Effects of Convection on Clouds and Water in the Tropical Tropopause Layer
Leonhard Pfister and Eric Jensen, NASA Ames Research Center
• Why are we interested in Clouds and Water in the Tropical Tropopause Layer? • What’s been done before? • What is our model formulation – how do we treat convection? • What are the water vapor and cloud distributions, and why? • What can Aura do for this problem? • Conclusions
Motivation
• TTL regulates water input to the stratosphere • Water in the TTL affects cloud distribution and global radiation budget • How are water vapor and cloud distributions in the TTL maintained?
Background and Previous Work
• Large areas of subvisible cirrus clouds near tropical tropopause (e.g. Wang et al) • Dehydration due to horizontal motion through cold regions (Holton, Gettelman, Haynes, and others) • Detailed microphysical modeling – (Jensen and Pfister) • 40 day back trajectory for 1995-1996 winter from a grid of points in the TTL • Evaluate vertical temperature profiles along these back trajectories (“temperature curtains”) • Initial water vapor imposed and .2-.5 mm/s updraft (clear sky radiation) • Use full 1-D microphysical model and time-varying T to calculate clouds and water along each trajectory. • Water vapor results show good agreement with HALOE obs (Randel, Rosenlof)
BUT – convection MUST BE important
• Isotopic water ratios cannot be explained solely by slow ascent/horizontal flushing (Kwang et al.; Webster and Heymsfield) • Convective turnover times are such that convection and slow ascent comparable at tropopause (Dessler, Gettelman et al) • Evidence that overall cold temperature maintained by convection (Salby, Dessler and Kim, Randel) • Connection of SVC to convection (Massie, Spang, Pfister)
SO
Convective Formulation
• Use existing temperature curtain trajectories • Move them through 3-hourly IR brightness Temps from ISCCP • Adjust brightness temps by 7K • Calculate cloud top altitude based on brightness temps in neighborhood of curtains • Change water vapor and clouds based on that cloud top altitude
Treatment of Convection in Model
ISCCP IR Image at 199512220300
240
200
180
IR Brightness T, K
220
Treatment of Convection in Model
ISCCP IR Image at 199512220600
240
200
180
IR Brightness T, K
220
Treatment of Convection in Model
ISCCP IR Image at 199512220900
240
200
180
IR Brightness T, K
220
Treatment of Convection in Model
ISCCP IR Image at 199512221200
240
200
180
IR Brightness T, K
220
Treatment of Convection in Model
ISCCP IR Image at 199512221500
240
200
180
IR Brightness T, K
220
Treatment of Convection in Model
Ice SMR, ppmv (dotted) 2 4 6
0 18.0
8
Initial Convective
17.5
17.0 Altitude, km
16.5
Temp(K)
16.0
15.5
ISMR(ppmv)
15.0 180 182 184 186 188 190 192 194 Temperature, K (solid)
Model Profiles
Treatment of Convection in Model
Ice SMR, ppmv (dotted) 2 4 6
0 18.0
8
Initial Convective 17.5 Post-Convective
17.0 Altitude, km
16.5
Temp(K)
16.0
15.5
ISMR(ppmv)
15.0 180 182 184 186 188 190 192 194 Temperature, K (solid)
Model Profiles
Treatment of Convection in Model
Ice SMR, ppmv (dotted) 2 4 6 Ice SMR, ppmv (dotted) 2 4 6
0 18.0
8
0 19.0
8
Initial Convective 17.5 Post-Convective
Water Vapor, ppmv
18.5
17.0 Altitude, km Altitude, km
18.0
16.5
Temp(K)
17.5
Temp(K) (solid)
16.0
17.0
15.5
ISMR(ppmv)
16.5
15.0 180 182 184 186 188 190 192 194 Temperature, K (solid)
ISMR(ppmv) (dotted) 16.0
180 182 184 186 188 190 192 194 Temperature, K (solid)
Model Profiles
STEP 1987, 870123
Sample hydration case
Temperature (K)
400 Pressure (mbar) 390 380 370 360 350 20
196 195 194 193 192 191 190 189 188 187 186 185
25 30 Time (days) 35 40
Ice Saturation Ratio
400 Pressure (mbar) 390 380 370 360 350 20
1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7
25 30 Time (days) 35 40
400 Pressure (mbar) 390 380 370 360 350 20
H2O mixing ratio (ppmv)
5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0
25 30 Time (days) 35 40
H2O mix. rat. (ppmv)
Si
Temperature
Sample dehydration case
Temperature (K)
400 Pressure (mbar) 390 380 370 360 350 20
196 195 194 193 192 191 190 189 188 187 186 185
25 30 Time (days) 35 40
Ice Saturation Ratio
400 Pressure (mbar) 390 380 370 360 350 20
1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7
25 30 Time (days) 35 40
400 Pressure (mbar) 390 380 370 360 350 20
H2O mixing ratio (ppmv)
5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0
25 30 Time (days) 35 40
H2O mix. rat. (ppmv)
Si
Temperature
Sample hydration with subsequent nonconvective dehydration
Temperature (K)
400 Pressure (mbar) 390 380 370 360 350 20
196 195 194 193 192 191 190 189 188 187 186 185
25 30 Time (days) 35 40
Ice Saturation Ratio
400 Pressure (mbar) 390 380 370 360 350 20
1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7
25 30 Time (days) 35 40
400 Pressure (mbar) 390 380 370 360 350 20
H2O mixing ratio (ppmv)
7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
25 30 Time (days) 35 40
H2O mix. rat. (ppmv)
Si
Temperature
Overall effect on water vapor distribution
Tropical mean final water vapor profiles
385 385
Convective turnover time
380 Potential Temperature (K) Potential Temperature (K)
380
375
No convective injection Instant ice removal 4-hour ice persistence
375
370
370
365
365
Dessler [2002] Gettelman et al. [2002] This work
360
360
355 2 3 4 5 Final H2O mixing ratio (ppmv) 6
355 0 50 100 150 Days 200 250 300
Proportions of parcels experiencing convection
380
All parcels experiencing convection Conv. Parcels with 4 hr ice persistence (no dehydration) Conv. Parcels with 0 hr ice persistence (no dehydration)
375
370 Theta(K) 365 360 355 0.0
0.2
0.4 0.6 0.8 Fraction of parcels experiencing Convection
1.0
Water Distribution
no convective input 360 K
20 10 0
-10 -20
Instant anvil ice removal
20 10 0
-10 -20
4-hour anvil ice persistence
20 10 0
-10 -20 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 Tropopause H2O Mixing Ratio (ppmv) 3.9 4.2 4.5
Water Distribution
no convective input
370 K
20 10 0
-10 -20
Instant anvil ice removal
20 10 0
-10 -20
4-hour anvil ice persistence
20 10 0
-10 -20 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 Tropopause H2O Mixing Ratio (ppmv) 3.9 4.2 4.5
Water Distribution
no convective input
380 K
20 10 0
-10 -20
Instant anvil ice removal
20 10 0
-10 -20
4-hour anvil ice persistence
20 10 0
-10 -20 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 Tropopause H2O Mixing Ratio (ppmv) 3.9 4.2 4.5
Cloud Distribution
no coninf 360-380 K
20 10 0 -10 -20
in situ clouds with convective injection
20 10 0 -10 -20
convective clouds
20 10 0 -10 -20
total TTL clouds
20 10 0 -10 -20
1
2
4
7
10
20
30
40
50
TTL Cloud Frequency (%)
Location and Effects of Convection reaching 365K
Open Squares -- convection locations Post-convective dehydration Final Parcel locations -- with PC dehydration Final Parcel locations -- no PC dehydration
Circulation of Convective Parcels reaching 365K
Open Squares -- convection locations Post-convective dehydration Final Parcel locations -- with PC dehydration Final Parcel locations -- no PC dehydration
Conclusions
• Effect of direct convective injection on water vapor distribution - Significant hydration below temperature minimum (20%) - Slight dehydration if instant anvil ice removal assumed - 10% hydration if anvil ice persists for 4 hours - Convective effects limited by subsequent dehydration • Convective hydration is reasonably well distributed in tropics • Cloud enhancement is confined to convective areas • How can Aura help? - Simple water vapor comparison for overall features - Convective output – water and temperature downstream of clouds - Gravity wave temperature perturbations abv T minimum - Cloud altitude distributions