U24A-01
The Planetary Boundary Layer Structure of the North American Monsoon and the Role of Land-Atmosphere Interactions as Depicted in Models and Observations
Vertical exchanges of heat, moisture, and momentum between the earth's surface and atmosphere directly affect the structure and evolution of the planetary boundary layer (PBL) whose stability profile determines the likelihood of convection and precipitation. Convective towers in turn have important radiative impacts and feedback on the energy budget of the surface. The complex interaction between the surface and the atmosphere involves physical processes that operate on a variety of scales whose quantitative importance is not well understood; coupled with the varied landscape of the region, this creates a particular difficult challenge in modeling the North American Monsoon. Using high-resolution observational data from the North American Monsoon Experiment supplemented with North American Regional Reanalysis data and output from several regional and global models of NAMAP2, this study examines the PBL structure of the monsoon region and illustrates model discrepancies in the representation of land-atmosphere processes important in determining the energy budget of the monsoon.
U24A-02
Observations of the transition from shallow to deep convection during the North American Monsoon
The regular development of deep convection over the mountains in southern Arizona makes them an ideal location to study the onset and transition from shallow to deep convection. The convection that occurs during the North American Monsoon develops over the peaks a few hours after sunrise and development occurs slowly and in stages despite the presence of sufficient convective available potential energy (CAPE). A number of explanations for the episodic development have been proposed. These involve a modification of the column above the mountain by shallow convection that conditions the atmosphere for deep convection. The modification is likely due to moistening and/or changes to the static stability by warming the profile. Details of the transition from shallow to deep convection were explored during summer 2006 with support from the National Science Foundation. The Cumulus Photogrammetric In-Situ and Doppler Observations (CuPIDO) program utilized a network of ten portable automated mesonet stations (PAM-III), four of which were equipped to measure surface latent and sensible heat flux, two GPS based mobile sounding systems (MGAUS), two stereo pairs of digital cameras and the University of Wyoming King Air (with 95 GHz Doppler radar) to examine the onset and evolution of monsoon thunderstorms. This talk will describe the CuPIDO 2006 field program and present some preliminary results. Stereo- photogrammetric techniques are utilized to determine the three-dimensional structure of the clouds during various stages of the development. Changes to the vertical profile during the course of the development are examined using the sounding data and augmented by in-situ data from the aircraft. One specific example will be presented in which the breakdown of a stable layer at about the 500 mb level coincides with the onset of deep convection. The breakdown appears to be associated with warming of the profile in the lowest portion of the inversion through the action of the shallow convection and cooling in the upper portion of the layer. The cooling appears to be due to evaporative cooling as dry environmental air is entrained in to the cloud tops. Data collected during CuPIDO will aid in our understanding of the interaction between cumulus clouds and the environment and the way this interaction contributes to the development of deep convection. An accurate representation of this process in forecast models will improve model performance and increase forecast skill.
U24A-03
Water Vapor Fluxes over the Intra-Americas Sea: Seasonal and Interannual Variability and Associations with Rainfall
The seasonal and interannual variability of moisture transports over the Intra-Americas Sea (including the Gulf of Mexico and the Caribbean Sea) is evaluated using the NCEP-NCAR global reanalysis. The seasonal variability of these moisture transports is consistent with previous studies and shows distinctive winter and summer regimes. Boreal winter moisture is mainly delivered to the central US from the Pacific with some contribution from the Gulf of Mexico. It is during the boreal summer that the moisture flow over the Intra-Americas Sea is most effective in supplying the water vapor to the central US via the northern branch of the Caribbean low-level jet. The increase of intensity of this jet during July is associated with an increase in evaporation over the Intra-Americas Sea, consistent with mid-summer drought conditions over this region. During both summer and winter, the interannual variability of the inflow of moisture from the Intra-Americas Sea into central US is associated with Caribbean low-level jet variability. The source of the varying moisture is mainly the Gulf of Mexico and the North Atlantic area just east of the Bahamas Islands and the sink is precipitation over east-central US. The main teleconnection pattern for these interannual variations appears to be the Pacific North American, although in boreal winter ENSO and possibly the North Atlantic Oscillation may also play a role. During boreal summer, associations with ENSO mainly involve the zonal moisture exchange between the Intra- Americas Sea/tropical Atlantic and the tropical Pacific.
U24A-04
Surface heat fluxes and oceanic heat balance in the southern Gulf of California during the NAME, June-August 2004
Oceanographic and meteorological data from two oceanographic cruises carried out in June and August 2004 as part of the NAME, are used to calculate the fluxes of heat across the sea surface and the heat balance in the southern Gulf of California. Understanding the oceanic heat balance is important to know the processes that control the evolution of SST in the gulf during the warm period. Collected data included: simultaneous velocity observations (Lagrangian and Eulerian); hydrography; and lower atmosphere data. During the period of observation, the heat balance given by the horizontal heat flux, the surface flux and the ocean heating, shows values with similar phase to those of the annual climatology; but the values are significantly different, reaching in some cases a difference of one order of magnitude. Although the oceanographic and meteorological climatological data resolves very poorly the southern Gulf of California during the warm period, the discrepancy can probably be explained mainly in terms of the oceanic mesoscale activity in the surface of the gulf which is not taken into account in the climatological calculations. Filaments, eddies and meanders are connected with the adjacent Pacific Ocean and play an important role in controlling the heat balance, and hence in the SST evolution during the NAM season
U24A-05
Structure of mesoscale eddies in the southern Gulf of California during cruise NAME-2 (August 2004)
As part of the 2004 NAME, the meteorological and oceanographic causes of the Gulf of California SST evolution during the Monsoon were investigated through two 17-day oceanographic cruises in the southern of the Gulf of California (22.5ºN-27.7ºN), and a number of surface drifters. This study focuses on the results from the second campaign (August 5-22, 2004). There were remarkable changes with respect to the conditions observed in the June 2004 campaign (NAME-1). The SST was practically homogeneous in all the southern Gulf, at ~30-31ºC in a 20 m surface mixed layer. But the subsurface thermohaline structure (T, S, O2, fluorescence) and the circulation pattern were dominated by a series of mesoscale eddies of alternating vorticity (three cyclonic and two anticyclonic) with diameters 80-100 km and up to 900 m deep. These eddies were mostly geostrophic, with speeds between 0.20 m/s and 0.45 m/s (measured with ADCP and drifters) in the top 100 m. These eddy caused much recirculation, evident in the similarity of hydrographic characteristics in the two sides of the Gulf.
U24A-06
Circulation and Hydrography in the Southern Gulf of California prior to the NAM onset (cruise NAME-1, June 2004)
The thermohaline structure and circulation in the entrance to the Gulf of California during June 2004 is described based on very detailed data (CTD, LADCP and surface drifters) collected in a 17-day survey, supported by coetaneous satellite data (winds from QuikScat and daily 1-km AVHRR). The AVHRR images show extensive mesoscale structures in the region (fronts, eddies, streamers, etc.), the most striking being: (a) a cool streamer extending from the California Current domain up to one fourth of the Gulf, (b) a warm intrusion along the mainland shelf, and (c) a narrow warm streamer attached to the tip of the Baja California peninsula. The ship data show that the latter was associated to a narrow outgoing current which carried Gulf of California Water; its core (0.2-0.4 m/s) was at 200 m, and reached down to 500 m depth. By contrast, there was a very strong current flowing into the Gulf on the eastern warm side of the thermal front created by the presence of the cool streamer, with speeds up to 0.60 m/s in the surface and 0.20 m/s at 200 m depth. There was also an ingoing coastal current on the mainland shelf, with weak surface currents but a well defined subsurface structure with speeds ~0.25 m/s at its core between 70-200 m. The two ingoing currents formed a very strong (speeds 0.40-0.80 ms-1) and narrow (~30 km) inflowing coastal current between the surface and 150 m depth that carried Tropical Surface Water in the upper 20 m, water from the Shallow Salinity Minimum, and Subtropical Subsurface Water below the 18 ºC isotherm. By resampling three cross-sections one week apart, it was possible to conclude that most of the observed changes in the temperature and salinity fields of the upper layers were dominated by advection, in agreement with previous indirect evidence from heat and salinity balances and limited observations.