The Role of Vortex and Environment Errors in Genesis Forecasts of Hurricanes Danielle and Karl (2010)

232 MONTHLY WEATHER REVIEW VOLUME 141 The Role of Vortex and Environment Errors in Genesis Forecasts of Hurricanes Danielle and Karl (2010) RYAN D. TORN AND DAVID COOK Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York (Manuscript received 16 March 2012, in final form 2 July 2012) ABSTRACT An ensemble of Weather Research and Forecasting Model (WRF) forecasts initialized from a cycling ensemble Kalman filter (EnKF) system is used to evaluate the sensitivity of Hurricanes Danielle and Karl’s (2010) genesis forecasts to vortex and environmental initial conditions via ensemble sensitivity analysis. Both the Danielle and Karl forecasts are sensitive to the 0-h circulation associated with the pregenesis system over a deep layer and to the temperature and water vapor mixing ratio within the vortex over a comparatively shallow layer. Empirical orthogonal functions (EOFs) of the 0-h ensemble kinematic and thermodynamic fields within the vortex indicate that the 0-h circulation and moisture fields covary with one another, such that a stronger vortex is associated with higher moisture through the column. Forecasts of the pregenesis system intensity are only sensitive to the leading mode of variability in the vortex fields, suggesting that only specific initial condition perturbations associated with the vortex will amplify with time. Multivariate regressions of the vortex EOFs and environmental parameters believed to impact genesis suggest that the Karl forecast is most sensitive to the vortex structure, with smaller sensitivity to the upwind integrated water vapor and 200–850-hPa vertical wind shear magnitude. By contrast, the Danielle forecast is most sensitive to the vortex structure during the first 24 h, but is more sensitive to the 200-hPa divergence and vertical wind shear magnitude at longer forecast hours. 1. Introduction Prediction System (NOGAPS) had varied success pre- dicting genesis in the western North Pacific over a 3-yr Despite significant advances in numerical weather period, with many failed predictions tied to large-scale prediction models, tropical cyclone (TC) genesis and features, such as vertical wind shear and humidity. intensity remain a significant challenge for these systems Moreover, Snyder et al. (2010) showed that ensemble (e.g., Rappaport et al. 2009). There are many potential genesis forecasts from the National Centers for Envi- reasons for the lack of improvement in TC genesis ronmental Prediction (NCEP) Global Forecast System forecasts, including that TC genesis is inherently less (GFS) during the National Aeronautics and Space Ad- predictable because of convective dynamics (e.g., Zhang ministration (NASA) African Monsoon Multidisciplin- et al. 2003; Zhang and Sippel 2009) and there is greater ary Analysis (NAMMA) campaign were characterized potential for large initial condition errors over the ocean by varying degrees of predictability, as measured by due to the lack of in situ data. Wang et al. (2012) sur- ensemble variance, which suggests that some forecasts mised that while convective processes may limit the of these systems could be particularly sensitive to initial predictability of TC genesis, the formation of the pro- condition errors. As a consequence, one of the purposes tovortex is largely controlled by synoptic and meso-a of the Pre-Depression Investigation of Cloud-System processes. To that end, Cheung and Elsberry (2002) in the Tropics (PREDICT; Montgomery et al. (2012)) found that the Navy Operational Global Atmospheric experiment was to evaluate the hypothesis that some genesis forecasts are particularly sensitive to initial condition errors by taking additional observations near Corresponding author address: Ryan Torn, University at Albany, State University of New York, ES 351, 1400 Washington Ave., pregenesis systems. Albany, NY 12222. There are multiple theories regarding how clusters of E-mail: [email protected] disorganized convection can develop into an organized DOI: 10.1175/MWR-D-12-00086.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 09/28/21 02:57 AM UTC JANUARY 2013 T O R N A N D C O O K 233 TC, which in turn provide hypotheses on how initial its north. Torn (2010a) and Sippel et al. (2011) found condition errors can influence genesis. Many recent that intensity forecasts associated with African easterly studies argue that genesis is a ‘‘bottom up’’ processes waves that eventually became Hurricane Helene and whereby localized cores of deep convection act to pre- Tropical Storm Debby (2006), respectively, depended condition the atmosphere by warming and moistening on the strength of the initial vortex and to the mid- an embryonic mesoscale vortex, with the circulation tropospheric moisture through which the vortex would forming by merger and axisymmetrization of the con- move through. In addition, Sippel et al. (2011) deter- vectively generated PV anomalies (e.g., Hendricks et al. mined that an ensemble containing a weaker set of storms 2004; Reasor et al. 2005; Montgomery et al. 2006). Other was more sensitive to the nearby moisture field and SST investigations have suggested that it is actually the vor- fields via track differences than an ensemble containing ticity on horizontal scales greater than individual con- stronger storms, suggesting that weaker pregenesis sys- vective updrafts that lead to the vortex buildup (e.g., tems could be more sensitive to their environment. Fang and Zhang 2010, 2011). Dunkerton et al. (2009) This manuscript explores how initial condition errors hypothesized that genesis can only occur in a region of associated with the pregenesis vortex and the surrounding approximately closed Lagrangian circulation, where the environment impact genesis forecasts by applying the embryonic vortex is to some extent isolated from the ensemble-based sensitivity technique (Ancell and Hakim surrounding environmental flow. This dynamical iso- 2007; Torn and Hakim 2008) to ensemble forecasts of lation would allow for the building of vorticity and two different TCs during the PREDICT experiment. gradual moistening of the column that are necessary for The initial conditions for this forecast ensemble are genesis to occur. All of the above theories require some obtained from a cycling ensemble Kalman filter (EnKF) sort of pregenesis synoptic-scale vortex and moistening system coupled to the Weather Research and Fore- of the free troposphere within it, which raises the pos- casting Model (WRF). The two TCs chosen for this sibility that genesis forecasts could be sensitive to the study (Danielle and Karl) were characterized by two initial wind and moisture fields associated with the different types of predecessor systems and locations, vortex. thus providing an opportunity to evaluate the robustness In addition to the pregenesis vortex, there is also ev- of the forecast sensitivities. idence that the genesis process depends on the envi- This manuscript proceeds as follows. Section 2 pro- ronment that the system is moving through; therefore, vides an overview of the data assimilation system and errors associated with these factors could also play a role model. A summary of the two TCs and their forecasts is in genesis prediction or predictability. In general, TC given in section 3. The initial condition sensitivity for genesis is less likely to occur in regions of greater vertical these two forecasts is given in section 4, while section 5 wind shear (e.g., Gray 1998) and low relative humidity tests the results from section 4 by perturbing the initial in the environment (e.g., Nolan 2007; Raymond and conditions in the most sensitive regions. Conclusions are Lopez-Carrillo 2010; Davis and Ahijevych 2012). These given in section 6. two factors are related in that large vertical wind shear acts to reduce the vertical alignment of the pregenesis 2. Model and data assimilation setup vortex, resulting in relative inflow of drier environmental air into the vortex (e.g., Raymond and Lopez- The role of initial condition errors in forecasts of Carrillo 2010; Tang and Emanuel 2010; Rappin et al. genesis for two TCs is evaluated using ensemble analy- 2010; Davis and Ahijevych 2012). This dry air can produce ses and forecasts taken from a cycling ensemble data downdrafts that dry out the boundary layer, produce assimilation system that was run in real time during lower-tropospheric divergence, weaker updrafts, and PREDICT. Much of the data assimilation setup is sim- thus less vortex stretching (e.g., Smith and Montgomery ilar to Torn (2010b) and is summarized here; the in- 2012). terested reader is directed to this paper for greater detail Previous studies have attempted to quantify the role on the methods outlined below. This data assimilation of initial condition errors on individual TC genesis system generates a 96-member analysis ensemble each events. Sippel and Zhang (2008) found that ensemble 6 h from 0000 UTC 1 August to 1800 UTC 7 November members that strengthened a nondeveloping system 2010 by combining observations with a 6-h forecast en- in the Gulf of Mexico were characterized by higher semble initialized at the previous analysis time (i.e., convective available potential energy (CAPE) and deep cycling) over a 36-km resolution domain that covered moisture in the initial conditions. Sippel and Zhang (2010) much of the Atlantic basin (shown in Fig. 1). For each showed that Hurricane Humberto’s (2007) genesis was system that the National Hurricane Center (NHC) is sensitive to similar factors

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