Low-level stratus in the East Pacific in JJA
2001 in the MRF
Glenn White
Glenn.White@noaa.gov
Low-level stratus clouds over the
eastern subtropical oceans play a crucial role in the ocean surface heat
balance, significantly reducing surface downward short wave radiation. During the northern hemisphere summer, such
clouds can be seen to the east of Baja California and off South America. Current numerical weather prediction analysis/forecast
systems have significant difficulties in producing low-level stratus clouds. Of the four reanalyses completed so far,
three failed to produce nearly enough low-level stratus clouds. Only the NCEP/NCAR reanalysis (NCEP-1)
produced enough low-level stratus clouds; NCEP-2, ERA-15 and GEOS-1 reanalyses
failed to produce enough. Recent tests
of the ERA-40 systems have also shown problems in producing low-level stratus
clouds.
This paper examines the ability of the
operational NCEP global analysis/forecast system to produce low-level stratus
clouds during June-August 2001, following the implementation of a new cloud
parameterization including cloud liquid water.
The
NCEP global system produced low-level stratus clouds, but displaced them away
from the coast in the eastern Pacific. (Changes introduced in mid-August decreased the magnitudes of
low-level cloudiness, but did not change the pattern.) This appears to be
associated with a local thermally direct circulation with strong rising motion
over land and intense sinking over the adjacent ocean, inhibiting the formation
of stratus clouds near land. Precipitation over land areas bordering the
east Pacific is larger in the NCEP system than in independent estimates and
climatologies. Thus, too strong
precipitation over tropical land areas in the MRF may be forcing a local
circulation that inhibits the formation of stratus clouds near the coast. Is the NCEP global model forcing too local a
vertical circulation, with intense descent adjacent to land rather than further
away over a broader area?
Clouds
Boundary layer
Clouds from the operational NCEP
global system's analysis cycle (0-6 hr forecasts) are compared to cloud
estimates from operational U.S. Air Force nephanalyses. The NCEP system has much less boundary layer
clouds than the nephanalyses in general and displaces oceanic stratus clouds to
the west away from the coast. It also
has much less boundary layer cloud in the Inter-tropical Convergence Zone. In NCEP forecasts clouds boundary layer
clouds decrease in the stratus regions in the forecasts at least through 5
days; increases in boundary layer clouds tend to dominate the 10 and 15 day
forecasts.
0-6
h and nephanalyses at 25N and 10S
Low-level clouds
Low-level clouds (above the boundary
layer) from the analysis cycle tend to look more like the nephanalyses than do
boundary layer clouds, but are still less than the nephanalyses in the ITCZ and
show a pronounced clearing along the coast.
Westward displacements of the stratus maximum are evident at 25N and
10S. Forecasts tend to displace the
clouds further to the west.
0-6
h and nephanalyses at 25N and 10S
Clouds in new system
A new global analysis/forecast system
was implemented in the middle of August 2001.
Parallel tests of the new and old systems in July 2001 showed that the
new system had fewer boundary layer clouds, but had a very similar pattern of
boundary layer clouds. Low-level clouds
were very similar in the new system, but tended to be less, especially in
five-day forecasts.
Boundary layer
Low level
Radiation
Surface downward solar radiation
Surface downward solar radiation in the
analysis cycle resembles the climatological estimate for June-August (prepared
by NASA Langley based on satellite observations of top of the atmosphere
radiation, ISCCP clouds and a radiative transfer model), but minima in the
eastern oceans are displaced to the west.
The NCEP system also has a stronger minimum in surface downward short
wave in the ITCZ. Downward short wave tends to increase along the coasts and
decrease further west in longer forecasts.
Differences in forecasts from 0-6h
Surface net solar radiation
Surface net solar radiation in the
analysis cycle resembles the climatological estimate for June-August (prepared
by NASA Langley based on satellite observations of top of the atmosphere
radiation, ISCCP clouds and a radiative transfer model), but minima in the
eastern oceans are displaced to the west.
The NCEP system also has a stronger minimum in surface net short wave in
the ITCZ. Net short wave tends to increase along the coasts and decrease
further west in longer forecasts.
Differences in forecasts from 0-6h
Net short wave in new model
A new global analysis/forecast system was implemented in the middle
of August 2001. Parallel tests of the
new and old systems in July 2001 showed that the new system has less of a
minimum in the ITCZ, in better agreement with the climatology, and has a weaker
minimum in the eastern subtropical Pacific.
The new system generally has more net short wave at the surface, in
worse agreement with the climatological estimate.
Langley
climatological estimate
Differences
between 0-6 hr and day 5 forecasts in old and new systems
Surface net radiation
Surface net radiation in the analysis
cycle resembles the climatological estimate for June-August (prepared by NASA
Langley based on satellite observations of top of the atmosphere radiation, ISCCP
clouds and a radiative transfer model), but minima in the eastern oceans are
displaced to the west. The NCEP system
also has a stronger minimum in surface net radiation in the ITCZ. Net radiation
tends to increase along the coasts and decrease further west in longer
forecasts.
Differences in forecasts from 0-6h
TOA OLR
The operational analysis cycle has more
top of the atmosphere out-going long wave radiation than the satellite
measurements; the greatest difference is in the eastern Pacific where the
operational system has too few stratus clouds and too much clearing. This may also imply that long wave cooling
is too low in the model atmosphere in the eastern Pacific.
Difference
between satellite and 0-6h
Vertical motion
The analyses and 1 day forecasts for
JJA 2001 show very intense rising motion over Central America and intense
sinking over the adjacent ocean at both 500 and 850 mb. The coastline at 25N is at 110W and at 10S
is at 77.5W. Longitude-height plots
show very intense low-level vertical circulations with rising over land and
sinking over the adjacent ocean. Strong
sinking can be seen in the regions where observations show low-level stratus
clouds next to the coast. In regions of
low level stratus clouds one would expect to see ascent in the boundary layer
with descending motion above; however, the cross-sections show no rising motion
in the boundary layer near 25N, 120W and 10S, 82.5W. At 25N the rising over land intensifies in the forecasts.
25N
Horizontal divergence
Plots of horizontal divergence also
show the low level vertical circulation.
At 25N outflow from ascent over land is apparent at two levels--600 and
200 mb. At 10S vertical ascent over
land is not apparent and may be occurring at a more northern latitude. However, strong surface divergence can be
seen over the ocean near the coast where observations but not the NCEP global
system display low-level stratus clouds.
The above horizontal divergence fields
are all valid at 000 GMT, afternoon in the east Pacific. Thus the entire diurnal cycle is not
included. The following plots of the
horizontal divergence of 10 m surface winds are sampled four times a day in the
analysis cycle (6 hr forecasts) and twice a day in the forecasts. Low-level convergence over land is clearly
weaker here than at 925 at 000 GMT, suggesting that the diurnal cycle does
dampen the signal, but does not reverse it.
For the month of August, the following
plots examine horizontal divergence in the east Pacific separately for fields
valid at 000 GMT and 1200 GMT.
25N
|
000 GMT |
1200 GMT |
10S
|
000 GMT |
1200 GMT |
925 mb
|
000 GMT |
1200 GMT |
Precipitation
The pattern of precipitation in
short-range NCEP global forecasts clearly resembles that found in satellite
estimates for JJA 2001 (OPI) and in satellite-based climatological estimates
for June-August, but the NCEP global model produces much more precipitation
over Central America than OPI and somewhat more precipitation than the
independent climatologies. Central
America suffered drought conditions during the summer of June-August 2001.
Vertical profiles
Vertical profiles from the NCEP model
of temperature and relative humidity in the regions where stratus clouds are
produced in the real atmosphere, but not the NCEP model atmosphere display low-level
inversions in temperatures and low-level maxima in relative humidity. The forecasts generally act to weaken or
dilute these features, but do not damp them very much. The forecasts do warm the lower atmospheric
layers, perhaps reflecting adiabatic warming associated with descending motion.
25N, 120W
10S, 82.5W
Temperature
Zonal asymmetric temperatures
The longitude-pressure cross-sections
of temperature with the zonal mean removed show an intense gradient of
temperature over the coast of Central and South America and a tendency over the
ocean next to the coast for cold low-level temperatures to be overlain with
warm temperatures.
Errors
In the stratus clouds regions, the NCEP global model displays warming
below 900 mb in the first 5 days of the forecasts and cooling above 800
mb.
925 mb
700 mb
Longitude-pressure cross-sections show
substantial low-level warming where the model produces too few stratus clouds.
Relative humidity
Longitude-pressure cross-sections of relative
humidity show a more and more shallow humid layer in the east Pacific as one
moves from west to east over the ocean.
A minimum in relative humidity can be seen over the ocean adjacent to
the coast. The minimum intensifies in
the forecasts, while the forecasts try to deepen the moist layer to the west.
25N
10S
925 mb
Zonal wind
Low-level
westerly maxima can be seen along the coast at 25N and 10S, as part of the
local thermally direct low level circulations apparent in the vertical motion
and horizontal divergence. The
forecasts act to increase low-level westerlies over the oceans adjacent to the
coast.
25N
10S
On-shore flow can clearly be seen in
the eastern oceans, with convergence over land and divergence over the oceans.
