Assimilation of Pseudo-GLM Data Using the Ensemble Kalman Filter
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SEPTEMBER 2016 A L L E N E T A L . 3465 Assimilation of Pseudo-GLM Data Using the Ensemble Kalman Filter BLAKE J. ALLEN Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, and NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma EDWARD R. MANSELL NOAA/OAR/National Severe Storms Laboratory, Norman, Oklahoma DAVID C. DOWELL NOAA/OAR/Earth System Research Laboratory, Boulder, Colorado WIEBKE DEIERLING National Center for Atmospheric Research, Boulder, Colorado (Manuscript received 30 March 2016, in final form 13 June 2016) ABSTRACT Total lightning observations that will be available from the GOES-R Geostationary Lightning Mapper (GLM) have the potential to be useful in the initialization of convection-resolving numerical weather models, particularly in areas where other types of convective-scale observations are sparse or nonexistent. This study used the ensemble Kalman filter (EnKF) to assimilate real-data pseudo-GLM flash extent density (FED) observations at convection-resolving scale for a nonsevere multicell storm case (6 June 2000) and a tornadic supercell case (8 May 2003). For each case, pseudo-GLM FED observations were generated from ground-based lightning mapping array data with a spacing approximately equal to the nadir pixel width of the GLM, and tests were done to examine different FED observation operators and the utility of temporally averaging observations to smooth rapid variations in flash rates. The best results were obtained when assimilating 1-min temporal resolution data using any of three ob- servation operators that utilized graupel mass or graupel volume. Each of these three observation operators performed well for both the weak, disorganized convection of the multicell case and the much more intense convection of the supercell case. An observation operator using the noninductive charging rate performed poorly compared to the graupel mass and graupel volume operators, a result that appears likely to be due to the inability of the noninductive charging rate to account for advection of space charge after charge separation occurs. 1. Introduction techniques in numerical weather prediction models. The upcoming launch of the Geostationary Lightning Map- Lightning location and frequency observations can per (GLM; Goodman et al. 2013) on board the Geo- pinpoint deep convective storms, and the availability of stationary Operational Environmental Satellite R-series lightning data in areas with poor radar coverage has the (GOES-R) will greatly extend the availability of total potential to supplement current radar data assimilation lightning data [intracloud and cloud-to-ground (IC 1 CG)] by providing continuous coverage over much of the Western Hemisphere. The GLM will provide in- Corresponding author address: Blake J. Allen, Cooperative In- stitute for Mesoscale Meteorological Studies, 120 David L. Boren formation on convective activity in mountainous terrain, Blvd., Norman, OK 73072. where radar coverage is poor, and over much of the E-mail: [email protected] Atlantic and eastern Pacific Oceans, where both radar DOI: 10.1175/MWR-D-16-0117.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 09/29/21 08:39 PM UTC 3466 MONTHLY WEATHER REVIEW VOLUME 144 coverage and the detection efficiency of ground-based study, the addition of total lightning data from LMA lightning detection networks are poor. measurements was found to improve the results over To test the feasibility of using GLM observations to using CG-only data. Pessi and Businger (2009) again help initialize and modulate storms in convection- showed improvements to a forecast when using CG resolving ensemble model frameworks, this study as- lightning data, converted to rainfall rates, to adjust the similated Lightning Mapping Array (LMA)-derived latent heating rate in a CPS. pseudo-GLM flash extent density (FED) observations In deterministic convection-resolving models, total using the ensemble Kalman filter (EnKF) to create an- lightning data in the form of Earth Networks Total alyses in a storm-scale model. In addition to demon- Lightning Network (ENTLN) flash origin densities, strating the value of assimilating such data, another goal converted to 9-km pixel-width data to mimic flash den- of this study was to further develop observation opera- sities that will be available from the GLM, have been tors to use with GLM data. To these ends, experiments used to initialize observed convection using two different were performed with two cases: a nonsevere multicell methods. The first method utilizes water vapor nudging storm that occurred on 6 June 2000 and shifted through in the mixed-phase region between the 08 and 2208C various morphologies during its lifetime, and a tornadic levels (Fierro and Reisner 2011; Fierro et al. 2012), and supercell that occurred on 8 May 2003. The analyses the second method acts by increasing the low-level tem- produced by assimilating the lightning data were com- perature in the model to the convective temperature pared both to observed characteristics of the storms and (Marchand and Fuelberg 2014). to analyses of the storms produced using the more More limited research has been done using ensemble established technique of radar radial velocity (Vr) data techniques. Using primarily CG lightning data con- assimilation. verted to rainfall rates and a synoptic-scale model, Previous studies have shown that total lightning ac- Hakim et al. (2008) was the first to assimilate lightning tivity is related to various measures of thunderstorm data using in an ensemble framework. As a direct pre- intensity (e.g., graupel and ice content, rainfall rates, and cursor to this study, Mansell (2014) performed an ob- updraft characteristics), and that total lightning activity serving system simulation experiment (OSSE) in which exhibits much stronger correlations and correspon- simulated total lightning data in the form of FEDs were dences with these thunderstorm characteristics than CG assimilated, using the EnKF, into the same storm-scale lightning activity alone. This has been shown in indi- model used in this research. In that study, the data as- vidual storms (MacGorman et al. 1989; Wiens et al. similation technique showed the ability to modulate the 2005; Goodman et al. 1988), in data aggregated over strength of convection and suppress spurious convec- multiple storms in a region (Deierling and Petersen tion. This study continues the research begun in Mansell 2008), and in globally aggregated data (Petersen et al. (2014) by working with real-data cases. 2005). It is also supported by modeling studies of indi- vidual storms (Kuhlman et al. 2006), aggregated storms 2. Data assimilation framework (Cohen 2008), and hurricane eyewalls (Fierro et al. a. Numerical model 2015). A large body of evidence suggests these correla- tions arise from the primary role rebounding collisions The model used for this work was the National Severe between graupel and cloud ice play in thunderstorm Storms Laboratory (NSSL) Collaborative Model for charge separation [the noninductive graupel–ice mech- Multiscale Atmospheric Simulation (NCOMMAS; anism; Reynolds et al. (1957); Takahashi (1978); Wicker and Wilhelmson 1995; Coniglio et al. 2006), Jayaratne et al. (1983)]. which is based on the equation set of Klemp and Several earlier studies have demonstrated benefits Wilhelmson (1978). The NSSL two-moment, four-ice from using nudging schemes to assimilate lightning data category bulk microphysics scheme of Mansell et al. into deterministic models, both in models with param- (2010), based on the scheme of Ziegler (1985), was used. eterized convection and in convection resolving models. The microphysics scheme predicts mass mixing ratio and Alexander et al. (1999) and Chang et al. (2001) used CG number concentration for six hydrometeor types: cloud lightning data converted to rainfall rates to adjust the droplets, rain, cloud ice, graupel, snow, and hail. Addi- latent heating rate in a convective parameterization tionally, the scheme predicts graupel and hail particle scheme (CPS), Papadopoulos et al. (2005) used CG densities. The electrification scheme of Mansell et al. lightning data to adjust humidity profiles and thereby (2005) was activated during the development of light- indirectly trigger a CPS, and Mansell et al. (2007) used ning observation operators (section 5); the electrifica- CG and total lightning data in the form of LMA source tion scheme includes graupel-ice noninductive charge point densities to directly activate a CPS. In the latter separation based on Saunders and Peck (1998) and Unauthenticated | Downloaded 09/29/21 08:39 PM UTC SEPTEMBER 2016 A L L E N E T A L . 3467 Brooks et al. (1997), as implemented in Mansell et al. all liquid water and ice water fields are initially zero in all (2010), and the scheme also includes the branched ensemble members. To introduce convection into the lightning simulation of Mansell et al. (2002). For the model and to develop ensemble spread, a combination lightning data assimilation experiments (section 6), of two methods was used. The first method adds ran- electrification was turned off except when using the domly placed thermal bubbles to the ensemble members noninductive charging rate observation operator. In in regions where radar reflectivity is absent in the en- experiments using that observation operator the non- semble mean but observations show nonzero FEDs. inductive charging parameterization was activated but This scheme is similar to the one used with radar data in lightning was not simulated. Tanamachi et al. (2013), and the use of the scheme with Horizontal grid spacing was set to 1 km for both cases, lightning data is facilitated by assuming an observed while vertical grid spacing varied from 200 m at low reflectivity of 40 dBZ wherever the observed FED is levels to a maximum of 700 m in the multicell case and nonzero. The bubble addition ended after the first was set to a constant value of 500 m in the supercell case 50 min of the simulations since continued forcing after (the cases are described in detail in section 3). The do- convection was established tended to degrade the re- main in each case had 50 vertical levels.