CHAPTER 1 Incremental
formulation of 3D/4D variational assimilation-an overview
CHAPTER 2 3D variational assimilation
CHAPTER 3 4D variational assimilation
CHAPTER 4 Background term
CHAPTER 5 Conventional observational
constraints
CHAPTER 6 Satellite observational
constraints
CHAPTER 7 Background, analysis
and forecast errors
CHAPTER 8 Gravity-wave control
CHAPTER 9 Data partitioning (OBSORT)
CHAPTER 10 Observation screening
CHAPTER 11 Analysis of snow
CHAPTER 12 Land surface analysis
CHAPTER 13 SST and sea-ice analysis
CHAPTER 14 Reduced-rank Kalman filter
REFERENCES
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The way the synoptic surface stations report mass observations (pressure
or geopotential height) is considered as bad if the
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• station altitude is above 800 m and station
reports mean sea level pressure |
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• station altitude is above 800 m and station
reports 1000 hpa level |
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• station altitude is above 1700 m and
station reports 900 hpa level |
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• station altitude is below 300 m and station
reports 900 hpa level |
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• station altitude is above 2300 m and
station reports 850 hpa level |
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• station altitude is below 800 m and station
reports 850 hpa level |
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• station altitude is above 3700 m and
station reports 700 hpa level |
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• station altitude is below 2300 m and
station reports 700 hpa level |
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• station altitude is below 3700 m and
station reports 500 hpa level |
The reporting practice is also considered as bad if the station reports
500 gpm, 1000 gpm, 2000 gpm, 3000 gpm or 4000 gpm level pressure, respectively,
and station altitude is more than 800 m different from the reported level.
For temp geopotentials the reporting practice is considered as bad if the
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• station altitude is above 800 m and station
reports 1000 hpa level |
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• station altitude is above 2300 m and
station reports 850 hpa level |
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• station altitude is above 3700 m and
station reports 700 hpa level |
The background quality-control rejection limits are applied more strictly
for some observation types than stated in Table 10.1. The special cases are the following
ones
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• airep wind observations with zero wind
speed are rejected if the background wind exceeds 5 m/s |
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• for airep and dribu wind observations
the rejection limit is multiplied by 0.5, and for pilot wind by 0.8 |
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• for satob wind observations the rejection
limit is multiplied by 0.1, except below 700 hPa level where it is
multiplied by 0.2 |
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• no background quality control is applied
for scatt winds |
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• for dribu surface pressure observations
the rejection limit is multiplied by 0.9, and for paob surface pressure
by 0.7 |
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• for airep temperature observations the
rejection limit is multiplied by 1.6 |
This appendix describes those parts of the ECMWF assimilation system which
involves some special code for the AMW case, i.e. the data selection and
the FG quality check. It refers to the operational status as from December
1996. A thinning procedure was introduced for high-density winds in Spring
1998.
There are several model independent checks which AMW data have to pass in
order to be considered for the assimilation process:
Check on longitude/latitude
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• AMW must be within a circle of 55°
from the sub-satellite point |
Check on levels depending on the computational method
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• WW CMW and WVMW must be above 400 hPa |
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• VIS CMW must be below 700 hPa |
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• IR CMW can be used at all levels. |
Check on land/sea
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• All AMW over sea are used |
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• AMW over land is not used north of 20ºN.
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• For Meteosat (0º mission) instead
of 20ºN this threshold is 35ºN to allow usage of AMW over
north Africa. |
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• For Meteost (63º mission) the use
of AMW has been extended over Asia if above 500 hPa. This is restriced
for longitudes east of 30ºE. |
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• AMW are blacklisted over the Himalayas
as a precautionary measure. |
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• AMW over land south of 20ºN (35ºN
for Meteosat) is used if above 500 hPa. |
Check on satellite (35ºN for Meteosat) is used if above 500 hPa.
This is a temporary selection on certain channels or satellites. At present
channels and satellite used are:
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• METEOSAT cloud tracked winds with 90
min temporal sampling |
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• METEOSAT IR (not at medium level), VIS,
WV |
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• METEOSAT HVIS, also at asynoptic times,
only if  ( |
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• GOES IR & WV (NOT at asynoptic times) |
The background quality check is based on a comparison of the AMW deviation
from the background. Observed wind components are checked together. The
AMW is flagged with j = 1 or 2 or 3 if this deviation squared is
greater than a predetermined multiple ERRLIM * ZREJMOD of its estimated
variance, as given by the following expression:
if [ D2 > (sfg 2 + sobs 2 ) * ERRLIMj * ZREJMOD] then flag= j
where D 2 = 1/2 (Du2 +D v2) with Du, Dv wind component deviations from background;
sfg std of the background wind component error (mean for u and v);
sobs std of the observation wind component error, 2  for levels below 700 hPa included, 3.5  at 500 hPa, 4.3  at 400 hPa and 5  for all levels above; ERRLIMj is 8 for j=1, 18 for j=2 and 20 for
j=3. The value of ZREJMOD depends on the level of AMW and normally its value
is:
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• ZREJMOD = 0.2 for low level |
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• ZREJMOD = 0.1 for all others levels |
A special check or asymmetric check is applied when the observed speed is
more than 4  slower than the background speed SPDfg. This
check has a more restrictive rejection limit:
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• ZREJMOD = 0.15 at low level |
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• ZREJMOD = 0.07 in the tropics |
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• ZREJMOD = 0.075 - 0.00125 * SPDfg all
others |
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• ZREJMOD = 0.0 if SPDfg > 60  (observation gets always flag j = 3) |
When the data is passed to the following variational quality control its
probability of being used depend on the
flag j. With flag j = 1 the data will be assimilated, with
flag j =2 it will be given an intermediate probability and might
be used or not and finally the analysis will reject all data with j
= 3
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