The Impact of Ice Microphysics on the Genesis of Hurricane Julia (2010) Stefan Cecelski 1 and Dr. Da-Lin Zhang Department of Atmospheric and Oceanic Science University of Maryland College Park 1: Research is funded by NASA’s Earth and Space Science Fellowship (NESSF)

Motivation A lack of focus on the impacts of ice microphysics for tropical cyclogenesis (TCG) and related processes – Need to properly represent ice microphysical properties for tropical cyclone (TC) structure and intensity (Ji et. al. 2014) Our previous work investigating the TCG of Hurricane Julia (2010) depicted: – Meaningful ensemble differences for upper tropospheric outflow, warming, and cloud ice content – Warming in the upper troposphere was responsible for meso-α- scale MSLP falls leading to TCG Are the upper-tropospheric dynamical and thermodynamical changes during TCG at all related to ice microphysical processes? 2

Objectives Examine the role of ice microphysics and related heating during the TCG of Julia – Focus on depositional heating since our previous work has shown thermodynamic changes to the upper troposphere during TCG Analyze the changes of deep convection and other parameters pertinent to TCG when ice microphysical processes are modified 3

Hurricane Julia (2010) Background Declared a tropical depression (TD) 0600 UTC 12 Sep 2010 (hereafter 12/0600) – Tropical storm 12 h later at 1800 UTC 12 Sep 2010 Formed within a potent African easterly wave (AEW) Prominent features during TCG: – Pronounced upper-tropospheric warming Hydrostatically induced surface pressure falls on the meso-α-scale – Persistent deep convection within the AEW closed circulation Created a storm-scale outflow in the upper troposphere that expanded warming with time Growth of low-level cyclonic vorticity occurred from the bottom-up with mesovortex merging Right: METEOSAT-9 IR imagery at 1200 UTC 10, 1200 UTC 11, 0600, and 1800 UTC 12 Sep

Methodology Conduct 2 WRF high-resolution sensitivity simulations – Modify the Thompson graupel 2- moment microphysics scheme (Thompson 2004, 2008) used in the control – Compare to the control simulation from Cecelski and Zhang (2013) WRF Details – 3 domains: 9 (D1), 3 (D2), and 1 km (D3 is a moving domain; see right) – 66-h simulation from 0000 UTC 10 to 1800 UTC 12 Sep 2010 – Genesis occurs 54 h into simulation WRF simulation setup. D1, D2, and D3 represent 9, 3, and 1-km horizontal resolution domains, respectively. D3 is a moving domain with initial and final positions drawn. NOAA OI SSTs are shaded (°C). 5

Sensitivity Simulations Experiment 1: “No Fusion” – Removes latent heat of fusion in deposition/sublimation – Thompson scheme definitions for various enthalpies (uses standard values at 0°C): Sublimation (L s ) = 2.834×10 6 J kg -1 Vaporization (L v ) = 2.5×10 6 J kg -1 Fusion (L f ) = 3.34×10 5 J kg -1 – Modification: L s = L v = 2.5×10 6 J kg -1 Still allows for portion of cloud water mass to become cloud ice; Only reduces amount of heating released into the environment by that of L f Experiment 2: “No HFRZ” – Removes any homogeneous freezing of cloud water Occurs with rapid transport of cloud water to upper troposphere via intense convective updrafts – In Thompson scheme, the temperature at which all cloud water must be frozen to become cloud ice is K – Modification: Temperature at which cloud water must turn into cloud ice is changed to K Effectively turns off any homogeneous freezing 6

First-Order Simulation Results Above: Comparison of track and MSLP intensity of experiments (blue and red) with the control (black); Right: Comparison of WRF-derived brightness temperatures (gray shades; K) and composite radar reflectivity (color shades; dBZ) 1800 UTC 11 Sep – first differences Also in extent and intensity of deep convection “No Fusion” has weaker deep convection at time of TCG 7

Upper-tropospheric differences Above: 100 km × 100 km area-averaged temperature difference from 0600 UTC 11 Sep (shaded, °C), absolute vorticity (black contours every 2×10 -5 s -1 ) and cloud ice mixing ratio (blue contours at 2, 5, 10, and 20 ×10 -4 g kg -1. Above: 200 hPa temperatures (shaded, °C) and co- moving wind vectors (reference vector is 10 m s -1 ) with MSLP overlaid (contoured every 1 hPa). Lack of fusion heating during deposition leads to: i)Minimal meso-α-scale upper-tropospheric thermodynamic changes a)Results in no prominent meso-α-scale hydrostatic MSLP falls ii)Weaker and less expansive storm-scale outflow Less warming near storm center in comparison to control Lack of low-level cyclonic vorticity growth versus control Minimal differences between HFRZ and control 8

Convective differences Above: Time-series of 200 km × 200 km area-averaged upper- level Brunt-Vaisala frequency (×10 -3 s -1 ), Rossby radius for deformation (km), composite radar reflectivity (dBZ), and upper-level cloud ice divergence (×10 -5 s -1 ). Right: Count of convective updrafts exceeding various thresholds (m s -1 ) within a 100 km × 100 km area for No Fusion (blue), No HFRZ (right), and the control (black). Weaker and less convective development near storm center in “No Fusion” during TCG Upper-troposphere has greater static stability in “No Fusion” Results in less potent storm- scale upper-level outflow 9 Limits expansion of upper-level warming with time

Conclusions The latent heat of fusion taking place during deposition impacts the TCG of Julia via: – Augmenting the warming of the upper troposphere – Limiting the growth of TD-scale MSLP disturbance – Modifying the strength and spatial extent of deep convection Research Implications – Could properly representing ice microphysics be a critical factor for TCG? – Can we reproduce this characteristic using other microphysics schemes and for other cases? – Are we able to obtain suitable observational data to observe cloud ice and other upper-tropospheric changes? 10