WP-2000 Calibration of EDA spread and adaptation of the observation error model

TitleWP-2000 Calibration of EDA spread and adaptation of the observation error model
Publication TypeReport
Date Published02/2022
Series/CollectionESA contract 4000130590/20/NL/IA
AuthorsLean, K, Bormann, N, Healy, S

In this second phase of this project to investigate potential future constellations of small satellites
carrying microwave (MW) sounding instruments, there are two subjects that have been explored. In
the first part of this report, real MW data have been used to establish links between the observation
impact using the Ensemble of Data Assimilations (EDA) method, described in the initial consolidation
phase, and in Observing System Experiments (OSEs). Reductions in the EDA spread, which
indicate a positive impact from the MW data, were compared with reductions in the standard deviation
of forecast error. The forecast error has been evaluated by comparing the short-range forecast
to a reference firstly provided by analyses from the operational system and secondly by radiosonde
observations, taking into account the observation error. There is overall good qualitative similarity
between EDA spread and both measures of the forecast error. Better agreement of the observation
impact is found in the lower/mid-troposphere with possible under-estimation of the impact using the
EDA spread in the stratosphere. While it is not useful to derive a quantitative relationship here, the
results show that the EDA method provides a basis to investigate the relative impacts from the future
simulated small satellite data and with the addition of reference points from the use of existing MW
The second part of the report presents the adaptation of the observation error model for the MW
temperature sounding channels needed due to the unavailability of key low frequency channels on
the small satellites. For channels with cloud sensitivity, the observation error model for the all-sky
assimilation inflates the observation error in cloudy regions by using an indicator of the presence of
cloud in the observations or model. The use of the 52.8GHz (channel 4 on AMSU-A) has been proposed
as the basis to construct an alternative cloud indicator forMWtemperature sounding channels.
Evaluation using AMSU-A observations showed that a similar structure to the current observation
error model can be achieved with the new indicator. Assimilation experiments showed some small
but significant negative impacts in short-range forecasts as measured by low peaking temperature
sounding channels of ATMS. There is also a small but not significant signal in some humidity sensitive
observations, however, impacts were generally neutral. Neutral impacts at longer forecast lead
times measured by verification against analyses showed that these short-range changes did not affect
the medium-range forecasts. The results confirm that 52.8GHz provides a viable alternative to use
for the small satellite temperature sounding observation error model. However, it is noted that the
potential impact from the loss of the lower frequencies is not fully captured as the channels are not
currently used in the operational assimilation of AMSU-A.

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