Moist Convection in the North American Southwest and the North American Monsoon Experiment (NAME)

Thunderstorms and moist convection are fascinating and Arizona and Northwestern Mexico are great places in the summer to study them. This past summer (2006) was the 4^th wettest in Tucson since 1949, producing 5 inches of rainfall in a 2 hour period in east Tuscon in late July, which set records for water flow in the Rillito river (and damaged our hallway ceiling). As a result Tucson looked more like northern Sinaloa than Arizona as Dave Gochis remarked at the August 2006 NAME meeting in Tucson.

The North American Monsoon Experiment

The North American monsoon produces about half of the annual rainfall in Tucson each year and a larger percentage further south in northwestern mainland Mexico. To first order, it results from heating of the continental interior that reverses the annual average direction of the pressure gradients and therefore the winds. The North American Monsoon Experiment is an ongoing NOAA sponsored effort to understand warm season precipitation in North America, and improving our ability to predict it on short term to interannual time scales.

During the summer of 2004, the North American Monsoon Experiment (NAME) Enhanced Observing Period (EOP, see Higgins et al., 2006 for details) was run in northwestern Mexico and the US southwest to develop better understanding of the mechanisms influencing warm season precipitation and ultimately to improve its representation and prediction in models. Current numerical weather prediction and climate models predict many feature of warm season rainfall rather poorly due to a strong dependence on small-scale dynamical processes, topography and rapid diurnal evolution. Convective parameterizations, which are crucial in some for predicting precipitation ranging from a few hours to decades often have difficulty accounting for such small scale, fast acting processes. Also, the monsoon affected region of the southwest U.S. and, particularly, northwest Mexico has historically been poorly observed which presents challenges for model initialization, validation and refinement.

Since precipitation condenses from atmospheric water vapor, understanding the patterns and movement of water vapor in the pre-storm environment is critical to improving precipitation forecasts. IR and visible satellite atmospheric measurements are limited to frequent cloud tops during the North American Monsoon (NAM) area and therefore cannot determine properties of the air below the cloud tops. Satellite microwave observations of PWV can only be made over large bodies of water because of surface emissivity variations over land.

To capture the diurnal water vapor variations in the critical mountain areas where the convection takes place, we implemented Global Positioning System (GPS) receivers and surface meteorological instrumentation, at 6 locations in the Sierra Madre Occidental Mountains (SMO) in Sonora and Chihuahua to measure precipitable water vapor (PWV) during the EOP. These data complement other datasets collected during the 2004 EOP, particularly the rain gauge observations collected at similar locations (e.g. Gochis et al., 2004). The data we acquired during the 2004 NAME EOP is summarized in our report to NSF.

I summarized our early findings in a talk at the NAME 8th Scientific Working Group meeting in Tucson August 17, 2006 which can be viewed as a pdf. A manuscript summarizing our initial findings is near completion and a second manuscript focused on the diurnal cycle of moist convection including PW, surface conditions and precipitation is underway.

Quantifying the sensitivity of convective precipitation forecasts to errors in the initial precipitable water conditions

One of our research goals for the NAME EOP was to assess the diurnal cycle of PW in the Weather Research and Forecasting (WRF) model run at very high resolution (1.8 km) to resolve moist convection without parameterizations. When we began evaluating the WRF model forecasts using our NAME data, we immediately discovered that the inaccuracies in the WRF moisture forecasts were tied more to the initial conditions defined by NOAA ETA analyses rather than inadequacies of WRF itself. My student, Walter Kolczynski, as his master’s research performed an initial sensitivity study to determine the sensitivity of WRF convection and precipitation forecasts to the accuracy of the PW estimates used to initialize WRF.

Walter and Carlos Minjarez assessed the accuracy of the ETA PW analyses via comparisons with our GPS PWV measurements. Walter’s results show that when the conditions are ripe for moist convection, errors of 5% in the initial PW field produce large and fundamental differences in the style of the moist convection in the WRF results. In comparison, we found that the 1-sigma errors in the 2003 ETA PW analyses were 7 -8% at Hermosillo and Puerto Penasco, Sonora, Mexico. The clear implication is that summertime severe weather prediction in the NAME region, particularly south of the border, is limited at present more by our knowledge of the water vapor distribution rather than our ability to model the convection. This situation could be improved upon dramatically by placing a small network of GPS receivers and associated surface meteorological stations in Northwestern Mexico (see below).

CUPIDO

This past summer (2006), we supported the CUPIDO field campaign over the Catalina mountains immediately north of Tucson. CUPIDO focuses on moist convection over mountains in the semi-arid southwest and how it depends upon, interacts with and alters free tropospheric moisture. Specifically working with Prof. Rick Bennett in UA Geosciences, we placed a GPS receiver on Mt. Lemmon to determine the diurnal cycle of precipitable water and its variations over the mountain and study the convergence of water vapor over the mountain under different conditions experienced this past summer

Future Data Assimilation ideas

I am interested in data assimilation which combines model with observations to produce a statistically optimal estimate of the state of the atmosphere (and other geophysical systems). I hope to create a quantitative, data assimilation characterization of moist convection in the NAM area using our GPS and surface observations combined with radar, rain gauge, lightning and satellite data and the high resolution WRF model explicitly resolving moist convection to better understand warm season precipitation and convection and ultimately their parameterization in climate models. The intent is to work with Jeff Anderson using the WRF Ensemble Filter data Assimilation tools in the Data Assimilation Research Tool (DART) produced by the NCAR Data Assimilation Initiative on this initiative.

Toward a permanent GPS and meteorological network in northwestern Mexico

A goal of Rick Bennett (UA Geosciences) and myself is to establish a permanent GPS receiver network in Northwestern Mexico to provide a multi-functional network for atmospheric and solid earth research as well as surveying and to provide a backbone for an internet network in Northwestern Mexico. Such a network could be assembled relatively inexpensively either from existing older GPS receivers or new internet ready receivers and low cost surface meteorological packages, some of which are already in place in Mexico (Dave Gochis pers.comm.). The internet could be paid for by the community where the internet would be placed as was accomplished in Mazatan during the 2004 NAME EOP. A quick summary of applications include

1) Atmospheric research and operational forecasting

a) Weather forecasting which will use the network to determine the upstream moisture before it flows into the semiarid southwestern North America.

b) Hydrology which will use observations of water in the gas phase to complement the rain gauge and radar network in the NAM area.

c) Climatic monitoring which will use a long term, precise and all-weather hydro-meteorological record of the important and relatively remote and certainly poorly sampled NAM area.

d) Measuring the behavior and evolution of summertime moist convection to provide critical constraints needed to refine/develop convective parameterizations that will work in the NAM region and other similar regions of the globe.

2) Solid earth applications for a high-rate GPS network in Mexico

a) High-precision tectonics characterized by measuring plate boundary deformation in and around the Gulf of California (a focus site for the NSF MARGINs program) and possible diffuse deformation within the Mexican Basin and Range province.

b) Seismology using surface waves (e.g., Larson et al., SCIENCE, 2003) and records of near-field displacements captured by high-rate GPS receivers.

c) a network complementing the US-based PBO facility by extending CGPS coverage into northern Mexico, and other relatively smaller-scale CGPS networks in southern Mexico.

3) Surveying and mapping applications such as exist in the Southwestern US in the greater Tucson and Phoenix metropolitan areas.

4) an Internet accessible phone/satellite network as required for data access which would provide a series of internet hubs at relatively remote locations across Northwestern Mexico, fulfilling the internet accessibility goal defined by the Mexican government.

References

Douglas et al., 1993

Higgins et al., 2006

Larson et al., SCIENCE, 2003