1. The Genesis of Hurricane Guillermo: TEXMEX Analyses and a Modeling Study
BISTER AND EMANUEL

2. Incipient disturbances are prevented from developing by convective downdrafts in their cores, which bring air of low into the boundary layer, suppressing further convection.
OVERCOME??
1) an increase of the in the middle troposphere
2) an increase of relative humidity so that evaporation of rain is suppressed
3) an increase of wind speed so that the sea surface fluxes keep replenishing the boundary layer

3. The main goal of the Tropical Experiment in Mexico (TEXMEX) was to test a hypothesis:
The elevation of in the middle troposphere just above a near-surface vorticity maximum is a necessary and perhaps sufficient condition for tropical cyclogenesis.
Assumption:
the elevation of is accomplished by deep convection bringing high to the middle troposphere.

6. The pre-Guillermo MCS was the object of IOP 5 from 2 August 1991 to 5 August 1991, during which six flights were flown. The first, third, and fifth flights, labeled 1E, 3E, and 5E, respectively, were flown with the Electra NCAR while the second, fourth, and sixth flights, labeled 2P, 4P, and 6P, respectively were flown with the WP-3D,NOAA. Data were excluded from the analysis if the magnitude of the vertical velocity exceeded 1 m/s, in order to minimize the effect of active convective updrafts and downdrafts on the analyzed fields. The differences of the temperature and the dewpoint temperature measured by the two aircraft were less than 0.3 K during both flights, and they were accounted for in the data analysis.

7. large-scale flow :
easterly wave

8. Flight 2P
0200 UTC 3 August

9. Flight 2P
0200 UTC 3 August
in the stratiform region
relative humidity peak
the vortex has a cold core at 3km and a warm core in the upper troposphere (thermal wind balance)

10. Flight 4P
0600 UTC 4 August

11. Strengthening rain band still northwest
Flight 4P
0600 UTC 4 August
Strengthening rain band still northwest
A small minimum inside the cold core
Vertical shear generally cyclonic, but with a anti-cyclonic in the north (a warm core developing near the region of the developing strong convection, also where maximum surface wind observed).

12. Flight 5E
2000 UTC 4 August
warm core at 3-km altitude is dominant with a reversal of the gradient of the virtual
potential temperature about 100 km from the center (also found in boundary layer).

13. between the cold-core stage and the development of the central warm core, the system’s main thermodynamic change is moistening of the boundary layer. (result not sensitive to square area within 15km)
The increase of and virtual potential temperature in both the boundary layer and at 3-km altitude while the tangential wind speed increases suggests that intensification owing to the feedback between the wind and the surface heat fluxes is underway.

14. An MCS with an extensive stratiform precipitation region forms
An MCS with an extensive stratiform precipitation region forms. Diabatic heating in the upper troposphere and cooling at and below the melting level lead to a formation of a midlevel vortex. Evaporation of rain increases relative humidity in the lower troposphere and leads to a downdraft that advects the vortex downward. Convection redevelops, leading to a further increase of vorticity below the maximum heating and a formation of a warm core.

15. neglected the effects of deep convection in obs
Simulation
A steady mesoscale ‘‘rain shaft,’’ emanating from a prescribed altitude, is switched on at the beginning of the simulation. Any latent heating associated with the formation of this rain is neglected.
Kessler microphysics
(No ice physics, melting would deepen the layer
of diabatic cooling by roughly a kilometer.)
horizontal resolution is 7.5 km
10s time step
2-D turbulence
No radiation or background wind
A positive anomaly in the relative humidity in the upper troposphere, to reflect the high relative humidity of the anvil, is maintained for 36 h.
rain shaft extends to 116-km radius, decreases linearly to zero.
Coriolis parameter is at latitude 12N
neglected the effects of deep convection in obs

16. Kessler microphysics

17. Kessler microphysics No ice physics, only supercooled water
Kessler microphysics No ice physics, only supercooled water. Twice the specific heat than ice (Water: 4200J/kg C, ice:2100J/kg C) Decrease the same temperature, more latent heat

18. Control
simulation
Anticyclonic tangential wind

19. Cold core first forms above 2km and be advected down to the boundary layer.
Extend to the surface within the innermost 90km.
Explains why shallow convection can develop in the middle of the rain shaft (not shown). A decrease of temperature by 1K corresponds to 2.5K decrease of saturated Evaporation efficiently reduces stability for shallow convection.

20. Negative RH anomaly to positive anomaly
Moist level deepening to the surface

21. At the beginning, divergent flow near the surface, anticyclonic motion in the boundary layer.
Downward advection of cyclonic wind which strengthens in both magnitude and horizontal extent.
More sea surface flux into the atmosphere.
The maximum wind speed isn’t at the lowest model level until 64h. There is still a cold poor but the core is not at the center.

22. Sensitivity Study

23. DI：
more evaporation, larger downdraft and T anomaly.
Maximum tangential wind not larger than control run, except in PBL.
Larger downdraft leads to stronger anticyclonic wind which increases sea surface fluxes at the beginning stage.
Outer convection occurs.
HI:
Warmer and drier PBL
No outer convection.
DA:
Maximum tangential wind stronger than control run. Anomalies over a larger area. Wind anomaly closer to the sea surface than control. Outflow convers a larger area. Outer convection occurs. Outer convection weakens, storm starts intensification.
HA:
The same anomalies in , smaller wind.

24. Outer-region convection can reduce the rate of intensification
Outer-region convection can reduce the rate of intensification. The outer convection seems to develop because of increased surface fluxes associated with the outflow and anticyclone at the model’s lowest level.
Sensitivity study:
Same as HDDI but no surface fluxes for r >340 km

25. Sensitivity study of midlevel humidity
The initial middle-tropospheric relative humidity was increased by 30% in one experiment and it was decreased by 30% in another experiment.
The results show that a dry middle troposphere favors the initial development of the cold-core vortex. However, a moist middle troposphere is slightly more favorable at later stages of the development. The overall development is not sensitive to the large-scale humidity in the middle troposphere.

26. Comparison between observation and model
Agreement:
Takes about 3~4 days to form a hurricane
A cold-core vortex with high relative humidity at the beginning
A warm core develops within the cold core
Disagreement:
It takes too long for model to form convection (due to lack of mean wind).

27. High humidity VS cold-core vortex
(68km in radius, from 2.5 to 12.5 km)
B2 contradicts simulation results of Emanuel (1995) showing that saturation of a mesoscale column is a sufficient condition for cyclogenesis.
Extend to 150km
a broad enough column of
saturated air by itself can result in a hurricane.

28. B2
Low
Cold PBL
Sea surface
PBL
Why B1 develops??
3K cold anomaly equals to 8K in , low level is more unstable
inertial stability prevents inflow of low
The downdraft in B3 brings smaller but the disturbance still develops.

29. For a depth of the layer of 4000 m, and vertical velocity of 0
For a depth of the layer of 4000 m, and vertical velocity of 0.1 m/s, the timescale is 10 h. W C W C C
PV anomaly will be diffuse,
vortex will not be strong W

30. A long lasting MCS forms
Evaporative cooling (and anvil heating)
results in a midlevel vortex
Vortex extends to lower altitudes, and the cold anomaly expands downward
Vortex wind enhances sea surface flux;
Cold core reduces PBL and favors convection

32. convection Cold core vortex Warm core Downward motion
Cold core downward
Vortex downward
Evaporation downward
More sea flux
convection
Humid lower layer
Warm core

33. Personal View
How to define the stage of cyclone genesis? (Can a warm core vortex die?)
Is it the only way to form a low level warm core vortex?
Ignore the convection and heating and asymmetric motion in the model.
How does a warm core develop inside a cold core (Or is shallow convection heating effective? Kessler?)
Can a vortex form near the surface without downward motion?
Stratification in stratification?