Method for the Validation of Precipitable Water (WVAPOR): 

SHEBA Radiosonde (GLAS) relative humidities were converted to specific humidities and integrated over the Path-P WVAPOR layers.  Daily averages are computed.  Path-P WVAPOR (precipitable water) amounts  are spatial averages of all retrieval footprints of an an overpass whose centers are within a radius of 50 km from the SHEBA camp. Spatial averages  (varying numbers per day) are then averaged in time to compute daily averages. Results are shown for NOAA-14 only. The analysis was conducted using Level 2 (orbital geophysical data) but minimize geolocation errors but the error statistics should be representative for Level 3 data (Gridded Path-P products). Tables below show the Root Mean Square Error (RMSE), the correlation coefficient (R) and the Mean Error (Radiosonde - TOVS). Coefficients for a linear fit (PWTovs=a+b*PWRadiosonde) are also shown.  Time series are shown as a 5-day running mean. 

Precipitable Water
(300-400 mb)
Variable:   WVAPOR
Level:  300 -400 mb
RMSE: 0.0494
R 0.814
Mean Error (R-T) -0.01
Fit Line (a,b) 0.03,0.077

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.
Precipitable Water
(400-500 mb)
Variable:   WVAPOR
Level:  400 -500 mb
RMSE: 0.123
R 0.80
Mean Error (R-T) -0.01
Fit Line (a,b) 0.075,0.703

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.
Precipitable Water
(500-700 mb)
Variable:   WVAPOR
Level:  500 -700 mb
RMSE: 0.66
R 0.77
Mean Error (R-T) 0.005
Fit Line (a,b) 0.36,0.692

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.
Precipitable Water
(700-850 mb)
Variable:   WVAPOR
Level:  700 -850 mb
RMSE: 0.82
R 0.83
Mean Error (R-T) 0.16
Fit Line (a,b) 0.35,0.70

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.
Precipitable Water
(850mb - Surface)
Variable:   WVAPOR
Level: 

850mb-Surface 
RMSE: 1.02
R 0.88
Mean Error (R-T) 0.39
Fit Line (a,b) 0.05,0.81

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.
Precipitable Water
(Total Column)
Variable:   WVAPOR
Level: 

Total Column 
RMSE: 2.2
R 0.88
Mean Error (R-T) 0.33
Fit Line (a,b) 0.7,0.82

Scatterplot and time series precipitable water from SHEBA radiosondes  and from corresponding Path-P retrievals.