David K. Adams

The North American Monsoon

The North American monsoon is an important feature of the atmospheric circulation over the continent, with a research literature that dates back almost 100 years. The authors review the wide range of past and current research dealing with the meteorological and climatological aspects of the North American monsoon, highlighting historical development and major research themes. The domain of the North American monsoon is large, extending over much of the western United States from its region of greatest influence in northwestern Mexico. Regarding the debate over moisture source regions and water vapor advection into southwestern North America, there is general agreement that the bulk of monsoon moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California, while the Gulf of Mexico may contribute some upper-level moisture (although mixing occurs over the Sierra Madre Occidental). Surges of low-level moisture from the Gulf of California are a significant part of intrase...

The Chuva Project: How Does Convection Vary across Brazil?

CHUVA, meaning “rain” in Portuguese, is the acronym for the Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (GPM). The CHUVA project has conducted five field campaigns; the sixth and last campaign will be held in Manaus in 2014. The primary scientific objective of CHUVA is to contribute to the understanding of cloud processes, which represent one of the least understood components of the weather and climate system. The five CHUVA campaigns were designed to investigate specific tropical weather regimes. The first two experiments, in Alcantara and Fortaleza in northeastern Brazil, focused on warm clouds. The third campaign, which was conducted in Belem, was dedicated to tropical squall lines that often form along the sea-breeze front. The fourth campaign was in the Vale do Paraiba of southeastern Brazil, which is a region with intense lightning activity. In addition to contributing to the understanding of clo...

CAPE and Convective Events in the Southwest during the North American Monsoon

Abstract The relationship between atmospheric stability, measured as CAPE, and deep precipitating convection has been widely studied but is not definitive. In the maritime tropics, CAPE and precipitation are usually inversely correlated. In continental convection (i.e., midlatitude and tropical), no consistent relationship has been found. In this study of the semiarid Southwest, a moderate positive correlation exists, approaching 0.6. Correlations based on radiosonde data are found to be sensitive to the parcel level of origin. The strongest correlations are found by modifying the preconvective morning sounding with the maximum reported surface temperature, assuming well-mixed adiabatic layers to the level of free convection with pseudoadiabatic ascent. These results show that the upper bounds on parcel instability correlate best with precipitation. Furthermore, the CAPE–precipitation relationship is argued to depend on the convective regime being considered. The North American monsoon convective regime r...

Deep Convection and Column Water Vapor over Tropical Land versus Tropical Ocean: A Comparison between the Amazon and the Tropical Western Pacific

AbstractThe relationships between the onset of tropical deep convection, column water vapor (CWV), and other measures of conditional instability are analyzed with 2 yr of data from the DOE Atmospheric Radiation Measurement (ARM) Mobile Facility in Manacapuru, Brazil, as part of the Green Ocean Amazon (GOAmazon) campaign, and with 3.5 yr of CWV derived from global positioning system meteorology at a nearby site in Manaus, Brazil. Important features seen previously in observations over tropical oceans—precipitation conditionally averaged by CWV exhibiting a sharp pickup at high CWV, and the overall shape of the CWV distribution for both precipitating and nonprecipitating points—are also found for this tropical continental region. The relationship between rainfall and CWV reflects the impact of lower-free-tropospheric moisture variability on convection. Specifically, CWV over land, as over ocean, is a proxy for the effect of free-tropospheric moisture on conditional instability as indicated by entraining plu...

The Amazon Dense GNSS Meteorological Network: A New Approach for Examining Water Vapor and Deep Convection Interactions in the Tropics

AbstractThe 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 G...

GNSS Precipitable Water Vapor from an Amazonian Rain Forest Flux Tower

AbstractUnderstanding the complex interactions between water vapor fields and deep convection on the mesoscale requires observational networks with high spatial (kilometers) and temporal (minutes) resolution. In the equatorial tropics, where deep convection dominates the vertical distribution of the most important greenhouse substance—water—these mesoscale networks are nonexistent. Global Navigational Satellite System (GNSS) meteorological networks offer high temporal/spatial resolution precipitable water vapor, but infrastructure exigencies are great. The authors report here on very accurate precipitable water vapor (PWV) values calculated from a GNSS receiver installed on a highly nonideal Amazon rain forest flux tower. Further experiments with a mechanically oscillating platform demonstrate that errors and biases of approximately 1 mm (2%–3% of PWV) can be expected when compared with a stable reference GNSS receiver for two different geodetic grade receivers/antennas and processing methods [GPS-Inferre...

Remarks on quasi-equilibrium theory

Abstract A recent article by J. I. Yano has indicated that there is an inconsistency in the original formulation of the quasi-equilibrium theory of Arakawa and Schubert. He argues that this inconsistency results from a contradiction in the two asymptotic limits of the theory; that is, the fractional area covered by convection, and the ratio of the convective adjustment and large-scale timescales cannot simultaneously go to zero, σ → 0 and τADJ/τLS → 0. Yano cites the heat engine theory proposed by Renno and Ingersoll as “formally establishing” this contradiction. It is demonstrated in this paper that the quasi-equilibrium framework originally developed by Arakawa and Schubert is perfectly consistent with the heat engine theory for steady-state convection, that is, when the timescale associated with the large-scale forcing τLS approximates the effective adjustment timescale of the large-scale ensemble of convective clouds τEFF. Indeed, the quasi-equilibrium framework states that, on the large scale, the at...

Mexican GPS Tracks Convection From North American Monsoon

The North American monsoon (NAM) is a dominant feature of the climate of northwest Mexico and the southwest United States. The annual monsoon, which usually lasts from July to September, contributes more than half of the yearly precipitation for much of the region. Thunderstorms brought on by the monsoon are routinely responsible for severe weather, including flooding, hail, wind, dust storms, and lightning. Tropical storms and hurricanes during the NAM season can also wreak havoc on a much larger scale.

The more extreme nature of North American Monsoon precipitation in the Southwestern United States as revealed by a historical climatology of simulated severe weather events

AbstractLong-term changes in North American monsoon (NAM) precipitation intensity in the Southwest U.S. are evaluated through the use of convective-permitting model simulations of objectively identified severe weather events during “historical past” (1950-1970) and “present day” (1991-2010) periods. Severe weather events are days when the highest atmospheric instability and moisture occur within a long-term regional climate simulation. Severe weather event day simulations are performed with convective-permitting (2.5 km) grid spacing, and these simulations are compared to available observed precipitation data to evaluate the model performance and verify any statistically significant model simulated trends in precipitation. Statistical evaluation of precipitation extremes is performed using peaks-over-threshold approach with a generalized Pareto distribution. A statistically significant long-term increase in atmospheric moisture and instability is associated with an increase in extreme monsoon precipitatio...

Long-Term Changes in the Climatology of Transient Inverted Troughs over the North American Monsoon Region and Their Effects on Precipitation

AbstractTransient inverted troughs (IVs) are a trigger for severe weather during the North American monsoon (NAM) in the southwest contiguous United States (CONUS) and northwest Mexico. These upper-tropospheric disturbances enhance the synoptic-scale and mesoscale environment for organized convection, increasing the chances for microbursts, straight-line winds, blowing dust, and flash flooding. This work considers changes in the track density climatology of IVs between 1951 and 2010. IVs are tracked as potential vorticity (PV) anomalies on the 250-hPa surface from a regional climate model that dynamically downscales the NCEP–NCAR Reanalysis 1. Late in the NAM season, a significant increase in IV track density over the 60-yr period is observed over Southern California and western Arizona, coupled with a slight decrease over northwest Mexico. Changes in precipitation are evaluated on days when an IV is observed and days without an IV, using high-resolution model-simulated precipitation estimates and CPC gri...