3A.3 Using Canadian Gem Output for Forecasts of Thunderstorm Initiation on the Canadian Prairies

3A.3 USING CANADIAN GEM OUTPUT FOR FORECASTS OF THUNDERSTORM INITIATION ON THE CANADIAN PRAIRIES Neil M. Taylor1∗ and William R. Burrows2 1Hydrometeorology and Arctic Lab, Environment Canada, Edmonton, AB 2Cloud Physics and Severe Weather Research Section / Hydrometeorology and Arctic Lab, Environment Canada, Edmonton, AB 1. INTRODUCTION Buban et al. 2007). An excellent review of convective initiation mechanisms is given by Predicting the location and timing of Weckwerth and Parsons (2006). Many of the thunderstorm initiation∗∗ (TI) is critical for processes described in the above studies vary on operational forecasters to issue timely and small spatial and temporal scales that are not accurate severe weather watches and warnings. readily resolved using operational observation The basic requirements for TI are sufficient networks. moisture, instability, and lift for air parcels to reach Sometimes, forecaster assessment of the pre- their level of free convection (LFC) and maintain storm environment identifies the potential for positive buoyancy through the troposphere. This, severe thunderstorms but delineation of a TI threat or course, is a very simplified conceptual model. area remains challenging. In Canada, contributing The potential for TI is sensitive to complicated factors to this can include surface observations processes both through the depth of the with coarse spatial and temporal resolution, troposphere and within the atmospheric boundary widely-spaced and infrequent rawinsonde and layer (ABL). AMDAR observations (in Canada most AMDAR Assuming the requirement for tropospheric data lack humidity observations), no operational instability (or criticality; Houston and Niyogi 2007) profiling network, and limited Doppler radar is met, the potential for TI will remain sensitive to coverage capable of detecting finelines via clear- ABL characteristics and processes. Observational air echoes. In short, access to observational data and modeling studies have examined the required to forecast TI can be limited. Canadian sensitivity of TI to ABL water vapour availability forecasters often rely on appropriate conceptual and depth (e.g., Mueller et al. 1993; Crook 1996; models and intelligent use of numerical weather Weckwerth et al. 1996; Craven et al. 2002; prediction (NWP) output to fill in data-void areas. McCaul and Cohen 2002), lift and depth of lift This paper describes work underway at along convergence lines (e.g., Wilson et al. 1992; Environment Canada’s (EC’s) Hydrometeorology Ziegler and Rasmussen 1998; Crook and Klemp and Arctic Lab (HAL) to generate real-time 2000; Wilson and Roberts 2006), and vertical wind forecasts of TI using Canadian operational NWP shear in / near the ABL (e.g., Rotunno et al. 1988; output. Wilson et al. 1992; Mueller et al. 1993; Crook The following sections discuss convective 1996; Crook and Klemp 2000; Markowski et al. forecasts in Canada, development of experimental 2006). Additional emphasis has been placed on model fields related to TI, formulation of TI the influence of mesoscale circulations on TI (e.g., forecasts, and subjective / objective verification of Wilson et al. 1992; Crook 1996) and interactions the forecasts. We conclude with a summary between circulations and boundaries (e.g., discussion and plans for future work. Weckwerth and Wakimoto 1992; Sills et al. 2004; 2. CANADIAN NWP FORECASTS OF ∗ CONVECTIVE PRECIPITATION Corresponding author address: Neil M. Taylor, Hydrometeorology and Arctic Lab, Environment Canada, #200 4999 – 98th Ave. Edmonton, AB, T6B 2X3; The primary source of NWP data for Canadian e-mail: [email protected] forecasters is the Canadian Meteorological Centre’s Global Environmental Multiscale (GEM) ∗∗ Thunderstorm initiation (TI) is used in place of convective initiation (CI) to avoid subtleties that may be associated with model (Côté et al. 1998). The GEM model is run the definition of CI. Results in this study are compared to regionally at 15-km horizontal grid spacing lightning so that TI is less ambiguous. (GEM15) over most of North America utilizing 58 forecast production system allows for generation levels in the vertical. Operational GEM15 forecasts of appropriate convective forecasts (including are produced every 6 h out to T+48 h with output watch / warning decisions) without depending provided to forecasters at temporal resolutions of 3 exclusively on objective NWP solutions (see Sills h or 6 h depending on data requirements. (2009) for an excellent discussion on the role of Forecasts of convective precipitation the human forecaster). (parameterized using the Kain-Fritsch scheme; Kain and Fritsch 1990) and other parameters are 3. EXPERIMENTAL GEM15 MODEL FIELDS available operationally via forecaster workstations or web-based tools. A variety of forecast parameters and indices Public forecasts are generated at specific have been developed to assist the meteorologist in geographic locations via forecaster interaction with assessing the potential for severe storms. These a meteorological database (SCRIBE) utilizing a include characterizations of stability (e.g., variety of direct and post-processed GEM15 MLCAPE, MUCAPE, 0-3 km CAPE, Normalized output. Using the SCRIBE graphical interface, CAPE), deep vertical wind shear (e.g., 0-6 km bulk meteorologists can edit forecast weather elements shear, Effective bulk shear), low-level shear and using the automatically-generated element in the helicity (0-1 km bulk shear, 0-1 km SRH, Effective database (or previous actual forecast) as a starting SRH) and composite indices (e.g., Energy-Helicity point. This allows the forecaster to consider trends Index, Vorticity Generation Parameter, Supercell in observational data and adjust the automatically- Composites, Significant Tornado Composites). generated weather element concept accordingly. Conversely, few forecast parameters are available Forecasts of thunderstorms are produced in that address the issue of TI specifically. To explore SCRIBE using a simple combination of GEM15 the use of GEM15 data for forecasting CI, a forecast Showalter Index (SI) and Updateable number of post-processed experimental NWP Model Output Statistics (UMOS; Wilson and Vallée fields have been developed representing model- 2002) forecasts of probability of precipitation simulated ABL characteristics. During summer, (PoP). If a forecast of SI < 0 and PoP ≥ 30% fields are produced in real-time using the latest occurs in a forecast region then a thunderstorm GEM15 REG model run and viewed via an internal forecast element is generated. Environment web page designed for that purpose. Output is Canada is exploring migration to an area-based produced for each forecast hour from T+1 h to forecast production system. Such a system will T+48 h over various domains in Canada. These require spatial ‘first-guess’ forecast fields for TI experimental fields may be grouped in terms of and other weather elements in addition to existing water vapour availability and depth, convergence point-based guidance. and convergence depth, and ABL vertical wind Objective NWP forecasts of the timing, location, shear. Selected experimental fields are introduced and intensity of warm-season convective below and illustrated for the T+6 h forecast valid precipitation tend to exhibit limited skill (e.g., 1800 UTC 30 July 2010 over southern Alberta Weckwerth et al. 2004; Wilson and Roberts 2006; (AB) and Saskatchewan (SK). Weisman et al. 2008). This is especially true for models with horizontal grid-spacing on the order of 3.1 Water Vapour Availability and Depth 10 km or greater (Weisman et al. 2008). While the operational GEM15 is not expected to pinpoint the Under similar conditions of instability, lift, and timing and location of convective storms, surface water vapour mixing ratio (qv), an characteristics of the general pre-storm environment with ABL water vapour mixed over a environment can often be reasonably simulated. deep layer should be more conducive to surface- This can even include model representation of based TI than if ABL moisture is confined to a convergence boundaries (e.g., the dryline in shallow near-surface layer (Mueller et al. 1993; Alberta). Following a rigorous assessment of McCaul and Cohen 2002). On the Canadian observational data, forecasters may consult and Prairies, especially in Alberta (Canada’s equivalent modify NWP output to fit observations and actual to the U.S. High Plains), ABL water vapour can expected trends. In this way, the forecaster can sometimes be a limiting factor for TI. leverage the strengths of NWP, utilize aspects that Operationally, forecasters routinely characterize are consistent with observations, adjust output for surface water vapour in terms of dewpoint (Td). spatial or temporal deficiencies, and generate or Anecdotal experience by the authors and modify a forecast. Utilizing the strengths of both forecasters is that GEM15 forecast surface Td the human forecaster and computers in the values are often higher than observed. Moreover, 2 model profiles suggest that surface values of Td decreasing values (10-12 °C) through the eastern are often not representative of a mixed moist layer portions of AB. Over the AB foothills, much lower in the ABL (i.e., skin-layer moisture). AVTD values predominate with a developing Two experimental fields discussed here dryline indicated by the gradient in AVTD characterize GEM15 representation of water separating the two areas. In Fig. 2, MMLD (qv) -1 vapour depth. The first is a depiction of the Td values are near 1000 m (10

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