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Northwestern Mexico experiences large variations in water vapor on seasonal time scales in association with the North American monsoon, as well as during the monsoon associated with upper-tropospheric troughs, mesoscale convective systems, tropical easterly waves, and tropical cyclones. Together these events provide more than half of the annual rainfall to the region. A sufficient density of meteorological observations is required to properly observe, understand, and forecast the important processes contributing to the development of organized convection over northwestern Mexico. The stability of observations over long time periods is also of interest to monitor seasonal and longer-time-scale variability in the water cycle. For more than a decade, the U.S. Global Positioning System (GPS) has been used to obtain tropospheric precipitable water vapor (PWV) for applications in the atmospheric sciences. There is particular interest in establishing these systems where conventional operational meteorological networks are not possible due to the lack of financial or human resources to support the network. Here, we provide an overview of the North American Monsoon GPS Transect Experiment 2013 in northwestern Mexico for the study of mesoscale processes and the impact of PWV observations on high-resolution model forecasts of organized convective events during the 2013 monsoon. Some highlights are presented, as well as a look forward at GPS networks with surface meteorology (GPS-Met) planned for the region that will be capable of capturing a wider range of water vapor variability in both space and time across Mexico and into the southwestern United States.

The complex interactions between water vapor fields and deep atmospheric convection remain one of the outstanding problems in tropical meteorology. The lack of high spatial–temporal resolution, all-weather observations in the tropics has hampered progress. Numerical models have difficulties, for example, in representing the shallow-to-deep convective transition and the diurnal cycle of precipitation. Global Navigation Satellite System (GNSS) meteorology, which provides all-weather, high-frequency (5 min), precipitable water vapor estimates, can help. The Amazon Dense GNSS Meteorological Network experiment, the first of its kind in the tropics, was created with the aim of examining water vapor and deep convection relationships at the mesoscale. This innovative, Brazilian-led international experiment consisted of two mesoscale (100 km × 100 km) networks: 1) a 1-yr (April 2011–April 2012) campaign (20 GNSS meteorological sites) in and around Manaus and 2) a 6-week (June 2011) intensive campaign (15 GNSS meteorological sites) in and around Belem, the latter in collaboration with the Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) Project in Brazil. Results presented here from both networks focus on the diurnal cycle of precipitable water vapor associated with sea-breeze convection in Belem and seasonal and topographic influences in and around Manaus. Ultimately, these unique observations may serve to initialize, constrain, or validate precipitable water vapor in high-resolution models. These experiments also demonstrate that GNSS meteorology can expand into logistically difficult regions such as the Amazon. Other GNSS meteorology networks presently being constructed in the tropics are summarized.

[1] In the tropics, understanding the shallow-to-deep transition and organization of convection on the mesoscale is made difficult due the paucity of long-term high spatial/temporal resolution data. In this paper, data from the world's first long-term equatorial Global Navigational Satellite System meteorological station in Manaus (Central Amazon) is used to create a new metric, a water vapor convergence time scale, to characterize the temporal evolution of deep convection over a tropical continental region. From 3.5 years of data, 320 convective events were analyzed using a compositing analysis. Results reveal two characteristic time scales of water vapor convergence; an 8 h time scale of weak convergence and 4 h timescale of intense water vapor convergence associated with the shallow-to-deep convection transition. The 4 h shallow-to-deep transition time scale is particularly robust, regardless of convective intensity, seasonality, or nocturnal versus daytime convection. This new result provides a useful metric for both high resolution and global climate models to replicate.

A dense Global Navigation Satellite System (GNSS) meteorological network (∼20 stations) in the central Amazon Basin in Brazil is being developed for long-term studies of deep convection/water vapor interactions and feedback. In this article, the network is described and preliminary results are presented: GNSS-derived precipitable water vapor is useful for tracking water vapor advection and in identifying convective events and water vapor convergence timescales. Upon network completion (early 2011), 3D water vapor field analyses and participation in the intensive field campaign GPM-CHUVA will provide unique data sets for initializing, constraining or validating high-resolution models or refining convective parameterizations. Copyright  2011 Royal Meteorological Society

Water vapor is one of the most important components of the Earth's atmosphere. It is the source of precipitation, and its latent heat is a critical ingredient in the dynamics of most major weather events. As a greenhouse gas, water vapor also plays a critical role in the global climate system: it absorbs and radiates energy from the sun and affects the formation of clouds and aerosols and the chemistry of the lower atmosphere. Despite its importance in climate and weather prediction, water vapor has been one of the most poorly measured and least understood components of the Earth's atmosphere. Researchers at FSL and elsewhere are utilizing recent technology to reverse this situation.

Since 1994, the NOAA Research-Forecast Systems Laboratory (NOAA/FSL) has been evaluating the utility of ground-based Global Positioning System (GPS) remote sensing techniques for operational weather forecasting, climate monitoring, atmospheric research, and other applications such as satellite calibration and validation. Techniques have been developed to acquire, process, distribute GPS integrated precipitable water vapor (IPW) retrievals and ancillary surface meteorological observations every 30-minutes with less than 15 minute latency. Techniques to assimilate these observations into the research version of the Rapid Update Cycle (RUC) numerical weather prediction assimilation/model system running hourly at NOAA/FSL have been developed, and the impacts of these observations on shortrange weather forecast accuracy have been evaluated since 1998 using a 60-km version of the system. These assessments consist of data denial experiments (parallel runs with and without GPS water vapor observations) to determine the impact that GPS-derived integrated (or total column) precipitable water vapor (IPW) retrievals have on short-range moisture and precipitation forecasts. The experiments have been conducted over a portion of the central United States that, from a meteorological perspective, is one of the best-observed areas on Earth. While this greatly facilitates the impact assessments, it also presents a special challenge to a new observing system under evaluation, such as GPS-Met, since relatively few measurements have to “compete” with an enormous number of other (conventional and nonconventional) observations of similar and related parameters. Despite this, five years of experiments in- dicate more or less continuous improvements in 3-hour relative humidity forecasts at pressure levels below 500 hPa. The greatest skill is seen during the cold season when moisture changes are dominated by synoptic-scale weather systems. Perhaps the most significant result is that the impact in improved forecast skill from assimilation of GPS-IPW data has increased each year as the number of stations has increased, suggesting that further increases in the network density over the United States will result in further forecast improvement.

[1] We report on precipitable water vapor (PWV) from a Global Positioning System (GPS) receiver and surface meteorological network during the 2004 North American Monsoon Experiment (NAME) in northwestern Mexico. The monsoon onset is evident as a large PWV increase over several days beginning July 1. Data in the Sierra Madre Occidental (SMO) foothills reveal a dynamical transition in mid-August from smaller, sub-synoptic scale to larger, synoptic scale moisture structure. During the Sub-synoptic phase in the SMO foothills, a positive feedback operates where near-daily precipitation supplied moisture maintains 15% higher surface mixing ratios that lower the lifting condensation level facilitating initiation of moist convection. Along the western edge of the SMO, precipitation typically occurs hours after the local temperature maximum, triggered by westward propagating convective disturbances. Precipitation is typically preceded by a rapid rise in PWV and sharp decrease in surface temperature, implying models must include moist convective downdrafts in the NAM area. Citation: Kursinski, E. R., R. A. Bennett, D. Gochis, S. I. Gutman, K. L. Holub, R. Mastaler, C. Minjarez Sosa, I. Minjarez Sosa, and T. van Hove (2008), Water vapor and surface observations in northwestern Mexico during the 2004 NAME Enhanced Observing Period, Geophys. Res. Lett., 35, L03815

An experiment conducted near the Nares Abyssal Plain broadcast LFM and cw signals to investigate propagation across two convergence zones (CZ). Air‐launched Difar sonobuoys were placed in the first and second CZs simultaneously, and cw bursts of about 1000 Hz, and LFMs covering 900 to 1100 Hz were recorded. A time‐delayed echo repeater, which rebroadcast received signals omnidirectionally, was located in the first CZ. These signals were recorded via an expanded videocassette recording system [S. G. Payne, N. R. Bedford, and L. A. Thompson, ‘‘The AEAS Poco Amigo Airborne Acoustic Measurement System,’’ ARL‐TR‐88‐28], which demultiplexes signals from each Difar sonobuoy into its omnidirectional, north–south, and east–west components. Matched‐filter processing was applied to these data to obtain a temporal arrival structure. In the case of the second CZ, all three components of the Difar buoy were separately recorded thus allowing horizontal directionality via beamforming. Using calibration information from directional hydrophones, vertical arrival angles were determined [M. B. Bennett, L. A. Thompson, and P. T. Eisman, ‘‘Moored Data Buoy for Shallow Water Acoustic Studies’’ (in preparation)]. The arrival structure of the primary source and the echo repeater will be contrasted. [This work is supported by the ASW Environmental Acoustic Support Program.]