1.- Goal of TPW product

Total Precipitable Water (TPW) is the amount of liquid water, in mm, if all the atmospheric water vapour in the column were condensed. High values of TPW in clear air often become antecedent conditions prior to the development of heavy precipitation and flash floods. When high TPW values areas present a lifting mechanism and warm advection in low levels, heavy precipitation often occurs. These data can provide to forecasters an important tool for very short range forecasting. Within the SAF NWC context, the main goal is to provide TPW data in clear air pixel by pixel in image format for Nowcasting purposes. 

 

2.- TPW algorithm summary description

The algorithms assume the lower and mid troposphere as a thick layer at temperature Tair overlying a surface with skin temperature Tsfc. The brightness temperature T for a window IR channel observed at Zenith angle is a combination of Tsfc and Tair weighted by the channel transmittance , which has a contribution from the carbon dioxide absorption and water vapour absorption .  

T = Tsfc * + Tair * ( 1 - )                              (1)

Assuming the transmittance difference proportional to the water vapour content:

= EXP [ - ( +* PW ) ] * sec                       (2)

And approximating the transmittance, assuming low carbon dioxide absorption:

= 1 - * PW * sec                                       (3)

Two different approaches for the estimation of the TPW can be derived:

a)      When the Surface Temperature can be accurately obtained (over SEA), by subtracting IR10.8 and IR12.0 channels brightness temperature in Equation (1) we have:

T11 - T12 = ( Tsfc - Tair ) *

 By applying Equation (3), the algorithm becomes the Split-Window Difference Algorithm called SD algorithm:

TPW = A + B * [ (T11 - T12) / (Tsfc - Tair) ] * cos                                (4)

Tair is the SEVIRI channel IR13.4 brightness temperature and Tsfc has been obtained using a Split Window SST algorithm .

 

b)      When the Surface Temperature accuracy cannot be assured, using Equations (1) and (2) applied to the IR10.8 and IR12.0 channels and eliminating Tsfc, the difference in brightness temperature is related to the differential absorption projected along the Zenith angle:

( T11 - Tai r)  /  ( T12 - Tair ) = EXP [ ( + * PW ) * sec ]

The resulting algorithm  for Total Precipitable Water over LAND called Logarithm Ratio or LR Algorithm is:

TPW = A + B * LN [ (T11 - Tair) / ( T12 - Tair) ] * cos                                    (5)

The Logarithm Ratio Algorithm has been proved to be good for land pixels and the Split-Window Differences Algorithm for sea pixels. Different coefficients have been obtained for land day/night, probably due to the differences in land surface temperature, and only one set of coefficients for sea. The algorithm used for sea makes use of a Split Window Sea Surface Temperature meanwhile a similar Surface Temperature for Land does not provide the sufficient accuracy without monitoring the emissivity.

In order to obtain these coefficients, datasets have been created using SEVIRI data matching radio-sounding data and ECMWF Numerical Model data. Three months of data (October, November and December 2003) has been used for the tuning.

In order to represent the seasonal evolution of the TPW values detected during the development phase with the NOAA AVHRR+HIRS and GOES GVAR prototypes, a seasonal factor has been applied to each algorithm.

 

3.- List of inputs for TPW
 

The input data to the TPW algorithm are described in this paragraph

Satellite imagery:

The following SEVIRI brightness temperature are needed at full IR spatial resolution:

T13.4 m	T12.0 m	T10.8m	T8.7m	T7.3m	T6.2m
Mandatory	Mandatory	Mandatory

The use of other SEVIRI channels (WV6.2, WV7.3, IR8.7) was preliminarily coded due to the good results obtained with SEVIRI simulated data. However, this approach consisting of adding linearly these satellite angle corrected channels have proved to be not useful for improving the algorithm performance when using real SEVIRI data. The current algorithm implementation foreseen the use of this information but the multiplicative coefficients are for the time being set to 0.  

The SEVIRI channels are input by the user in HRIT format and extracted on the desired region by SAFNWC software package.


Coefficients file:
 

• Coefficients table for LAND DAY/NIGHT and SEA.

             This file is included in the SAFNWC software package.

Angles associated to SEVIRI imagery:
 

• Satellite Zenith angle
• Sun Zenith angle

             This information is mandatory and computed by SAFNWC software package.

Auxiliary data:
 

• TPW from previous slot for quality control
• CMa product in SAFNWC/MSG format to define clears sky areas

             This information is mandatory and computed by SAFNWC software package.

Ancillary data sets:
 

Land/sea atlas
 

4.- Coverage and resolution

5.-Description of TPW outputs

        Range [0-7] reserved to:

            0       specific value for zenith angles > zenith angle threshold

            4       one band is missing

            6      TPW<0 or TPW>70 or algorithm calculation error

Range [8-127] reserved  for Cloud-free pixels containing:

             Total Precipitable Water with 119 levels from 0 to 70 mm.

 TPW (mm) = Scale * Counts _8bits + Offset

 Where:

                    Scale = 70/119

            Offset = -8*70/119

Range [128-255] reserved for cloudy pixels containing:

                     IR10.8 (configurable) SEVIRI degraded to 7 bits will be stored

• 1 bit to define the viewing conditions:

The TPW output will be an image with TPW data scaled in 70 values from 0 to 70 mm in clear areas and the enhanced IR10.8 SEVIRI channel brightness temperature over cloudy areas (configurable). The image displayed in Figure 1 has been obtained with SEVIRI data. Grey scaled areas with enhanced SEVIRI IR10.8 fit the cloudy areas and coloured areas are the TPW values following the bottom scale. 
 

Fig. 1 TPW from SEVIRI   11 MAR 2004 12:00:00 UTC

Click on thumbnail for full-size this image