Improved water vapour retrieval from AMSU-B and MHS in the Arctic

Abstract. Monitoring of water vapour in the Arctic on long timescales is essential for predicting Arctic weather and understanding climate trends, as well as addressing its influence on the positive feedback loop contributing to Arctic amplification. However, this is challenged by the sparseness of in situ measurements and the problems that standard remote sensing retrieval methods for water vapour have in Arctic conditions. Here, we present advances in a retrieval algorithm for vertically integrated water vapour (total water vapour, TWV) in polar regions from data of satellite-based microwave humidity sounders: (1) in addition to AMSU-B (Advanced Microwave Sounding Unit-B), we can now also use data from the successor instrument MHS (Microwave Humidity Sounder), and (2) artefacts caused by high cloud ice content in convective clouds are filtered out. Comparison to in situ measurements using GPS and radiosondes during 2008 and 2009, as well as to radiosondes during the N-ICE2015 campaign and to ERA5 reanalysis, show the overall good performance of the updated algorithm

Variability and trends of Arctic water vapour from passive microwave satellites Special role of Polar lows and Atmospheric rivers

Water in the vapour phase is the most important component of the hydrological cycle. It is formed by processes of evaporation and sublimation during which a lot of energy as latent heat is absorbed from the atmosphere. Through atmospheric large and small scale circulation, this energy is transported and released elsewhere through the process of condensation. Water vapour is the most important greenhouse gas (GHG) due to its abundance and its effectiveness in absorbing longwave radiation. In the light of global climate change, it is of great importance to identify trends of water vapour amounts in the atmosphere and its variability. Climate change in terms of the near-surface temperature is most pronounced in the Arctic, known as Arctic Amplification. Since most of the Arctic are either open ocean or sea-ice covered surfaces, only sparse ground-based observations, mostly confined to land areas are available. Therefore, one must resort to usage of the satellite based observations which offer a great advantage by their large spatial coverage. For water vapour assessment, passive microwave satellites are well suited due to their ability to sense water vapour under clear and cloudy sky conditions independent of sun light. A number of products of integrated water vapour (IWV) from various satellites are available. However, these are often inconsistent and prone to have biases due to various assumptions and uncertainties of a priori data included in the retrieval algorithms. According to the Clausius-Clapeyron relation, water vapour is constrained by the saturation vapour pressure which is constrained only by the temperature. Therefore, this thesis investigates the hypothesis that brightness temperatures (Tbs) from spaceborne passive microwave instruments can be used as a proxy for water vapour trends. To test this hypothesis, satellites based Tbs are compared to synthetic Tbs derived from the Arctic System Reanalysis (ASR). To enable the comparison, the ASR has been evaluated in Tb space by employing the Passive and Active Microwave TRAnsfer forward model (PAMTRA). Moreover, Tbs from sounding channels were correlated with corresponding IWV based on the weighted absolute humidity profiles peaks. The hypothesis is tested for the dry, cold and sun-absent winter season (January) and the sun-return transitional spring season (May). The results show that Tbs from frequency channels can explain trends in the corresponding IWV columns derived from ASR for regions with significant positive trends for both, Tb and IWV since high correlation coefficients, reaching 0.98, have been found. This is true for different time scales, daily, monthly and for the period of 17 years (2000-2016). The exception to this has been found for May for daily time scale for frequency channel dominated by the signal from the upper troposphere lower stratosphere (UTLS). For this combination of Tbs and IWV correlations tend to be weaker and at some locations even negative. This is consistent with theoretical calculations and observational studies which report a cooling in the UTLS region for increasing IWV. However, Tbs from the corresponding channel seem less reliable in explaining trends of the corresponding IWV derived from the ASR. This indicates the importance of other processes relevant in the UTLS region during spring. Furthermore, this thesis investigates synoptic features which are associated with water vapour transport and precipitation. Previous studies have shown that Arctic cyclone activity during winter has a large impact on the sea ice melt in the following seasons making them important players in the complex feedback mechanism of the climate change in the Arctic. However, the life cycle of the most intense of such cyclones, also known as polar lows (PL) are not yet fully understood. To analyse their dynamics, this thesis investigates different environmental conditions (and their combination) between genesis and maturity stage of January PLs. PLs with overall lower thermal instability between the surface and 500 hPa during formation stage are typically accompanied by higher and steeper lapse rates throughout the boundary layer. Therefore these PLs were fostering convective development. However, as observed for a few cases, a decreased thermal instability alongside a simultaneous decrease of convection coincides with high relative humidity (mostly above 90%). Furthermore, higher relative humidity at lower levels during genesis stage promoted stronger winds at the maturity stage. Besides water vapour turnover associated with Arctic cyclones, atmospheric rivers (ARs) transport major amounts of moisture from tropical and extratropical regions into the Arctic. Studies have shown that about 90% of the total mid-latitude vertically integrated water vapour transport (IVT) is related to these synoptic features. To study the influence of ARs on PL precipitation, an event with a coupled AR and PL is compared to an event which featured only a PL. The AR had a strong influence on the PL resulting in higher snow amounts on the order of ∼ 4 kg/m2 higher wind speeds and a longer distance traveled during its life cycle, compared to the PL only case

Polar Lows: their climatology, interaction with the ocean and response to climate change

Polar lows (PLs) are intense mesoscale cyclones that form at high latitudes during winter. Their high wind speeds and heavy precipitation can substantially impact offshore infrastructures and coastal communities over regions such as Scandinavia, Russia and Japan. However, large uncertainties regarding their climatology, interaction with the ocean and response to climate change still remain. Using an automatic tracking method and specific identification criteria, a reliable long-term climatology of PLs and their environment is derived from two atmospheric reanalyses. The mean number of PLs differs significantly between reanalyses, however the inter-annual variability of PL numbers is highly correlated between both datasets. PLs activity from these reanalyses is found consistent with observations and literature. The large-scale environment of PLs is found to play a role in the inter-annual variability of PL numbers. The possible impact of PLs on the ocean circulation over the Nordic Seas is investigated using high resolution simulations from a coupled global climate model. As seen in previous studies based on an ocean model with parametrized PLs, this thesis shows, in high resolution climate model simulations, a clear positive link between the ocean surface heat fluxes and PL occurrences. However, in this study, no evidence is found that PLs influence on the ocean density is sufficient to destabilize the water column and trigger deep ocean convection over the Nordic Seas. Finally, for the first time, the representation of PLs and their environment are assessed in a high resolution atmosphere-only global climate model, for both present climate conditions and a future climate scenario. Furthermore, the impact of the resolution of the model on the representation of PLs is assessed using simulations from three different horizontal resolutions for both climate conditions. Overall the PL numbers are expected to decrease in the future, mainly due to an increase in static stability. However, regional differences appear and new areas for PL occurrence emerge over the Arctic Ocean. The horizontal resolution of the climate model is found to affect the mean numbers of PLs but not their activity

Quantifying various thunderstorm characteristics during high impact events using radar and satellite observations

Climate change is expected to change the intensity and frequency of heavy storms. Thus, understanding different characteristics of this phenomena (i.e., intensity, size, speed, direction, etc.) is vital for the effective climate adaptation. Many extreme storms have small areas and short lifetimes (sub-daily/hourly) and can have destructive impacts, especially over urban areas. Therefore, it is vital to understand the nature of changes in these extremes to reduce the risk of their destructive impacts on cities. The overarching goal of this thesis is to quantify various storm characteristics, including their changes, using radar and satellite observations. Using an object-based technique, I compare the Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) and ground radar based Multi-Radar Multi-Sensor Quantitative Precipitation Estimates (MRMS) over the United States and show that the object-based storm properties are not sensitive to the observational platforms. However, there are differences that are statistically significant. Secondly, I investigate the error sources associated with different types of contributing data in the IMERG during the hurricane days occurred in 2016-2018 with MRMS as the reference. The results show that IMERG have better agreement with MRMS during the passive microwave (PMW) observations compared to rainfall estimates come from the combination of the interpolation techniques and infrared observations (morph/IR). Also, the quality of morph/IR estimates deteriorates with the longer absence of PMW observations. Thirdly, I establish an object-based climatology of rain systems using radar data near Sydney, Australia. The results show that rain systems in different seasons have distinct object-based characteristics, and these differences are dependent on their source of origins and also their positions over land and ocean. Using a two-step clustering algorithm, I have found five system types over Sydney peaking in different seasons. While overall rainfall statistics don't show any link to climate modes, links do appear for some system types using a multivariate approach. Finally, I show that there is a robust increasing trend of 20% per decade in sub-hourly extreme rainfall in the Sydney region over 20 years, despite no evidence of trends on hourly or daily scales. I am able to obtain this new result via a novel analysis of long-term radar data, including cross-checking between neighboring radars