Secondary Circulation Of Tropical Cyclones In Vertical Wind Shear: Lagrangian Diagnostic And Pathways Of Environmental Interaction

Transcription

1 Secondary Circulation Of Tropical Cyclones In Vertical Wind Shear: Lagrangian Diagnostic And Pathways Of Environmental Interaction M. Riemer and L. Laliberté, JAS, 72, ATM 741 discussion led by Rosimar Rios-Berrios

2 Motivation Vertical wind shear affects tropical cyclones by promoting ventilation Intrusion of environmental, low-entropy air Previous studies described three distinct ventilation pathways: Midlevel entropy fluxes (Tang and Emanuel 2010, 2012) Shear-induced downdrafts (Riemer et al. 2010) Warm core erosion (Frank and Ritchie 2001) The relative importance of each pathway to tropical cyclone intensity changes is unknown

3 Objectives of this study Present a Lagrangian description of the secondary circulation of sheared tropical cyclones Objectively identify ventilation pathways and their relative impacts on tropical cyclone intensity changes

4 Approach Idealized numerical simulations with the Regional Atmospheric Modeling System (RAMS) 5-km grid spacing No radiation Warm-rain microphysics Parameterized surface fluxes vs. shear experiments wind speed à time à

5 Approach Approximately half a million trajectories integrated 6 h backward and forward Seeding locations needed to fulfill three criteria: 1) robust ascent, 2) near saturation, and 3) high inertial stability 20 m s -1 shear

6 Approach Parcel trajectories were mapped into entropytemperature space using the following thermodynamic transform: This vector measures the rate of change of mass along the entropy-temperature space

7 Results: Lagrangian circulation Shear ß T ß θ e Evident in-up-out circulation in both cases Lower entropy air enters the inflow in the shear case Entropy decreases above the inflow (entrainment) Updrafts reach lower outflow temperatures in the shear case

8 Results: Identifying ventilation Ventilation = parcels that intrude into the secondary circulation from the environment Shear ß T ß θ e [% of initial parcels]

9 Results: Identifying ventilation Ventilation = parcels that intrude into the secondary circulation from the environment Frictional inflow Freetropospheric ventilation ß T ß θ e Low-level ventilation

10 Results: low-level ventilation Shear organizes the intrusion of low-entropy air from the environment into the inflow layer Shear

11 Results: frictional inflow Shear prevents an entropy increase before the parcels start rising Shear

12 Results: free-tropospheric ventilation Shear does not appear to increase midlevel ventilation Shear Hypothesis: intensifying versus weakening TC (cause/effect?)

13 Results: Identifying detrainment Detrainment = parcels that exit the textbook secondary circulation and end in the environment Shear ß T ß θ e [% of end* parcels] *only parcels that end with T > 215 K

14 Results: Identifying detrainment Detrainment = parcels that exit the textbook secondary circulation and end in the environment Uppertropospheric detrainment Midtropospheric detrainment Eye detrainment *only parcels that end with T > 215 K

15 Results: Mid-tropospheric detrainment Shear induces a greater entropy reduction when air starts rising above the boundary layer Shear *This process could also be associated with midlevel ventilation

16 Results: Upper-level detrainment Shear increases the amount of parcels that undergo upper-level detrainment (important for intensity changes?) Shear *This process is reminiscent of warm-core erosion

17 Conclusions Vertical shear acts as a constraint on the thermodynamics of tropical cyclones The most prominent impact of shear *in this set of experiments* is the intrusion of lowentropy air through the inflow layer Shear does not appear to increase midlevel ventilation, but shear increases midlevel entrainment Shear also increases upper-level detrainment, thus hinting at a warm-core erosion process Low-level ventilation Midlevel ventilation Midlevel detrainment Upper-level detrainment Outflow detrainment

18 Results: Outflow detrainment Shear increases outflow detrainment Shear *This process consists of entropy loss at cold temperatures followed by a temperature increase