1. UARS MLS Observations of Cloud Ice

A 205GHz radiometer has been flown on UARS MLS to measure O3, ClO and other constituent profiles in the middle atmosphere. UARS MLS has continuous observation periods between Septerber 1991 and November 1994, and sparse coverages in 1995-1997. In the window channels at ~203GHz, i.e., far away from major spectral lines, the limb radiances are contributed mostly by dry air and water vapor continuum emissions, and can be used to retrieve upper-tropospheric humidity [Read, et al., 2002]. Exessive radiances from the clear-sky background, called cloud-induced radiances, can be used to deduce cloud ice mass.  At high tangent heights, the cloud-induced radiances are proportional to a field-of-view averaged IWC near the tangent height [Wu et al., 2005], whereas at low tangent heights the cloud-induced radiances are proportional to a column of cloud ice (hIWP) along the slant path of MLS line-of-sight. Since MLS normally does not see through the limb at low tangent heights, these hIWPs represents a column above the saturation point, which may vary with frequency. Radiances that saturate at higher altitudes will measure higher columns of cloud ice.

Cloud-induced limb radiances

Ice clouds can significantly alter the microwave limb radiances at high or low tangent heights. At high tangent heights (e.g., 12-18km for 203GHz) cloudy radiances are warmer than normal clear-sky backgrounds (20-100K). At low tangent heights (e.g., 0-8km for 203GHz) cloudy radiances are colder than normal clear-sky backgrounds (180-260K). Separation of clear and cloudy radiances Two methods have been applied to compute cloud-induced radiance dTcir: one relies completely on the radiative transfer (RT) model for finding clear-sky radiance limits whereas the other is a purely statistical approach based only on the data themselves. Retrieval of ice water content (IWC) from cloud radiances We have developed a research algorithm to retrieve near-tropopause IWC from monthly averaged cloud radiances at 203GHz. Uncertainties in MLS IWC retrieval
The largest uncertainty for the IWC retrieval is associated with assumptions on particle size distribution and particle shape. Depending on the model parameters used, the retrieval IWC may be differed by a factor of 2-3.

2. Results from UARS MLS

In the following we present some UARS MLS results with emphasis on cloud ice measurement near the tropopause region. In particular, we are interested in MLS cloud ice in relation to H2O near the tropopause during the dry (January-March) and moist (July-September) periods as shown in MLS stratospheric H2O measurement.

UARS MLS ch61 radiances

Ch61 of UARS MLS has the best sensitivity to H2O at 100hPa among all the MLS channels. Because the H2O weighting function sharply peaks at the tangent pressure of the pointing, the ch61 radiance at 100hPa may be used as a proxy for H2O abundance at 100hPa. Large variability is seen in the monthly maps of 100hPa ch61 radiances.
Seasonal variations of cloud ice and H2O near 100hPa
There is a strong correlation in the time series between 100hPa cloud ice and 147hPa or 215hPa H2O variations but the correlation with the 100hPa H2O variation is poor.
Cloud ice maps for the dry (January-March) and moist (July-September) periods
Similar to the seasonal variations, the 100hPa cloud ice shows better correlation with 147 and 215hPa H2O than the ch61 proxy for H2O at 100hPa. Cloud IWC can reach 1-2mg/m3 in some overshooting regions at 100hPa, which is equivalent to 10-20ppmv in H2O. The H2O at 100hPa reveals some features that might be caused by horizontal transport.
Cloud ice amount and occurrence frequency at 100hPa in 1991-1997
Strong seasonal variations are associated with 100hPa cloud ice within narrow latitudinal bins but no significant annual and seasonal variations are found in total (30S-30N) zonal mean cloud ice at 100hPa during 1991-1997.

3. Summary

A research algorithm has been applied to UARS MLS radiance measurements to infer cloud ice content near the tropopause region. This approach takes advantage of microwave limb techniques at ~200GHz with adequate cloud sensitivity and penetration ability to retrieve cloud physical properties (IWC and occurrence frequency) from cloud-induce radiances. The retrieval uncertainty is primarily limited by poor knowledge about ice particle sizes and shapes. Further improvements in these research areas will make MLS cloud ice measurement more reliable.

The initial results of MLS cloud ice and H2O at 100hPa reveal many features of interest to water transport in the TTL region. MLS observations suggest that both convective and advective processes are likely moistening air at 100hPa, and the horizontal transport may be more important in distributing H2O at this level.

Although 100hPa cloud ice varies coherently with tropical convective activity at lower altitudes, there is no significant seasonal or annual variations in the total ice mass (30S-30N). On average, MLS observations show that the total zonal mean cloud ice at 100hPa are maintained at ~0.2mg/m3 level (or ~0.1 K in 203GHz radiance, or ~2 ppmv if converted to water vapor).

4. Future Work

We will develop and implement an algorithm to retrieve cloud ice from cloud-induced radiances at low tangent heights. Different from the radiances at high tangent heights, these radiances are also sensitive to ice particle scattering from some of the low (8-12km) clouds, which can be used to infer IWP down to ~5km in high ice density cases.

The upcoming Aura MLS experiment will provide more cloud data for this study. Beside the similar channels (in the 190GHz radiometer) to UARS MLS, radiances at other frequencies (near 118, 240, 640GHz) are also planned to use for the cloud studies.

References

Contact Information

Dr. Dong L. Wu
Email: dwu@mls.jpl.nasa.gov; Phone: 818-393-1954; FAX: 818-393-5065
Mail: M/S: 183-701, 4800 Oak Grove Drive, Pasadena, CA 91109-8099