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This paper investigates the relationship between seasonal precipitation over Iran and low‐level moisture, in terms of vertically integrated specific humidity (VISH) from the surface to 850 hPa. The VISH is calculated from ERA5 data for the domain (10°N–60°N, 15°E–80°E), and the precipitation is calculated from 50 stations across Iran, both for the period 1968–2023. Canonical correlation analysis (CCA) is applied to examine the spatial–temporal relationship between seasonal averages of moisture and precipitation during January–March (JFM), April–Jun (AMJ), and October–December (OND). VISH and precipitation are considered as the simultaneous predictor and predictand fields in the CCA, respectively. The CCA time series are correlated to global sea surface temperatures to assess the connections to large‐scale, potentially predictable modes of variability. The CCA spatial patterns indicate that there is a strong relationship between low‐level moisture and seasonal precipitation, with VISH over the Persian Gulf, Oman Sea, Arabian Sea, and Red Sea positively correlated with precipitation over most areas of Iran, while VISH over the Caspian Sea and Black is negatively correlated. Generally, these relationships are notably low over northwestern areas of Iran and the coastal regions of the Caspian Sea and the prediction skill of CCA remains limited over these regions. In OND, the leading CCA time series exhibits the well‐known connection to the El Niño–Southern Oscillation (ENSO). However, the highest CCA skill is found for JFM precipitation, which does not exhibit an ENSO connection, and so may present an additional source of skill. The first canonical pair of vertically integrated specific humidity (VISH) from the surface to 850 hPa and precipitation for the JFM season, with the VISH pattern in (a), the precipitation pattern in (b). This pair of spatial patterns indicates a positive association between the spatial pattern of the VISH over the Persian Gulf, Oman Sea, Arabian Sea, southern parts of Red Sea, and precipitation for JFM over most areas of Iran. However, the VISH over northern latitudes (38°N–60°N) has a negative association with JFM precipitation over most areas.

The impacts of the low level jets that form through the gaps in the topography in the Limpopo and Zambezi River Valleys (LRV and ZRV) on southern African climate are investigated. ERA-5 reanalysis data and numerical experiments using regional climate models reveal that the two valleys act as main gateways for southwest Indian Ocean-sourced moisture inflows into southern Africa. The effects of the LRV jet are stationary and mostly confined to south of Limpopo. By blocking the LRV, the moisture convergence in the interior of the subcontinent reduces and leads to a weakening in both the Angola Low and the ridging high over southeastern Africa. These are unfavorable conditions for the southward transport of moisture, causing a decrease of up to 50% of the subtropical southern African total summer rainfall. In contrast, the influence of the Zambezi jet varies with the season. During early austral summer, a blocked ZRV results in an overall deficit in rainfall in the subcontinent. It then leads to a weaker Mozambique Channel Trough, intensified Angola Low, hence increase in rainfall during January and February. Toward the end of the austral summer season, the absence of the Zambezi jet allows more moisture from the tropical Atlantic to penetrate into the mainland which then converges with moisture transported from the Indian Ocean and triggers excess rainfall over the tropical areas. These results have important implications for understanding the intraseasonal and interannual rainfall variability over the subcontinent during the summer half of the year.

Because they are most sensitive to atmospheric moisture content, radar refractivity observations can provide high-resolution information about the highly variable low-level moisture field. In this study, simulated radar refractivity-related phase-change data were created using a radar simulator from realistic highresolution model simulation data for a dryline case. These data were analyzed using the 2DVAR system developed specifically for the phase-change data. Two sets of experiments with the simulated observations were performed, one assuming a uniform target spacing of 250 m and one assuming nonuniform spacing between 250 m to 4 km. Several sources of observation error were considered, and their impacts were examined. They included errors due to ground target position uncertainty, typical random errors associated with radar measurements, and gross error due to phase wrapping. Without any additional information, the 2DVAR system was incapable of dealing with phase-wrapped data directly. When there was no phase wrapping in the data, the 2DVAR produced excellent analyses, even in the presence of both position uncertainty and random radar measurement errors. When a separate pre-processing step was applied to unwrap the phase-wrapped data, quality moisture analyses were again obtained, although the analyses were smoother due to the reduced effective resolution of the observations by interpolation and smoothing involved in the unwrapping procedure. The unwrapping procedure was effective even when significant differences existed between the analyzed state and the state at a reference time. The results affirm the promise of using radar refractivity phase-change measurements for near-surface moisture analysis.

A simple diagnostic model was used to derive afternoon (0000 UTC) mixing-layer depths from gridded zero hour output from the Canadian Global Environmental Multiscale (GEM) numerical weather model interpolated to approximately 200 locations in the Prairie provinces of Canada. Knowledge of the spatial pattern of monthly mean afternoon mixing-layer depths was enhanced by associating the annual cycles of monthly means with eco-climatic regions - regions with fairly uniform terrain and soils, and where similar vegetation indicate that the area experiences, on average, similar synoptic-scale weather and produces similar surface fluxes of heat and moisture. When the mixing-layer depth is used to assess the dilution of airborne pollutants or to determine the specific humidity of the boundary layer, the eco-climatic region rather than proximity to the location-of-interest should be the primary factor in the selection of an appropriate radiosonde site. [PUBLICATION ABSTRACT]

During the summer, the midwestern United States, which covers the main U.S. corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to the northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn-belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO₂ and the associated warming increase evapotranspiration while precipitation reduces in the region, leading to further reduction in precipitation minus evaporation in the future. At the same time, the poleward transient moisture flux increases, leading to enhanced atmospheric moisture export from the corn-belt region. However, storm-track intensity is generally weakened in the summer because of a reduced north–south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as the storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region as a result of the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future the U.S. Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export, leading to drier soils in the region.

The South American low‐level jet (SALLJ) is a major source of moisture transport to southeastern South America, influencing rainfall, agriculture, and hydropower. While past research emphasized atmospheric controls, we examine the role of antecedent soil moisture in modulating jet dynamics. We focus on strong Chaco jets, a southward‐extended branch of the SALLJ that transport 37.9 Gt of water daily—twice the Amazon River's discharge. Using reanalysis data, we identify 54 Chaco jet events and categorize them based on antecedent soil moisture over northern Argentina; 63% occur under drier‐than‐normal conditions. Dry soils are associated with enhanced surface sensible heating, lower‐tropospheric warming, and a deepened thermal low, which intensifies the Chaco jet (2.8 times stronger than in wet cases) during the 5 days before its peak. These results highlight the importance of land‐atmosphere interactions in modulating SALLJ dynamics and suggest that antecedent soil moisture information could improve jet forecasts in the region.

In this study, we investigated the moisture sources for precipitation through a Lagrangian approach during the genesis, intensification, and dissipation phases of all tropical cyclones (TCs) that occurred over the two hemispheric sub-basins of the Indian Ocean (IO) from 1980 to 2018. In the North IO (NIO), TCs formed and reached their maximum intensity on both sides of the Indian Peninsula, to the east in the Bay of Bengal (BoB), and to the west in the Arabian Sea (AS). The oceanic areas where TCs occurred were their main moisture sources for precipitation associated with TCs. Additionally, for TCs over the BoB, continental sources from the Ganges River basin and the South China Sea also played a notable role; for TCs over the AS, the Somali Low-Level jet (along the African coast in a northerly direction) also acted as an essential moisture transport. In the South IO (SIO), the western, central, and eastern basins were identified as the preferred areas for the genesis and development of TCs. During TC activity, the central IO and the Wharton and Perth basins mostly supplied atmospheric moisture. The Mascarene High circulation was the main moisture transport mechanism for the precipitation of TCs formed in the SIO basin. In both basins, during their intensification process, TCs gained more moisture (even more intensely when reaching the hurricane category) than during the genesis or dissipation stages. Additionally, the modulation during monsoonal seasons of the moisture contribution to the TCs was more noticeable over the NIO basin than for the SIO. Overall, the moisture uptake for precipitation from the sources for TCs occurred slightly faster in the NIO basin than in the SIO basin.

The Colombian Andes (CA) are located over the northwestern corner of tropical South America (NW‐TropSA), where land and atmosphere interchange moisture and energy in a complex way owing to the orographic influence on the recycling of moisture over land. We aim to understand where and when water vapor evaporated from land turns into rainfall through moisture recycling, using the Water Accounting Model‐2layers (WAM‐2), an offline model to track atmospheric moisture forced with data from ERA‐5 at its native 0.25° resolution during 1980–2020. We define the spatiotemporal distribution of sources (high evaporation recycle ratios, ϵC) and receptors (high precipitation recycle ratios, ρC) of continental moisture at diverse timescales, including monthly, seasonal, annual and interannual (ENSO). Referring to the regional runs of WAM‐2 over NW‐TropSA (4°S–12°N/80°W–66°W), at elevations above 1,000 m.a.s.l., the CA has a mean annual ρC of 11% (ranging 6%–16%) and a mean annual ϵC of 35% (ranging 27%–40%). Moisture recycling in the CA exhibits a strong annual cycle over the region. The seasonal dynamic of moisture recycling shows two clear‐cut sources of moisture: the eastern foothills of the Eastern and Central ranges of the CA. Both foothills are also regions of high rainfall, although moisture recycling mechanisms differ. Sources of continental moisture grow spatially during September‐October‐November and March‐April‐May. The seasonal availability of moisture recycled coincides with regions where orography interacts with low level jets sourcing humidity. At interannual timescales, sources and receptors of continental moisture in the CA are modulated by the extreme phases of ENSO. Key Points The orography of the Colombian Andes emerged as a critical factor shaping the interaction between land and atmospheric moisture fluxes Foothills and valleys of the Colombian Andes play a pivotal role in continental moisture recycling, in terms of convergence and divergence ENSO modulates both sources and receptors of continental moisture at interannual timescales

Elevated spring and summer rainfall in the U.S. Midwest is often associated with a strong Great Plains low-level jet (GPLLJ), which transports moist air northward to the region from the Gulf of Mexico. While the intensity of hourly precipitation extremes depends on local moisture availability and vertical velocity, sustained moisture convergence on longer time scales depends on horizontal moisture advection from remote sources. Therefore, the magnitude of moisture convergence in the Midwest depends in part on the humidity in these moisture source regions. Past work has identified the time-mean spatial distribution of moisture sources for the Midwest and studied how this pattern changes in years with anomalous rainfall. Here, using reanalysis products and an Eulerian moisture tracking model, we seek to increase physical understanding of this moisture source variability by linking it to the GPLLJ, which has been studied extensively. We find that on interannual time scales, an anomalously strong GPLLJ is associated with a shift in the distribution of moisture sources from land to ocean, with most of the anomalous moisture transported to—and converged in—the Midwest originating from the Atlantic Ocean. This effect is more pronounced on synoptic time scales, when almost all anomalous moisture transported to the region originates over the ocean. We also show that the observed positive trend in oceanic moisture contribution to the Midwest from 1979 to 2020 is consistent with a strengthening of the GPLLJ over the same period. We conclude by outlining how projected changes in a region’s upstream moisture sources may be useful for understanding changes in local precipitation variability.

The impactful 04 July 2025 Central Texas extreme rainfall event is examined to understand how surface conditions influence storm development. Utilizing convection‐permitting model simulations, we evaluate the sensitivity of this event to Gulf of Mexico sea surface temperature anomalies (SSTAs) and antecedent soil moisture distributions. The precursor wet soil conditions enhanced storm rainfall, whereas warm coastal and central Gulf SSTAs suppressed rainfall through perturbations of the low‐level circulation, including the Great Plains low‐level jet, which modified moisture transport and moisture convergence. When compared with climatological conditions, SST and soil moisture anomalies produced a rainfall reduction, indicating SST forcing dominated the combined response. These results suggest that this extreme storm would have produced higher rainfall totals had SSTs been closer to their recent climatological average.