Transcript Document
Relationships between wind speed, humidity and
precipitating shallow cumulus convection
Louise Nuijens and Bjorn Stevens*
UCLA - Department of Atmospheric Sciences
*Max Planck Institute for Meteorology
Outline
Motivation, main idea and questions
what is the nature of the observed relationship between winds,
humidity and precipitation?
Large Eddy Simulation - preliminary results
Bulk analysis
Summary and thoughts
RICO observations
Nuijens, Stevens and Siebesma (2009)
Ideas and questions
A column of air moving at a greater speed:
enhanced upward transport of moisture, deeper clouds, hence more rain?
Betts and Ridgway ('89), Bellon and Stevens ('05), Stevens ('06)
is the relationship between wind speed and humidity purely one
reflecting enhanced surface fluxes and moisture transport into the
cloud layer?
how does the cumulus cloud ensemble change with wind speed?
in equilibrium, can similar surface fluxes be maintained at different
wind speeds?
analogy to precipitating deep convection?
Back and Bretherton (2005), Raymond (2003, 2005)
Large Eddy Simulation
Initial profiles and forcings GCSS RICO Intercomparison
12.8 x 12.8 x 5 km domain, 50 x 50 x 40 m resolution
Interactive surface fluxes, shifted geostrophic wind profiles
Time series and profiles
BL depth h
cloud fraction
after 60 hrs:
Flux behavior
w ' 's w ' 'lcl Qr
0
t
hlcl
hlcl
d w ' '
q w ' q 's - w ' q 'lcl
0
t
hlcl
q
dz
const.
w ' q 'lcl
w ' q 's
1
Sensitivity to wind speed?
Stronger winds lead to enhanced evaporation, more humid and
deeper cloud layers
Surface fluxes show a different behavior than expected
The surface buoyancy flux and sub-cloud layer depth for different
wind speeds are very similar
Entrainment fluxes of temperature and humidity are larger for
stronger winds
A first approach: bulk analysis
1) what constrains the buoyancy flux and sub-cloud layer depth?
2) what is the influence of cloud layer air (via entrainment)?
Bulk analysis (1)
w ' v 's hlcl Qr w ' v 'lcl
(1 ) w ' v 's hlcl Qr
B w ' v 's
dB
dz
const.
hlcl
we M ws 0
t
we M
1
M ac w* ac (hB) 3 ,
U
B
B
h
h
ac 0,
M
1 h
1B
M
M 0
3 h
3 B
Bulk analysis (2)
fixed, q varies to keep B constant
v
q
SH
LH
B
hLCL
we
M
Summary and questions
Wind speed may considerably affect cloud and boundary layer properties
Understanding its impact seems interesting and challenging enough
(and we have not even considered precipitation) …
More evaporation, deeper clouds, more mass flux, more drier
downdrafts?
Is a change in the jump in virtual potential temperature across the
transition layer necessary to explain the behavior?
How about shear? How about relations between wind speed, updraft
speed, and precipitation?
Can we generalize our results? (how specific is the RICO case?)