SSMIS Radiance Monitoring Statistics for the ACCESS NWP Systems


Please note that on 25 April 2013, SSMIS instrument suffered a failure of its 56.4GHz oscillator and hence would no longer be monitored.



LINK TO GLOBAL SSMIS RADIANCE MONITORING PLOTS


Sensor Overview

Special Sensor Microwave Imager (SSMIS) instrument was first launched aboard the DMSP-F16 platform. The instrument is currently flown aboard the DMSP-F16, -F17 and -F18 platforms, with follow-on SSMIS instruments planned for launch on two additional DMSP platforms: F-19 and F-20. The primary mission of the SSMIS instrument is to combine and extend the imaging and sounding capabilities of three previously separate DMSP microwave sensors: the SSM/T-1 temperature sounder, the SSM/T-2 moisture sounder, and the SSM/I. With improved temperature sounding capabilities, the SSMIS is capable of profiling the mesosphere (10 to 0.03 hPa). As such, the SSMIS is currently the only operational passive microwave sensor that can collect temperature measurements in the 40 to 80 km altitude range. In addition, SSMIS offers capabilities associated with radiometer channels having common fields of view, uniform polarizations, and fixed spatial resolutions across the active scene scan sector, making it thus far the most complex and unique operational satellite passive microwave imager/sounder flown.

In the ACCESS system, only SSMIS data from DMSP-F16 satellite are assimilated for the time being.

Table 1: Channel characteristics of SSMIS sensor.

Channel Number

Channel Centre Frequency (GHz)

Polarization

Channel Application

Foot Print (km)

Sensitivity

1

50.3

V

LAS

37.5

Surface

2

52.8

V

LAS

37.5

Temperature

3

53.596

V

LAS

37.5

Temperature

4

54.4

V

LAS

37.5

Temperature

5

55.5

V

LAS

37.5

Temperature

6

57.29

RCP

LAS

37.5

Temperature

7

59.4

RCP

LAS

37.5

Temperature

8

150

H

IMA

12.5

Humidity

9

183.31+/-6.6

H

IMA

12.5

Humidity

10

183.31+/-3

H

IMA

12.5

Humidity

11

183.31+/-1

H

IMA

12.5

Humidity

12

19.35

H

ENV

25

Surface

13

19.35

V

ENV

25

Surface

14

22.235

V

ENV

25

Surface

15

37

H

ENV

25

Surface

16

37

V

ENV

25

Surface

17

91.655

V

IMA

12.5

Surface + humidity

18

91.655

H

IMA

12.5

Surface + humidity

19

63.283248 +/- 0.285271

RCP

UAS

75

Temperature

20

60.792668 +/- 0.357892

RCP

UAS

75

Temperature

21

60.792668 +/- 0.357892 +/-0.002

RCP

UAS

75

Temperature

22

60.792668 +/- 0.357892 +/-0.0055

RCP

UAS

75

Temperature

23

60.792668 +/- 0.357892 +/- 0.016

RCP

UAS

75

Temperature

24

60.792668 +/- 0.357892 +/- 0.050

RCP

LAS

37.5

Temperature

 

*As indicated in Table 1, the SSMIS channels correspond to four main categories of measurement parameters:

Lower Atmospheric Sounding (LAS)
LAS temperature channels (1 7) provide weighting functions spanning those of SSM/T-1 and AMSU-A1 (Advanced Microwave Sounder Unit, Temperature). The moisture channels (8 11) coincide with SSM/T-2 frequencies of 150, and 183 ±6.6, ±3, and ±1 GHz. There is also a close correspondence with AMSU-B moisture channel frequencies. Polarization remains constant throughout scans.

Upper Atmospheric Sounding (UAS)
The SSMIS Upper Atmospheric Sounding (UAS) channels (19 24) utilize the oxygen absorption line complex near 60 GHz. In order to have sufficient absorption and emission to retrieve atmospheric temperature above 40km, the SSMIS UAS channel passbands are collocated with several of the strongest transitions within the 60 GHz oxygen complex. The channel bandwidths are very narrow (~1 30 MHz) in order to achieve reasonable vertical resolution at the required altitude.

Environmental (ENV)
The SSMIS offers five Environmental channels (window channels 12 16) to continue the capability of the DMSP Special Sensor Microwave Imager (SSM/I) in monitoring and retrieving atmospheric and surface parameters such as precipitation, sea ice, ocean surface wind speed, land surface temperature & emissivity, and soil moisture.

Imaging (IMA)
91.655 GHz imaging channels in SSMIS replaces the 85.5 GHz channels in SSM/I and has different resolution and spectral frequency to the 85.5 GHz SSM/I channel. Additionally, the SSMI/S added a new 150 GHz channel plus three humidity profiling channels, which consists of pairs of narrow passbands on either side of the H2O absorption line centre at 183.3 GHz similar to those on the Advanced Microwave Sounding UnitB/ Microwave Humidity Sounder (AMSU-B / MHS) instruments.


Channel Selection

Channel selection are done in ACCESS 4DVAR to guarantee that the assumed linear relationship between brightness temperature and atmospheric temperature are met. In the current configuration, SSMIS channels 1, 8, 19, 20, 21, and 22 are not assimilated in ACCESS. Channel 1 is omitted because its weighting function peaks very low (~ 1 km altitude) and hence is highly sensitive to the surface. The surface sensitive channels are usually eliminated because of the model uncertainties in background skin temperatures over land, snow and ice. Further, a microwave land surface emissivity model is not implemented in the RTTOV-7, the fast RT model used in this study, to account for the land surface variability in space and time. Channel 8 is also sensitive to surface and in addition exhibits a high sensitivity to scattering from ice/solid hydrometeors. Channels from 19-22 have their weighting function peaks in the mesosphere (~40-80 Km). At this layer of atmosphere, radiative transfer (RT) modelling is more difficult because of the interaction of oxygen molecule absorption spectrum with geomagnetic field; which leads to Zeeman splitting of the oxygen absorption lines. This will effectively shift the peak of weighting functions in altitude depending on geomagnetic field. Thus, in the data assimilation of radiances from this high peaking SSMIS channels, brightness temperatures and their Jacobians (or gradient with respect to temperature) must be computed with a fast RT scheme that takes into account the Zeeman-splitting effect. RTTOV-7, however does not include the variation of the Zeeman effect with magnetic field strength for this high peaking SSMIS channels. Only a constant correction factor is included, which is independent of latitude and view angle. This may not remove all the associated variability of transmittance due to the geomagnetic field. Hence as a cautionary step, these channels are not considered for assimilation.

Further, channels are rejected by OPS if it is "MW Cloudy". i.e., when cloud liquid water exceeds a specified threshold.



Disclaimer: These plots are experimental. The Bureau of Meteorology accepts no responsibility for actions taken on the basis of these monitoring plots.


For queries, please contact:

Dr. Vinod Kumar
Centre for Australian Weather & Climate Research
Bureau of Meteorology
Melbourne, Victoria - 3008
Australia.
Mail to Vinod Kumar