Summer Precipitation across New Mexico:Regimes and Variability Kerry Jones & Deirdre KannNWS Albuquerque
Topics Covered Summer Precipitation in New Mexico North American Monsoon System Backdoor Cold Fronts Variability of Summer Precipitation Summer 2003 vs. Summer 2004
Mechanisms Responsible for Summer Precipitation • North American Monsoon System • Backdoor Cold Fronts • General Terrain Forced Convection (diurnally-driven)
Precipitation across the U.S.:July – August – September (% of Annual)
(inches) Average Summer Precipitation for New Mexico
North American Monsoon System (NAMS) Portions of the southwest United States, including New Mexico, are influenced by the North American Monsoon System (NAMS), which is also referred to as the Southwest Monsoon. Many locations in New Mexico receive 40 to 50 percent of the annual precipitation during the period from July 1 through August 31 and much, but not all, of the summer rainfall can be attributed to the Southwest Monsoon.
North American Monsoon System (NAMS) • Important elements of the NAMS include: • a temperature contrast between the land and adjacent sea surface, • low pressure at the surface with high pressure aloft, and a transport of moisture at low levels of the atmosphere. • Monsoon climates have a distinct and dramatic increase in precipitation associated with the onset of monsoon circulations.
North American Monsoon System (NAMS) Although New Mexico is not impacted by the NAMS until July, development of the system takes place over Mexico during May and June.
The switch from a westerly flow to a southwest/southerly flow can be noted in monthly-averaged sounding data.
Clockwise circulation around the upper level high transports moisture from Mexico to the SW U.S.
Annual Distribution of Precipitation at Deming In the SW corner of the state, onset is readily apparent, with an average start date of July 3.
Annual Distribution of Precipitation at Albuquerque The sudden onset is not as apparent at Albuquerque, where the average start date is July 7.
Moisture Source • Numerous authors have attempted to identify the primary source of moisture for the monsoon in the SW U.S. • While it remains unclear, there is a consensus that most of the low-level moisture arrives from the northern Gulf of California. • The precipitation is enhanced by diurnal heating.
Changes in Upper Level Circulations Once the westerlies shift to the north and moisture is in place, circulations associated with the monsoon may change, but diurnally-driven convection can still produce convective precipitation.
Lightning data illustrates both the onset of the monsoon and the influence of terrain on summer convection
Lightning data also indicates the importance of heating in our diurnal distribution of precipitation
Backdoor Cold Fronts • At least 4 events in 2003, and 7 or 8 in 2004 • Most in 2003 were weak & shallow, 2 stronger events in 2004 (moisture made it to eastern Arizona) • Gap winds (ABQ) • Low level convergent boundary between Continental Divide and Rio Grande Valley • Generally favor Eastern Zones
Significant Convective Events at Albuquerque in 2003 July 20 July 26 August 25 July 23 (mdt) August 14 (mdt) August 16 (weak) August 28-30 (stg) No Backdoor Front 49% ABQ JJA Precip!! Backdoor Fronts
Significant Convective Events at Albuquerque in 2004 July 20 July 27 August 2 June 29 (mdt) July 11(weak) July 18 (strong) July 23-24 (strong) August 10 (weak) August 30 (strong) No Backdoor Front 68% ABQ JJA Precip!! Backdoor Fronts
500 mb Analysis 12Z July 22, 2003 12Z July 23, 2003
Increased Afternoon/Evening Convection West Central New Mexico July 22, 2003 July 23, 2003
KABX WSR-88D -- Higher dew points spreading westward-- Enhanced low level convergence
You know the routine… August 4, 2004 ABQ Sunport: Trace ABQ Foothills: 2.01” ABQ Sunport 10E: 2.08”
Summary While the NAM system is responsible for much of our summer precipitation, back door cold fronts as well as diurnally-driven convection contribute to summer precipitation events Precipitation across the Southwest, including the summer periods, exhibits substantial variability. While ENSO can explain some of this variability, much of the variance cannot be explained and is the topic of current research projects.