Performance simulations for a submillimetre-wave satellite instrument to measure cloud ice

Abstract

ABSTRACT:The performance of a conically scanning satellite instrument for the measurement of cloud ice was studied. The instrument measures radiances in 12 channels placed around the 183, 325 and 448 GHz water vapour lines and the 243, 664 and 874 GHz window channels, and is designed to provide estimations of ice water path (IWP), the equivalent sphere diameter (DME), and the median ice mass height (ZME). Overall median relative errors of around 20% for IWP, 33 µm for DME, and 240 m for ZME for a midlatitude winter scenario, and 17% for IWP, 30 µm for DME, and 310 m for ZME for a tropical scenario were found. Detection limits (relative retrieval error reaching 100%) of around 2 gm −2 were estimated for both scenarios. The performance of a five-receiver instrument, where either the 664 or 874 GHz channel is dropped, was close, but with increased errors for very thin and high clouds. A trade-off between having the 874 GHz receiver or two infrared channels at 10.7 and 12 µm emerged, as very similar performance was found between the six-receiver instrument and the five-receiver instrument with the infrared channels. Another trade-off between receiver selection and noise was also apparent, with some of the four-receiver selections operating at half noise levels being able to compete with the standard six-receiver instrument. Dual-polarized measurements were also tested, but they did not significantly improve the retrievals of IWP or DME.

A concept for a satellite mission to measure cloud ice water path, ice particle size, and cloud altitude

“…As discussed in more detail in Jiménez et al (2007), this results in an instrument with slightly poorer performance for those situations with thin and high clouds containing very small particles, as is sometimes the case for the tropical simulations. As demonstrated by Figure 11, adding the IR channels improves notably those retrievals, and results in an overall better retrieval performance than the instrument with all submillimetre channels but without additional IR channels.…”

“…Tables V and VI summarize the key parameters of the instrument. Retrieval simulations were used extensively to test the performance of the prototype instrument and many other instrument configurations (Jiménez et al, 2007). Figure 11 shows a summary of the retrieval performance, relative to the scientific mission requirements.…”

“…The result is presented here. The methodology of the retrieval simulations is described in detail in Jiménez et al (2007). Figure 10 shows the channel positions of the prototype instrument concept, relative to the atmospheric spectrum for clear and cloudy conditions.…”

“…(The rationale for that change is discussed in section 4.2.) A further difference is that the dualpolarization capability of the highest-frequency channels of the original concept was dropped, because retrieval simulations did not provide sufficient proof that polarized measurements give a significant benefit (Jiménez et al, 2007). Table V lists the channel positions explicitly, along with the performance estimates that were derived by the engineering team in the ESA study (Jarret et al, 2007), under the lead of Sula Systems Ltd, UK.…”

“…The training data contained random cloud profiles, including random sizes and shapes of ice particles, with statistics consistent with radar and in situ aircraft data. The methodology and results of these simulations are discussed in detail in Jiménez et al (2007). Here, we will show only the performance simulation for the selected prototype configuration, and compare the simulated performance to the scientific mission requirements.…”