Upper tropospheric humidity gridded data derived from satellite observations

Both datasets provide global satellite-based observations of upper tropospheric humidity (around 500-200 hPa) on a regular daily grid, enabling monitoring of water vapor distribution in this key atmospheric layer.

It is mainly used to analyze climate processes (such as greenhouse effects and water vapor-cloud feedbacks) and to study long-term climate variability using a consistent multi-satellite time series.

Key Strengths

• Multiple sensors: The UTH datasets are based on multiple passive microwave humidity sensors onboard different satellites combined into a harmonised long-term dataset that covers a period of 23 years (1999-2021) for UTH v1.0 and almost 25 years (1994-2018) for UTH v2.0. UTH v1.0 contains separate observations per satellites whereas UTH v2.0 is a merged product.
• Complete coverage and good accuracy in 60S-60N: Both datasets are available globally, and they have a good accuracy between 60ºS and 60ºN. Missing data are mainly due to gaps between orbits and due to the presence of deep high clouds. Other reasons for missing data are very dry atmosphere, or regions of high surface elevation.
• Reduced cloud contamination: The channel 183.31+/-1.00 GHz selected for the retrieval process minimizes the impact of clouds compared to IR retrievals. In this context, cloud contamination refers to situations where precipitation and deep clouds contribute to the measured signal and may be erroneously interpreted as atmospheric humidity. An additional cloud-clearing selection is implemented to further reduce any cloud interference.
• Uncertainty information: UTH v2.0 includes detailed uncertainty information extensively discussed in the documentation.

Key Limitations

• Pressure level of UTH observations: The UTH retrieval method is a transformation of radiance to a more humidity unit. Thus, UTH is not an average relative humidity in a fixed vertical layer of the troposphere and the width of the layer depends on the moisture burden of the atmospheric column.
• Reduced accuracy in polar regions: The UTH exhibits surface contamination and sensitivity to the lower atmosphere in polar regions because of the very small amounts of water vapour in the upper troposphere. Consequently, it is advisable to exercise caution when working with data from polar regions.
• Constant lapse rate assumption: The retrieval algorithm is based on the assumption that the lapse rate and RH are uniform in the upper troposphere, an assumption valid for the tropical atmosphere but less suitable elsewhere. A change in lapse rate due to the surface temperature trend could produce systematic changes in the retrieved upper humidity.

Example Applications

• Atmospheric processes: Both products can be used to examine the atmospheric processes which control or influence the UTH. For instance, these datasets can be used to examine how general atmospheric circulation impacts UTH (S. A. Buehler et al., 2008). Another example is exploring the relationship between UTH and high clouds formed by deep convection or in situ processes (Fabrizio Sassi et al., 2001, Zhengzhao Luo et al., 2007). These types of analysis are possible because the data records are obtained and provided at an instantaneous scale.
• Climate variability dependence: These products are ideal for investigating how UTH are affected by climate variability at regional scales. One potential research focus could be examining the regional impact of ENSO conditions on UTH (Lei Shi et al., 2022).

Further recommendations

• Compare with other products: The UTH datasets are not an average relative humidity in a fixed vertical layer of the troposphere. Therefore they cannot be directly compared with other datasets such as models or radiosondes. In order to compare with different products, users should conduct radiative transfer simulations to generate the brightness temperatures at 183.31 +/- 1.00 GHz, which can be directly compared using equation 3-2 of the Product User Manual.
• Quality requirement: The products provide the valid number of observations per gridbox. Users are recommended to review this variable and consider applying a minimum threshold in their analysis to ensure the selection of high-quality UTH measurements.