RAL | Convective Weather

Conceptual Approach

Short-term storm prediction builds upon several components, as outlined in the figure below, including both observations and numerical modeling. The analysis of a wide range of observations is key to monitoring storms. Weather radar plays a chief role in the identification and tracking of storm cells. At very short lead times a storm forecast is obtained primarily by advection of existing radar echoes based on their recent movement (i.e., extrapolation). The skill of extrapolation forecasts, however, decreases rapidly with increasing lead times, thus making it important to account for storm evolution (i.e., growth and/or decay). Moreover, new storms may initiate, which requires recognition and monitoring of characteristic features in the boundary layer (e.g., frontal boundaries created by a storm outflow or advancing sea breeze) and the thermodynamic conditions of the storm environment. Model-based information provides crucial information for identification of regions ripe for new storms to initiate. The ultimate storm forecast is generated based on blending extrapolation, evolution, initiation and modeling information, where the weight shifts from extrapolation at very short lead times to mostly model for longer outlooks.

Expert Systems

AutoNowCaster (ANC)

The ANC provides regional analyses and 0 – 2 h nowcasts of thunderstorms, their initiation, growth and decay. The ANC distinguishes itself from other nowcasting systems that primarily focus on storm extrapolation and trending (e.g., TITAN), in that the ANC is able to forecast initiation of new storms. The ANC is an expert system that mimics much of what is normally done by a human (albeit without the time stress)—i.e., review and assimilate a wide range of disparate observations and model results within the context of a forecaster’s knowledge of how the atmosphere works. The ANC uses a data fusion procedure to assimilate data from radar, satellite, surface stations, soundings, and numerical weather prediction (NWP) models for analysis and calculation of predictor fields. These predictor fields provide information about the current storms and environmental conditions, including: cumulus cloud detection and vertical development based on satellite data; boundary-layer convergence and stability based on radar, surface stations and NWP information; and storm characteristics based on radar data. A fuzzy logic routine is used to combine the predictor fields that are based on observations, a numerical boundary layer model and its adjoint (i.e., VDRAS), feature detection algorithms, and optional forecaster input, to create nowcasts that are issued at regular intervals (typically every 5 min) and that are based on a conceptual understanding of how storms initiate, grow and dissipate. For further details see Mueller et al. (2003).
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Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN)

TITAN is a real-time algorithm for automated identification, tracking, and extrapolation-based short-term forecasting of thunderstorms utilizing volume-scan weather radar data. For each time step, defined by the radar scanning strategy, TITAN identifies a “storm” as a contiguous region exceeding tunable thresholds for reflectivity (e.g., 35 or 40 dBZ) and size (either area or volume). A combinatorial optimization scheme is employed to match the storms at one time with those at the following time, with some geometric logic to deal with mergers and splits. The short-term forecast of both position and size is based on a weighted linear fit to the storm track history data. Besides identification and tracking of storm cells, TITAN also calculates a wide range of storm attributes, including echo area extent and volume, echo top, height of the maximum reflectivity, and storm motion (speed and direction), among many others more. A detailed description of TITAN is provided by Dixon and Wiener (1993).
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Data Assimilation and modeling

VARIATIONAL DOPPLER RADAR ASSIMILATION SYSTEM (VDRAS)

VDRAS uses a cloud-scale model with its adjoint to retrieve boundary-layer winds and thermodynamics from Doppler radar, surface stations, and sounding data by means of a four-dimensional variational (4DVar) analysis procedure. VDRAS is used as a data assimilation and analysis system, but can also be employed as a short-term forecasting tool for convective storms. VDRAS is an essential component of the ANC to provide analyses of the boundary-layer wind field characteristics, which has been highly successful in capturing regions of new storm initiation. The high-resolution (2 – 5 km in the horizontal, 100 – 400 m in vertical) VDRAS analyses are available every 5 – 10 min, depending on the temporal resolution of radar volume scans, and provide the ANC system with information needed to characterize boundary-layer stability and convergence that may lead to thunderstorm development. VDRAS is the first real-time to diagnose low-level wind and temperature over a wide region using four-dimensional data assimilation of Doppler radar data. A detailed description can be found in Sun and Crook (2001).
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VARIATIONAL LIDAR ASSIMILATION SYSTEM (VLAS)

VLAS represents the Doppler lidar variant of VDRAS. VLAS provides very high-resolution wind information at the neighborhood scale and has been used to study atmospheric transport and diffusion in urban environments.
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RT-FDDA: REAL-TIME FOUR-DIMENSIONAL DATA ASSIMILATION

The RT-FDDA system was developed to provide high-resolution analyses and short-term (0 – 12 h) forecasts, although recent advances in computing power enable longer outlooks to be generated. RT-FDDA employs a time-continuous assimilation of a variety of synoptic and asynoptic observation data to provide real-time local-scale analyses and short-term forecasts in a cycling fashion. The RT-FDDA is built upon a high-resolution MM5 numerical weather prediction model (future versions will be based on the WRF model) and the data assimilation makes use of a Newtonian relaxation (i.e., nudging) scheme. The characteristics of the RT-FDDA system generally contribute to superior analysis and forecasts compared to a twice daily MM5 forecast system, especially for shorter forecast lead times. The RT-FDDA system is used in a variety of both winter and summer weather hazard applications.
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Blending of expert system and model output

NCWF

The NCWF product combines meteorological observations, feature detection algorithms, and numerical weather prediction model output to provide a diagnosis of the current locations of convective hazards to aircraft as well as a probabilistic depiction of future locations of existing convective hazards for lead times of 30, 60, 90, and 120 min. Both the convective hazard detection field and the forecasts update every 5 minutes. The current operational version of NCWF runs at the NWS Aviation Weather Center (AWC) and shows the convective hazard detection field and a binary forecast of storm location with a 1 h lead time (NCWF-1). This operational product was first available in 1998 as an experimental product and became operational in 2000; it is available on the Aviation Digital Data Service (ADDS) Convection webpage. Current efforts are geared toward extending the forecast lead time to two hours (NCWF-2) and a continuous 0 – 6 h probabilistic forecast (NCWF-6). These preliminary products are available on the Experimental Aviation Digital Data Service (ADDS) website.
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Niwot

Named after the Native American Chief Niwot, this new short-term forecasting system is being developed based on a blending of observation extrapolation technology and numerical weather prediction model output fields. The promise is that the blending of numerical forecasts with expert system-based extrapolations will benefits from the skills of the latter at short time scales while weighing in more on the numerical prediction skills for extended outlooks, and by doing so yield improved short-term predictions. This tool, which is aimed at 0 – 6 h forecasts of aviation impacting convection, will be flexible enough to accommodate improved extrapolation algorithms and numerical model output as they become available, and allow for forecaster input. It is anticipated that the Niwot and NCWF efforts will eventually be combined into one next-generation short-term prediction system.

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