Objectives
ACES addresses three primary science objectives:
1.) Lightning Imaging Sensor (LIS) Validation
2.) Lightning-Storm Relationships
3.) Storm Electrical Budget
Overview
The validation effort will provide detailed characterization of lightning type, cloud-top optical energy, and power statistics that is needed to better interpret the global lightning database collected by LIS.
The ALTUS electrical measurements and ancillary ground-based measurements, from the extensive electrical and meteorological observing systems already in place at KSC, will provide detailed information on cloud properties throughout the thunderstorm life cycle. The relationships between storm electrical and kinematic properties is of particular interest as they might be used to discriminate severe from non-severe storms. How mesoscale boundaries (e.g., land/ocean) affect the development and evolution of these properties will also be explored.
Finally, ACES electrical measurements will enable us to uniquely address important questions about the electrical budget of thunderstorms, the global electric circuit, and the electrodynamic interaction with the upper atmosphere.
Lightning Imaging Sensor (LIS) Validation
The UAV measurements provide an uninterrupted depiction of the storm growth and decay life cycle-from the very first indication of electrification to the first lightning through thunderstorm dissipation. The timing of the initial electrification and the complete documentation of total lightning activity provide important validation data for newly developed three-dimensional storm electrification models with explicit (and detailed) microphysics (Mansell, 2000). Such models make explicit forward predictions of the co-evolving microphysics, kinematics, and the total flash rate partitioned into in-cloud and Cloud to-Ground (CG) lightning components, including flash polarity. The ability of the UAV to stay aloft for an extended period also offers an opportunity to observe convective storms as they transition from multi-cellular storms into organized mesoscale convective weather systems having well defined convective and stratiform precipitation regions and electrical coupling to the upper atmosphere
The combined UAV and ground-based observations will provide a necessary measurement set that yields more physically realistic cloud models, which in turn can be expected to benefit the forthcoming higher-resolution research and operational mesoscale forecast models. The connection of the measurements to the enabled science and thus to NASA's science themes, depicted in the figure below, is clear.
Storms are the fundamental elements of the global water and energy cycle and the agents of severe weather, flash floods, and wild fire initiation. The unique set of observations afforded by the ability of the UAV to continuously observe storms for extended periods of time will improve our understanding of the process physics and lead to improved models of individual thunderstorms and convective weather systems.
Lightning-Storm Relationships
Both theory and observations show that the processes that lead to the production of lightning are tightly controlled by the cloud updraft and the formation of ice (Baker, 1995; Dye, 1986). Lightning initiates soon after the onset of strong convection, after significant cloud mass and ice have formed in the upper regions of the thunderstorm. It is this physical coupling that enables us to use lightning to study strong convection and ice development. Developing these lightning relationships is important because lightning is often easier to measure than most convective parameters and lightning measurements can be easily made from space.
Storm Electrical Budget
An opportunity to obtain the current budget in a thunderstorm will present itself if we conduct a field campaign at Patrick Air Force Base (PAFB), Florida. The complete current budget consists in determining the vertically directed currents flowing above, within, and beneath a thundercloud. By determining the current budget, it will be possible to test support for the convective theory (Vonnegut, 1963) of thunderstorm charging since this theory places specific constraints on the expected storm currents.
