AIRS science team meetingCamp Springs, 25-27 February 2003

Holger Vömel

University of Colorado

and NOAA/CMDL

Upper tropospheric humidity validation measurements

Overview

• The NOAA/CMDL frost-point hygrometer

• Sample profile

• Available data

• Issues• Hygrometer

• Vaisala Humicap data

• Snow White

• Summary

• Vertical Range:Lower troposphere to 28 km

• Sensitivity:Detection limit < 0.7 ppmvAccuracy : 10%

The NOAA frost-point hygrometer

Frost-point hygrometer data

• Frost-point instruments measure frost-point temperature. We use a saturation vapor pressure formulation (Goff Gratch) to calculate vapor pressure, RH, mixing ratio etc.

• All data shown here are as relative humidity with respect to liquid water.

Sample profile

Data files contain:

• Vaisala P + T

• Vaisala humicap RH

• Snow White FP temp

• NOAA FP temp

• ECC ozone mr

Available data

Location Instruments # soundingsSan Cristóbal, Galapagos FP/O3/SW/PTU 2

San Cristóbal, Galapagos PTU 19

San Cristóbal, Galapagos* O3/PTU 4

Hilo, HI FP/O3/SW/PTU 7

Huntsville, AL ** FP/O3/SW/PTU 2

Boulder, CO FP/O3/SW/PTU 1

Sodankylä, Finland*** FP/O3/SW/PTU 2*) SHADOZ data **) Simultaneous BBAERI observations ***) SAGE III validation sounding

All data are located at: /dom/files/ops/correl/point/YYYY/MM/val_voem/

Data issues: NOAA Hygrometer

Filtered data gaps :

• Gain change

• Mid level clear

• Frost-point controller not optimized for lower troposphere oscillations

Data issues: NOAA Hygrometer

Hygrometer data files contain ascent and descent data.

Time stamp tells, whether the data are ascent or descent.

In some cases there are significant real differences between the ascent location and the descent location.

Data issues: Vaisala humicap

Vaisala UTH issues:

• Two Humicap types:Humicap-H: Most common type Humicap-A: Old sensor (military still uses it)

• Time lag! Important at T < -40°C

• Contamination of the sensor. Important issue for sondes manufactured before 2002. New sensor protective cap introduced in 2001 improves this potential source of error.

• Dry bias (for humicap type-A). Only few of these sondes are sold Not an issue for AIRS validation measurements.

The Meteolabor Snow White sonde

• Vertical Range:Lower troposphere to upper troposphere

• Accuracy: 0.1°C (company spec.)

Data issues: Snow White low RH limit

Snow White may loose frost-coverage on the mirror

Loss of frost-point control

• May affect the entire profile

Data issues: Snow White low RH limit

Snow White may loose frost-coverage on the mirror

Loss of frost-point control

• May affect the entire profile or shallow layers after the dry layer

• RH min ≥ 5-8 %

Data issues: Dew vs. ice ambiguity

Humidity differences at low altitudes

Data issues: Dew vs. ice ambiguity

Humidity differences at low altitudes

Due to the ambiguity of ice or dew coverage of the mirror

Need to use different vapor pressure curve

Snow White upper tropospheric limit

Upper tropospheric limit depends on frost-point temperature

At TFP < -75°C the Snow White does no longer measure reliably

• NOAA/CMDL frost-point hygrometer • measures accurately up to the middle stratosphere

• Few data gaps due to oscillations, mirror clear, gain change

•Vaisala humicap sensor• Assume only Vaisala Humicap-H is used

• Time lag is an important issue in the upper troposphere (T < -40°C)

• Contamination probably not a serious issue for newer sondes (new cap)

•Snow White• Has a lower limit of 5-8% RH

• Ambiguity of ice/dew T > -25°C

• Limit to tropospheric measurements with TFP > -75°C

•All data files have a field that contains the ‘Best MR’, which combines the best data from the three sensors into one field.

Summary

NOAA/CMDL/MLO AIRS Validation

Mauna Loa Lidar, Water Vapor

0

2

4

6

8

10

12

14

16

0 25 50 75 100 125

Relative Humidity, Ice (%)

Alt

Ab

ov

e S

ea

Le

ve

l (k

m)

2002/10/11, clear

2002/10/2, cirrus 15 km

2002/9/9, cirrus 16 km

Three examples of water vapor observations, integration time 2 hours for Upper Trop, cirrus measured with lidar aerosol channels at 532 and 1064 nm.

NOAA/CMDL/Mauna Loa Observatory Validation of AIRS for First Year

Proposed Actual

Water Vapor

Raman Lidar 14 17

Radiosonde 7 7

Frost-Point 5 for lidar calibration 2 for lidar calibration3 for AIRS Validation

Temperature

CMDL Radiosonde 7 7

Ozone

Ozonesonde 7 7

Temperature note: The NWS releases daily radiosondes at 2:00 AM and PM local time which measure accurate temperature profiles.

PWV and Cloud Conditions

MLO Lidar, Water Vapor above 4.1 km

0

1

2

3

4

5

200 220 240 260 280 300 320 340

Julian Day

PW

V (m

m)

Clear

Subvisible Cirrus

Visible Cirrus

SnowWhite - NOAA comparisons:Dry limit

SnowWhite - NOAA comparisons:Dry limit

Data issues: Humicap-A dry bias

SnowWhite - NOAA comparisons:Dry limit