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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Lightning applications in hydrology and extreme weather monitoring

Themis Chronis (Hellenic Center for Marine Research)

Lightning represents a robust and unique diagnostic tool; these attributes emanate from the fact that lightning is a unique proxy to cloud microphysics, severe weather and at the same time can be monitored at high accuracy and frequency, on a global basis. Its presence guarantees certain ongoing microphysical and dynamical processes; from a microphysical point of view, cloud electrification expressed via the Cloud-to-Cloud or Cloud-to-Ground discharges signifies the existence of an electrostatically charged cloud and a combination of various solid [ice, graupel, and hail] and super-cooled aqueous phases [Latham, 1981]. Lightning is also a conditional counterpart to precipitation. The electrical activity in precipitating clouds has a direct relation to the thermodynamic variables, which further control condensation.
Severe/extreme weather monitoring and forecasting
Recent upgrades in the data assimilation and numerical weather prediction have significantly improved flash-flood forecasts and the severe weather alert status. Unlike most variables used in assimilation (e.g. brightness temperatures or ground data) lightning assimilation can be applied on a real-time basis and can be as frequent in space and time as the computational needs of the numerical model. Papadopoulos et al., [2005] has documented a direct assimilation algorithm that has employed lightning ground-based observations over Europe and Africa. The main idea behind this implementation is that the observed lightning flash rates are used to generate moisture profiles that are closer to the real state of the atmosphere, forcing the model to re-distribute the precipitation amount.
Recent observations over the African continent have revealed that lightning is also a good indicator of extreme weather dynamics; Chronis et al. [2007] has documented that several tropical storms formed off the West African coast showed good agreement between their embedded electrical activity and intensification stages. The Zeus network was the first network to make dedicated and continuous lightning observations over Africa. are used to generate moisture profiles that are closer to the real state of the atmosphere, forcing the model to re-distribute the precipitation amount.
Continuous, global and high temporal resolution precipitation estimates have significantly advanced since the first geo-synchronous (Infra Red-IR) satellites were launched. Due to current technological limitations, geo-synchronous satellites offer worldwide continuous coverage but lack the direct insight of the atmospheric column physical parameterization.
The passive and active microwave sensors have unequivocally improved the inherited weaknesses of the geo-synchronous spaced-based observations. Passive and active microwave radiometers onboard the Low Earth Orbiters provide the direct microphysical dimension needed in any precipitation retrieval algorithm, although lack the continuous global coverage. Most of the precipitation retrieval algorithms combine all available (visible to microwave) frequencies and encompass the so-called "merged" precipitation products [Levizzani, 2003]. From an observational perspective, lightning has the ability to abridge the advantage trade-off between the geo and sun-synchronous satellites; the monitoring is global, continuous and the relayed information has direct physical significance to atmospheric processes. Although lightning observations show high co-variability with passive microwave observations, their combinatory use in precipitation retrieval is far from redundant [Boccippio, 1995]. The major advantage that lightning has to offer is the delineation of the convective and stratiform precipitating areas within the IR/microwave outlined cluster. Recent studies show that lightning-based precipitation retrieval algorithms show improved performance in terms of reducing the bias especially at high rain-rates [Chronis et al., 2004].