# Presentation: 431505

## Water Vapor Comparisons

- Implications for Satellite Validation
- 			Karen H. Rosenlof
- 			NOAA Aeronomy Laboratory
- 			Boulder, CO

*EOS Aura Science Team Meeting*
*Pasadena Convention Center*
*Feb 28-Mar 3, 2005*

## Two issues that can impact validation of satellite water vapor observations are:

- Spatial/temporal variability (related to the need for exact space/time matches)
- Absolute accuracy (related to how well different correlative measurements agree)

## Through examining past long-term records, we can get some idea of what the variability of water vapor is for a specific area.

- The longest continuous 1 station record we have for water vapor extending into the stratosphere is the CMDL frost point balloon in Boulder (S. Oltmans)
- Looking at seasonal variability estimated over 25 years of data gives some idea as to optimum times to sample for satellite validation.

## Slide 4

## Slide 5

## Summary: variability

- At NH midlatitudes, equinox is good time to find minimum variability over a large altitude region.  Minimum values are in Jan/Feb, and cover an altitude region that should be large enough for the satellite to resolve. Max values are in summer, but so is maximum variability.   It may be harder to do validation during summer in that the need for exact location/timing matches is likely to be more important.

## SPARC H2O Assessment (2000)

| Upper Troposphere | Stratosphere |
| --- | --- |
| MLS-UTH
SAGE-II
TOVS/HIRS UTH
MOZAIC
LASE (DIAL Lidar)
Vaisala RS80-A&H
CMDL Frostpoint
LMD Frostpoint
LaRC/ARC DLH
OHP Raman Lidar | HALOE
SAGE-II
POAM III
ILAS
MLS
ATMOS
MAS
MkIV
FIRS-2
WVMS
WASPAM
CMDL Frostpoint
LMD Frostpoint
MIPAS
FISH (Lyman-alpha)
AL (Lyman-alpha)
Harvard (Lyman-alpha)
JPL TDL |

**SPARC H****2****O Assessment (2000)**

***Topic 2: water vapor accuracy***

## Direct comparisons cluster within

- a 10% range in middle and upper
- stratosphere,with over 20%
- differences not uncommon in the
- lower stratosphere.
- Extreme range of direct
- comparisons exceeds 30%.
- Indirect comparisons differ
- by as much as 50%.
- (Figure 2.73 from WAVAS 2000)
- Largest % differences at the levels of
- smallest water vapor content.

## Balloon intercomparisons, 1978 & 1983

*From WMO Atmospheric Ozone, 1985*

## At low mixing ratios, differences still exist with current instruments

## Fall 1999, NASA WB-57F based in Houston, Ly- TDL, FP

*Fall 1999, NASA WB-57F based in Houston, Ly-**** TDL, FP*

## Kiruna, Sweden  (SOLVE)

## Close coincidence

**Close coincidence**

## Relative humidity

*WB-57F fights from 3S-5N, Jan 2004*
*Pitch angle < 3 deg,*
*Jan 30, 2004 SC Balloon,*

**~380K**

## Differences between instruments at low (stratospheric) mixing ratios are on the order of 1-2  ppmv.  

***Does this matter?***
- For trend analysis:  yes (can’t combine instrumental records)
- For degree of supersaturation in the UTH: yes
- For assessing mechanisms related to control of stratospheric entry levels:  yes
- For examining temporal and spatial variability: no
- For examining radiative transfer:  probably not

## Some comparisons with Aura

## Slide 17

## Slide 18

## Significant differences exist between water vapor instruments in the UTLS.  Many of these are planned to be used for Aura validation in the region.  Note, these differences are large enough that interpretations as to processes occurring in the atmosphere can change by simply using data from different instruments.

- Assessing the reasons for these differences and establishing what the accuracies of the measurements actually are should be a priority before using such measurements for validation.

## Possibilities to consider for fp/lyman-alpha differences

- Other species condensing on the mirror and changing the indicated frost point temperatures
- Other species (or aerosols) possibly fluorescing.
*Preliminary lab studies are starting, but real funding and a multi-institution study is really needed to assess accuracy in flight at low mixing ratio.*
**Current correlative instruments agree to 1-2 ppmv in the UTLS.  That may be good enough for satellite validation.  It’s not good enough for the study of certain processes,  and some resources should be invested in assessing why differences exist.**