are not known in real time, and produce dispersion patterns with an ash cloud edge defined in a look-up table based on a qualitative relationship between unit source model output, the height of the eruption above the volcano summit, and the size of observed ash clouds for various historical eruptions. Operationally, the forecaster can set a different model output value to be the ash cloud edge based on the size of the observed ash cloud and forecaster knowledge and experience. The HYSPLIT model output product is a set of maps that show areas of ash based on the look-up table, not the model-calculated concentration relative to the unit source.
Quantitative forecasts are needed because parts of the aviation sector are moving from an ash avoidance approach to a risk-based approach. Quantitative output requires knowledge of source parameters such as the mass eruption rate and ash column height as a function of time. An empirical relationship in the literature between eruption column height and mass eruption rate has been used to produce quantitative output; however uncertainties in concentration using this approach are very large especially for weak plumes. More research is needed to determine if other methods will produce smaller uncertainties in a quantitative forecast and if so which method is best suited to which kind of eruption. We have longer-term plans to use satellite analyses, when available, and inverse modeling with HYSPLIT to better define the source term, which is expected to lead to better forecasts of the ash transport and dispersion.
The aviation community expects consistent VAAC products no matter which VAAC issues the products. To promote consistency in VAAC products we are developing a method to facilitate exchange of model outputs and model inputs among VAACs as well as suggesting standards for model output and estimation of eruption source parameters when they are not known. When comparing one VAAC's model output to another, the model inputs need to be as similar as possible to allow a meaningful comparison. Outputs also need to be produced in forms which can be directly compared and are useful to the forecaster. We envision VAACs may exchange information for eruptions in which more than one VAAC has an interest in the lead VAAC's products, for example during a very large eruption and/or when the ash is approaching another VAAC's area of responsibility and operational responsibility will soon be handed over to the adjacent VAAC. Operational information exchange during both real events and exercises should promote inter-VAAC collaboration and standardization of VAAC products.
The model evaluation database will consist of dispersion model source term information, satellite ash analyses, analyses meteorology, and evaluation statistics for several historical eruptions which will represent a range of eruptive scenarios. One statistic that compares the ‘footprints' of the model output and the observation is the Critical Success Index (CSI). For example, a preliminary analysis shows that when compared to one time period of satellite ash analyses of an eruption of Kasatochi, Kamchatka, Russia (2008), the HYSPLIT output, using a given meteorology and a given source term, has a CSI of 0.4. The model evaluation data base will aid dispersion modelers in testing model performance for different types of eruptions. It will also help provide users of the Volcanic Ash Advisory forecasts knowledge of the typical accuracy of the models that underlie the forecasts for large eruptions.
This R&D effort will be described, and some results will be shown.
This research is in response to requirements and funding by the Federal Aviation Administration (FAA).