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.

 

0-6h and nephanalyses for JJA01

Difference between 0-6h and nephanalyses

 

0-6 h and nephanalyses at 25N and 10S

 

0-6h and 15 d clouds

1 day and 3day fcsts minus 0-6h fcsts

5 day fcsts minus 0-6h fcsts

10 day and 15day fcsts minus 0-6h fcsts

 

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.

 

0-6h and nephanalyses for JJA01

Difference between 0-6h and nephanalyses

 

0-6 h and nephanalyses at 25N and 10S

 

0-6h and 15 d clouds

1 day and 3day fcsts minus 0-6h fcsts

5 day fcsts minus 0-6h fcsts

10 day and 15day fcsts minus 0-6h fcsts

 

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

 

0-6 hr

4-5d

Differences between new and old

 

Low level

 

0-6 hr

4-5d

Differences between new and old

 

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.

 

0-6 hr and 15 day forecasts

Langley climatological estimate

 

Differences in forecasts from 0-6h

 

1 and 3 day forecasts minus 0-6h

5 and 10 day forecasts minus 0-6h

15 day forecasts minus 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.

 

0-6 hr and 15 day forecasts

Langley climatological estimate

 

Differences in forecasts from 0-6h

 

1 and 3 day forecasts minus 0-6h

5 and 10 day forecasts minus 0-6h

15 day forecasts minus 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

 

0-6h forecasts

Day 5 forecasts

Day 10 forecasts

 

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.

 

0-6 hr and 15 day forecasts

Langley climatological estimate

 

Differences in forecasts from 0-6h

 

1 and 3 day forecasts minus 0-6h

5 and 10 day forecasts minus 0-6h

15 day forecasts minus 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.

 

850 mb

 

500 mb

 

25N

 

Analyses and 1 day forecasts

3 and 5 day forecasts

1 and 3 day differences from analyses

 

10S

 

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.

 

925 mb

25N

10S

 

          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.

 

10m 0-6 and 0-24 h

10m 2-3 and 4-5 day

10m 9-10 and 14-15 day

0-24 h and 48-72h minus 0-6 h

96-120h minus 0-6 h

9-10d and 14-15d minus 0-6 h

 

          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

Analyses and 1 day fcsts

Analyses and 12 hr fcsts

3 and 7 day fcsts

2.5 and 6.5 day fcsts

10 and 15 day fcsts

9.5 and 14.5 day fcsts

1 and 3 day fcst minus anl

12 and 60 hr fcst minus anl

5 and 7 day fcst minus anl

4.5 and 6.5 day fcst minus anl

10 and 15 day fcst minus anl

9.5 and 14.5 day fcst minus anl

 

10S

 

000 GMT

1200 GMT

Analyses and 1 day fcsts

Analyses and 12 hr fcsts

3 and 7 day fcsts

2.5 and 6.5 day fcsts

10 and 15 day fcsts

9.5 and 14.5 day fcsts

1 and 3 day fcst minus anl

12 and 60 hr fcst minus anl

5 and 7 day fcst minus anl

4.5 and 6.5 day fcst minus anl

10 and 15 day fcst minus anl

9.5 and 14.5 day fcst minus anl

 

925 mb

 

000 GMT

1200 GMT

Analyses and 1 day fcsts

Analyses and 12 hr fcsts

3 and 7 day fcsts

2.5 and 6.5 day fcsts

0 and 15 day fcsts

9.5 and 14.5 day fcsts

1 and 3 day fcst minus anl

12 and 60 hr fcst minus anl

5 and 7 day fcst minus anl

4.5 and 6.5 day fcst minus anl

10 and 15 day fcst minus anl

9.5 and 14.5 day fcst minus anl

 

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.

 

0-6 and 0-24 hr forecasts

OPI

CMAP climatology

GPCP climatology

 

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

 

Temperature

Temperature error

Relative humidity

Relative humidity error

 

 

10S, 82.5W

 

Temperature

Temperature error

Relative humidity

Relative humidity error

 

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.

 

25N

 

10S

 

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

 

1 and 3 day

5 day

10 and 15 day

 

700 mb

 

1 and 3 day

5 day

10 and 15 day

 

          Longitude-pressure cross-sections show substantial low-level warming where the model produces too few stratus clouds.

 

25N

10S

 

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

 

Analyses and 15 day forecasts

1 and 3 day forecast errors

5 day forecast error

 

10S

 

Analyses and 15 day forecasts

1 and 3 day forecast errors

5 day forecast error

 

925 mb

 

Analyses and 15 day forecasts

1 and 3 day forecast errors

5 day forecast error

 

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

 

Analyses and 5 day forecasts

1 and 3 day forecast errors

5 day forecast error

 

10S

 

Analyses and 5 day forecasts

1 and 3 day forecast errors

5 day forecast error

 

925 mb streamfunction

 

          On-shore flow can clearly be seen in the eastern oceans, with convergence over land and divergence over the oceans.