Tropical Cloud Systems [In the ‘tradition’ of GCSS Plenary talks, a broad and misleading title]
Ed Zipser
University of Utah
AcknowledgementsResearch Associates:• Chuntao Liu (U of Utah); Baike Xi (U of Utah, now UND)Current and Former Students:• Haiyan Jiang (U of Utah, now GSFC/UMBC)• Yaping Li (U of Utah)• Steve Nesbitt (Texas A&M and U of Utah, now CSU),• Dave Yorty (U of Utah, now modifying the weather) • Dan Cecil (Texas A&M, now NASA MSFC/UAH)• Rick Toracinta (Texas A&M, now Ohio State U)• Karen Mohr, Chris Lucas, Brad Ferrier, Gary McGaughey, Robbie
Hood, Gerry Heymsfield, +participants in numerous field programsTRMM Program Scientists: Joanne Simpson, Chris Kummerow, Bob
AdlerErich Stocker and TSDIS Funding mostly NASA TRMM Science, also NASA CRYSTAL-FACE,
NASA Earth System Science Fellowships
Precipitation Features
What is a precipitation feature?
Contiguous area at least 4 pixels in size (75 km2) with:
PR “near surface” reflectivity ≥ 20 dBZ
(identify near surface rain)
orTMI 85 GHz PCT ≤ 250 K
(identify anvils and be consistent with previous work using the SSM/I, e.g. Mohr and Zipser
1996)
The TRMM Database (5 yr; 12M PFs)can be used as observational validation of
properties of rain-producing systems
• Where are the MCSs and how much rain do they contribute?• Where do small PFs contribute most? :Large PFs?• How much rain is contributed by PFs reaching 12 km? 15 km?
Where? • How much rain is contributed by PFs containing lightning?• How much rain falls from mesoscale PFs with strong convection?• How much rain falls from mesoscale PFs without strong convection?• What is the convective vs. stratiform partitioning of rain and how
does it vary around the globe?
• [….and how well do GCMs do answering these questions….?]
Fractional rain (%) contributed by large MPFs vs. small PFs
Very large
Small
Fractional rain (%) contributed by PFs with specified properties
Fractional rain (%) contributed by MPFs with 40 dBZ height </> 6 km
Fractional rain (%) contributed by MPFs with convective fraction </> 50%
Proxies for Convective Intensity
• Lowest IR brightness temperature - gives indication of cloud top height but poorly related to convective vertical velocity- might even be only thin cirrus
• Highest altitude with >20 dBZ radar echo - good• Highest altitude with >40 dBZ radar echo - better• Lightning flash rate - generally good (but indirect and also
may be related to aerosol properties of environment)• Lowest brightness temperature at 85 GHz or 37 GHz - good
How good? Which are best?
Next slides show global distribution of proxies for convective vertical velocity
Logarithmic color scale by rarity of event (i.e., black 10 X rarer than purple, purple 10 X rarer than orange, etc. (Cecil et al)
CDF of PFs by flash rate
Green: > 2/minuteOrange: > 30/minute Purple: > 120/minuteBlack triangle: > 300/minute
Each color step signifies that the event is 10 X rarer!
Why does convective vertical velocity matter?
• It matters a great deal in the microphysics of the mixed phase region
• Weak updrafts => low graupel mixing ratios, absence of large graupel, little electrification, low dBZ aloft
• Rain over tropical oceans comes from weak updrafts…..
• Really strong updrafts (undilute, adiabatic cores) may exhaust huge numbers of small ice crystals, with huge ice water content, at and above the tropopause
Data from oceans
Data from land
20°N-20°S Intensity Proxies: Deep tropics--no influence of intense subtropical systems
Total rainfall is not closely related to flash rate or other intensity proxies (but it is, of course, closely related to convective mass flux)
Challenge
• Challenge to modelers: Can we simulate convective intensity as well as rainfall?
• Opportunity: Seven years of TRMM observables are now available. Why not use them to evaluate models?
• Example: Distribution of overshooting radar tops
Overshooting PFs are defined using 5 different reference heights
(I): Constant 14 km (Alcala and Dessler, 2002)
(II): Potential temperature > 380 K
(III): NCEP reanalysis tropopause height
(IV): Level of Neutral Buoyancy (LNB) from NCEP sounding (using e surface) (V): Level of Neutral Buoyancy (LNB) from NCEP reanalysis
(using maximum of e 925mb and e 1000mb)
(Tropopause height and LNB for each one of 5 million PFs in 5 years are interpolated from 2.5o x2.5o, 6 hour interval NCEP reanalysis data.)
Location of overshooting towers(total # of PFs with 4 pixels between 20N-20S in 5 years: 5,208,813)
Overshoot distance > 2km (blue) >3km (red)
Diurnal variation of overshooting tower occurrence
Land
Ocean
Overshooting area distribution from 5 reference heights
14km
=380k
Tropopause
LNB (e 925&1000mb)
LNB (e surface)
Summary
• TRMM database is now 7+ years and may be as valuable for convective properties as for rainfall
• The longer TRMM stays in orbit, the more valuable the statistics become; e.g., more stable diurnal cycles, more sampling of rare events, …
• All proxies for convective intensity [w] show land much stronger than ocean, with intriguing regional differences
• Why are equatorial Africa storms stronger than Amazon storms?
• Stratospheric penetration of convection is greatest over Africa. Why? Significance for transport of (name it!)?
January 19, 2005 Department of Meteorology Seminar Series, University of Utah
Thank you!Thank you!
January 26, 2005 Department of Meteorology seminar series, University of Utah
Time-height maximum radarreflectivity and vertical velocity Kwajalein MCS of11-12 Aug. 1999
Max reflectivity
Max upward speed
dBZ
m/s
Time-height maximum radar reflectivity and vertical velocity
Time-height maximum radar reflectivity
Graupel is major contributor of reflectivityin the ice region
Max reflectivity from Rain Water
From Graupel
From Snow
Max reflectivity
dBZ
Kwajalein MCS of11-12 Aug. 1999
Time-height maximum graupelmixing ratio and vertical velocity
Max upward speed
Max Graupel mixing ratio g/kg
m/s
Kwajalein MCS of11-12 Aug. 1999
Comparison of the simulated results with actual observations
20 dBZ
40 dBZ
Maximum height of
Storm total CFADs of convective region reflectivity
(From Yuter et al. 2005)
Research radar observations
Kwajalein MCS of 11-12 August 1999
Comparison of the simulated results with actual observations
Maximum reflectivity at Minimum PCT of
85 GHz
37 GHz
6 km
9 km
Comparison of the simulated withthe actual TRMM observation statistics
Comparison of simulated & actual TRMM observation statistics