# Presentation: 203065

## Changes in Total Precipitable Water Vapor – Infrared(MOD-05)

**Suzanne Wetzel-Seemann, Jun Li, Liam Gumley, **
**and Paul Menzel**
**22 Jul 2002**

## Earth emitted spectra overlaid on Planck function envelopes

- H20

- CO2

- O3

- CO2

**Notes:**
This slide shows an observed infrared spectrum of the earth thermal emission of radiance to space.  The earth surface Planck blackbody - like radiation at 295 K is severely attenuated in some spectral regions.  Around the absorbing bands of the constituent gases of the atmosphere (CO2 at 4.3 and 15.0 um, H20 at 6.3 um, and O3 at 9.7 um), vertical profiles of atmospheric parameters can be derived.  Sampling in the spectral region at the center of the absorption band yields radiation from the upper levels of the atmosphere (e.g. radiation from below has already been absorbed by the atmospheric gas); sampling in spectral regions away from the center of the absorption band yields radiation from successively lower levels of the atmosphere.  Away from the absorption band are the windows to the bottom of the atmosphere. Surface temperatures of 296 K are evident in the 11 micron window region of the spectrum and tropopause emissions of 220 K in the 15 micron absorption band.  As the spectral region moves toward the center of the CO2 absorption band, the radiation temperature decreases due to the decrease of temperature with altitude in the lower atmosphere.
IR remote sensing (e.g. HIRS and GOES Sounder) currently covers the portion of the spectrum that extends from around 3 microns out to about 15 microns. Each measurement from a given field of view (spatial element) has a continuous spectrum that may be used to analyze the earth surface and atmosphere.  Until recently, we have used “chunks” of the spectrum (channels over selected wavelengths) for our analysis. In the near future, we will be able to take advantage of the very high spectral resolution information contained within the 3-15 micron portion of the spectrum.  From the polar orbiting satellites, horizontal resolutions on the order of  10 kilometers will be available, and depending on the year, we may see views over the same area as frequently as once every 4 hours (assuming 3 polar satellites with interferometers).  With future geostationary interferometers, it may be possible to view at 4 kilometer resolution with a repeat frequency of once every 5 minutes to once an hour, depending on the area scanned and spectral resolution and signal to noise required for given applications. 

## 	Total Water Vapor Retrieval from MODIS IR Radiances

- 	                                            ps
- 	I  =  sfc B(T(ps)) (ps) -     B(T(p)) [ d(p) / dp ]  dp .
- 	                                           o
- 	I1, I2, I3, .... , In are measured with MODIS
- 	P(sfc) and T(sfc) come from ground based conventional observations
- 	(p) are calculated with physics models
**	**
**	**Regression relationship is inferred from (1) global set of in situ radiosonde
- 	    reports, (2) calculation of expected radiances, and (3) statistical non-linear
- 	    regression of observed Raob TPW and calculated MODIS radiances
- 	   (brightness temperatures)
**	**
**	**Need RT model, estimate of sfc, and MODIS radiances

##            Mitigating Problems in MODIS TPW Algorithm

**           ****Mitigating Problems in MODIS TPW Algorithm**

- * Changed predictors band 24 and 25 (4.4 and 4.5 um) brightness temperatures to their BT difference to remove surface effects:	                    	Old algorithm had 12 predictors:
- 		individual bands 24, 25, and 27 through 36.
- 	New algorithm has 11 predictors:
- 		band 25 - 24 BT difference and
- 		individual bands 27 through 36.
- *  Separated training into seven regression BT zones to include a broader range of moisture regimes
- * Implemented global radiance bias corrections.
- * Applied post-launch NEdT in place of pre-launch.

- December 18, 2001               						         MODIS Science Team Meeting

**Notes:**
1) Changed the predictors used for the statistical regression: The operational algorithm uses 12 individual infrared 
bands, with wavelengths 4.4-14.2m (MODIS bands 24,25,27-36).  The new algorithm uses these same 12     
bands but uses the difference between the 4.4 and 4.5m bands (MODIS 24, 25) instead of the individual bands.
2) Separated the statistical training data set into 5 brightness temperature classes: < 254, 254-275, 275-286, 286-
297, > 297 oK.  For example, a retrieval for a scene with MODIS brightness temperature < 254 oK uses only the 
subset of the training data for which a forward model calculation from the profile predicts a brightness 
temperature < 254 oK.
3) Updated the estimates of NEdT: Operational NEdT was based on pre-launch values.  New, post-launch estimates were computed and are used in the new algorithm.
4) Implemented a radiance bias correction: The operational MODIS TPW algorithm does not include any radiance 
bias correction.  The new radiance bias adjustments account for errors introduced in both the measurements and the algorithm, including the radiative transfer calculations.  

## Previous MOD07 v3.0 (left) and new MOD07 v3.1.0 (right)

- for the north African deserts on June 2, 2001 at 0830-1155 UTC

##         MODIS new

**        ****MODIS new**
**            ****TPW**
**20 Aug 2001 18 UTC**
-                  **GOES -8**** **

**MODIS wo BT11 **
**partitioning**

## Average (Observed-Calculated) brightness temperature for MODIS IR bands 24, 25, 27-29, and 31-36 from 60 clear sky cases at the SGP ARM-CART site from April 2001 to June 2002.  Red bars indicate radiance calculations used skin temperature observed by the IRT; regression-derived skin temperature was used for the calculated radiances in the blue bars.  No bias is computed for band 30 because of insufficient ozone observations for input to the forward model 

- Average (Observed-Calculated) brightness temperature for MODIS IR bands 24, 25, 27-29, and 31-36 from 60 clear sky cases at the SGP ARM-CART site from April 2001 to June 2002.  Red bars indicate radiance calculations used skin temperature observed by the IRT; regression-derived skin temperature was used for the calculated radiances in the blue bars.  No bias is computed for band 30 because of insufficient ozone observations for input to the forward model

## The global biases are separated into twelve groups: 

- six latitude zones:
- 	north tropical (latitude 0o to +30o),
- 	south tropical (0 o to -30o),
- 	north mid-latitude (+30o to +50o),
- 	south mid-latitude (-30o to -50o),
- 	north polar (50o to 90o),
- 	south polar (-50o to -90o),
- each for land and ocean.
- The average global biases for north mid-latitude land agree fairly well with the CART site biases.
- Bias values may vary seasonally so the bias corrections calculated from four days in June may need to be updated.

## Comparison of TPW from MODIS regression (red dot), GOES-8 (blue diamonds), and radiosonde (black cross) with SGP ARM-CART microwave radiometer (MWR) in mm for 64 cases from April 2001 to June 2002 

- Comparison of TPW from MODIS regression (red dot), GOES-8 (blue diamonds), and radiosonde (black cross) with SGP ARM-CART microwave radiometer (MWR) in mm for 64 cases from April 2001 to June 2002

## TPW (mm) for 02 June 2001 over North America retrieved by MODIS regression (left), MODIS physical (center), and GOES-8 and GOES-10 (combined, right).  Day on top row and night on bottom row.

## MODIS TPW (top) and SSM/I f-14 TPW (bottom) on 22 May 2002.   Ascending and descending passes were averaged. 

- MODIS TPW (top) and SSM/I f-14 TPW (bottom) on 22 May 2002.   Ascending and descending passes were averaged.

**Notes:**
The SSM/I (resolution 12.5 km) retrieves products for clear or cloudy skies over ocean only, and uses the 22 and 37 GHz microwave channels.  MODIS and SSM/I show similar patterns of TPW distribution and similar magnitudes, however MODIS retrievals are somewhat less moist over tropical oceans.  Some of the differences can be attributed to the fact that MODIS does not retrieve cloudy pixels and, thus, does not capture the moist environment around clouds.  This can affect the results even where MODIS retrievals were performed since the retrieval only requires that 5 of the 25 pixels in a 5x5 MODIS field-of-view area be clear.  The remaining cloudy pixels are excluded, however the retrieval is still performed using only the clear pixels.  Other differences may be attributed to the time differences between the two satellite overpasses.  

## MODIS (top) and TOMS (bottom) total column O3 (Dobson units) for 02 June 2001

- MODIS (top) and TOMS (bottom) total column O3 (Dobson units) for 02 June 2001

**Notes:**
MODIS total column ozone retrievals are compared with ozone from the NASA/GSFC Total Ozone Mapping Spectrometer (TOMS) (Bowman and Krueger 1985; McPeters et al. 1996, 1998) ozone measurements from the Earth Probe (EP) satellite for 22 May 2002.  The general distribution of ozone from TOMS is similar to that from MODIS.  In order to predict the evolution of ozone on time scales of a few days to a week, reliable measurements of ozone distribution are needed.  However, the TOMS instrument measures backscattered ultraviolet solar radiation and can not provide measurements at night.  High spatial resolution IR radiance measurements at 9.6m from MODIS allow ozone estimates during both day and night.