# Presentation: 134195

## Suppression of downdraftsand cyclone development

- Brian Mapes
- University of Miami
- Radar collaborator: Jialin Lin
- Scatt/sat: Ralph Milliff, Jan Morzel

## Outline

- Background: tropical convection
- radiative-convective equilibrium
- rotating: Hadley / hurricanes
- how do downdrafts fit in?
- Observations: wind divergence
- statistics: “typical” convection
- case: developing depression Ivo (2001)
- statistics: scatterometer winds
- Conclusions, future work

## Why tropical atm. convects

- Infrared radiation cools air
- Solar radiation heats surface
- mainly ocean
- Linking the two:
- evaporation: heat from ocean
- condensation: heats air
- 2 mm  rain <~>  1 K heating

- 20N-20S mean rad. cooling
- Bergman & Sardeshmukh 1998

- 1-2 K
- per day
- cooling

- tropical mean rain
- ~3 mm/d

## but condensation is localized

- but condensation is *localized*

-  latent heating = flow across isentropes
- circulation redistributes heat to cooled regions

- IR
- cooling

- tropical
-  surfaces
- (K)

- p (mb)

- high- air

- low- air

- latent heating

**Notes:**
latent heating occurs in vertical motions (flow across isentropes)
mass continuity implies inflow-outflow (horizontal wind divergence) 
low-q air enters, high-q air exits
widespread subsiding motion balances widespread radiative cooling 

## Profile of vertical mass flux

- levels ~~ equally spaced in p (mass)
- [d/dt ]rad ~~ constant with height
- mass flux ~~ const, PBL-200mb

- [d/dt ]rad

- ~[d/dt ]rad

- latent heating

- “square”
- circulation

## Some tropical circs. ARE square

- 15S             Eq.            15N

- JJA

-          Hadley:
-                             Hurricane:

## Dynamics or thermodynamics?

- 15S             Eq.            15N

- inflow

- Frank
- 1977

- a frictional boundary layer?
- driven by SLP or SST gradients?
- a thermally driven flow (hot air rising)?
- All?

## An incomplete picture

- latent heating

- =

## Forgot about rain, cooling,  downdrafts

- Zipser 1969

**mid-level**
**convergence**

**splat!**

## Tropical heat budget

- deep convective *systems* with stratiform rain & downdrafts

- “Square” circulation -- requires shallow latent heating component

## Outline

- Background: tropical convection
- radiative-convective equilibrium
- rotating: hurricanes and Hadley cells
- how do downdrafts fit?
- & how do they get out of the way?
- Observations: wind divergence
- statistics: “typical” convection
- case: developing depression Ivo
- statistics: satellite (scatterometry)
- Conclusions and plans

## Observations

- Details of convection very complex
- Need to *average* - as large-scale flow does (by definition)
- horizontally
- over time (life cycle, many systems)

## Vergence of horizontal wind

- Vergence of horizontal wind

- horizontal wind
- divergence
- at some altitude

- convergence
- (negative divergence)

## The divergence theorem

- Vn

- Any area A

- Normal component of wind along perimeter Vn

- Perimeter length increment *dl*

*dl*

**Notes:**
\int \int_A (\vec \nabla_p \cdot \vec V_{}) dA = \oint V_n dl

## A circular disk of air with a Doppler radar in the middle

- [Vr] = azimuthal mean
- of radial velocity

- radial velocity
- Vr

- raindrops

## Velocity-Azimuth Display (VAD)

- At fixed r & z,  Vr vs. azimuth

- Vr

- azimuth (deg from N)

- 0

- 360

- 0

- wd

- ws

- Mean wind =
-     sine wave

## 2 altitudes

- S

- N             E

- W             N

- [Vr] < 0
- at 925 mb

- [Vr] > 0
- at 125 mb

## research radar deployments

- several 10s of days = many 100s of hours in each field deployment
- Example: EPIC,_x0018__x0013_ September 2001

## Data: a layer cake of histograms

******r = 8 km **
******az = 15°  **
******z = 500 m **
**	****-->********p = 50 hPa**
******t = 1 hour**

- r

## EPIC 2001 ship radar

## regress (p) on surface rainrate R

- regress (p) on surface rainrate R

- all
- data

- rain
- >0.5 only

- characteristic divergence at each altitude, _per unit surface rainrate _

- Q: why no correlation???

## A: There are 2 flavors of rain: Convective and Stratiform

- Tangled together in space (convection often confined to bands or zones).
- Can separate them by local horizontal texture

- 200 km

- rain map

## Why is rain not correlated w/ low-level convergence? (hourly, simultaneous, ~100km scale)

*Convective** **& **stratiform* rain have *opposite* sfc. wind divergence

- all
- data

- rain
- >0.5 only

- • div>0
- ° div<0

- convergence in convective rain

- divergence in stratiform rain

## 
Multiple linear regression isolates pure div. signatures

Con. & Strat. rain linked: 
C leads S

- Multiple linear regression isolates pure div. signatures
- Con. & Strat. rain linked:
- C leads S

- 3 hours

## Con-strat evolution can also be seen in lag regression using total rainfall rate 

- Divergence (cloud top) rises as convection develops upward
- Downdraft fills lower troposphere, lagging rainrate 1-12 hours

**splat!**

## Mean divergence regression 8 field deployments around worldthousands of hours of datadozens of rain events

**splat!**

## Not square...

- How does this                 become this
- with cyclone development?

- (or average out to this in the zonal mean)?

## Do TCs lack stratiform rain? 

- Not by echo-structure measures!

- RAINEX

## Is cyclone convection (& its stratiform rain) somehow special? 

- Less downdrafts for some reason?
- (Or more updrafts?)

## Ivo

- during EPIC field campaign

## pre-Ivo tropical wave

- Mexico

- Yucatan

- Caribbean

- Pacific

## 1315 UTC Sept. 10, 2001

## 1545 UTC Sept. 10, 2001

## 1815 UTC Sept. 10, 2001named as depression: 15.2N, 101.4W 29kts

- 1815 UTC Sept. 10, 2001named as depression: 15.2N, 101.4W 29kts

## 2045 UTC Sept. 10, 2001

## 2315 UTC Sept. 10, 2001Depression - 15.7N, 102.7W  35kts

- 2315 UTC Sept. 10, 2001Depression - 15.7N, 102.7W  35kts

## More zoomed in on radar

- Convective to stratiform evolution
- albeit on a moving ship in moving weather

## 09Z

- 20m/s easterlies

## 12-13Z (dawn)

- 16Z

- upper easterlies
- almost gone

## 16Z

*Low res:            1 h, 15**o** az, 8km range *

- Instantaneous cross section

- Instantaneous map

- 20Z

- Typical stratiform profile_x0010_, but weak

## Z-R estimated rain total: 

- Z-R estimated rain total:
- 50-60 mm
- for various radii

## Mean from all 8 field programs

## Typical mean MCS vs. Ivo44 km radius

- time integrated divergence

- typical

- Ivo

## Is depression convection somehow special? 

- Apparently... 1 case

## ITCZ convection is special too...

- GATE

## caution!

## NICAM output: ITCZ, or lacks downdrafts? (does it get too many cyclogenesis events?)

- GATE
- overlay

## Is depression convection somehow special? 

- Apparently... 1 case
- More data!
- satellite survey

## Outline

- Background: tropical convection
- radiative-convective equilibrium
- rotating: hurricanes and Hadley cells
- how do downdrafts fit in?
- Observations: wind divergence
- statistics: “typical” convection
- case: developing depression Ivo
- statistics: satellite (scatterometry)
- Conclusions, future work

## Surface wind divergence

- scatterometer winds over NW Pacific
- multi-sat precipitable water (PW)
- gridded datasets used for convenience
- 6 hourly wind, 12 hourly PW
- composite divergence and PW fields around big convection events in IR
- subdivide according to vorticity

## Divergence, w/ 4-degree-scale vorticity < 0.5x10-4 s-1

- Divergence, w/ 4-degree-scale vorticity < 0.5x10-4 s-1

- 10 x 10 degree boxes
- 6 hourly
- centered on cold cloud

**splat!**

## Divergence, 4-degree scale vorticity > 10-4 s-1

- Divergence, 4-degree scale vorticity > 10-4 s-1

- 10 x 10 degree boxes
- 6 hourly
- centered on cold cloud

## Divergence, high vort subtract time mean

- Divergence, high vort *subtract time mean*

- 10 x 10 degree boxes
- 6 hourly
- centered on cold cloud

## Divergence, low vort subtract time mean

- Divergence, low vort *subtract time mean*

- 10 x 10 degree boxes
- 6 hourly
- centered on cold cloud

## QuikScat surface divergence(anomalies) 

- QuikScat surface divergence(anomalies)

- 10 x 10 degree boxes
- 6 hourly
- centered on cold cloud

- Low vorticity cases

- High vorticity cases

- t+6h

- t+6h

- t-6h

- t-6h

## Outline

- Background: tropical convection
- radiative-convective equilibrium
- rotating: hurricanes and Hadley cells
- how do downdrafts fit in?
- Observations: wind divergence
- statistics: “typical” convection
- case: depression -> T.S. Ivo
- statistics: satellite (scatterometry)
- Mechanisms, conclusions, future work

## Why is depression convection special? 

- too humid for rain evap?
- different thermal structure?
- some non-thermo dynamics?

## rotating cases more humid high PW => less rain evap. ?_x0018__x0013_

- rotating cases more humid high PW => less rain evap. ?_x0018__x0013_

- low vort

- high vort

- only 3mm more

## PW and vorticity correlated(but some counter-cases)

## 2. Maybe lapse rate is the key difference in depressions?ambient T affects all parcels’ buoyancies

- Mapes and Houze 1995
- (based on rainband obs.)

- anomalous div. looks like anti downdrafts

- cool

- convective

- stratiform

*all *parcels are
- extra buoyant

## 3. Maybe horiz. dynamics, not thermo?

- friction-governed
- flow

- Wide updrafts become hydrostatic - a whole different regime, driven by    HPGF, not b_x0018__x0013_?

- depression updraft sampled by aircraft

- strong vortex

- weak vortex

## Conclusions 

*Surface* wind convergence into convection - dominant in the Hadley cell and mature tropical cyclones - is largely canceled by “downdraft splat” in common deep convection (MCSs)
- Rotating convection (Ivo case, high-vort tercile) seems to lose this splat - a positive feedback mechanism for cyclogenesis
- (surface spinup <=> air-sea interaction)
- ITCZ convection also seems different (GATE). Why? Flow driven by SST-gradient, not pulled by convection? Undersampled in radar database.

## Future work

- More field observations
- ITCZ obs needed - new GATE? (old GATE?)
- Satellite obs
- more composites using existing scatt. data
- (no *case* is covered)
- TRMM PR profiles --> strat. rain evaporation
- depression/TC vs. regular MCSs
- TropSat: new rain-wind scatterometer w/ passive MW in low-inclination orbit: 100-minute storm resampling
- Models: microphysics - CRM - GCM param.

## Conclusions 

- NICAM aquaplanet convection - weak downdrafts apparently.
- Why?
- Not rotating! (equatorial).
- ITCZ - driven by SST gradient?
- Too wide - hydrostatic regime?
- excessive cyclogenesis in other runs?
- Microphysics issues?
- Shallow convection under-resolved?

## NICAM- hydrostatic convection?

- weak downdraft

- ~ hydrostatic updraft

## Vortex stretching factor

- If this were the only term in the vorticity equation,

- This would be its solution

- defining the *vortex stretching factor*

**Notes:**
[\frac{\partial \zeta_{abs}}{\partial t}]_{str} = -\zeta_{abs} (\nabla \cdot V)
\zeta_{abs}(t) = \zeta_0 e^{- \int \nabla \cdot V dt }

## Vortex stretching by 50mm rain(if same air felt time-integrated convergence) 

## Microwave images from NRL 

- 4 times during ship sampling

## 0252 SSMI

- 02Z hour radar

- 1042 TRMM

- 10Z radar

- 18Z radar

- 1855 TRMM

- 01Z radar

- 9/11 0107 SSMI

## Precipitable water (column water vapor)

- sept 10 daily composite

## spare images

- don’t delete, or powerpoint chokes? Mac-Microsoft mayhem

## Slide 82

## 1815 UTC Sept. 10, 2001

## 2315 UTC Sept. 10, 2001

## Tropical heat budget

- deep convective *systems* with stratiform rain & downdrafts

- “Square” circulation requires lack of downdrafts, or another latent heating process to compensate