Extreme Orographic Rainfall in the Eastern Andes Tied to Cold Air Intrusions

Abstract

Long-duration localized heavy rainfall (> 6 h, < 10 km 2) was recorded by raingauges on October 4, 2013 at mid-elevations (∼ 1, 500 m) in the Peruvian Andes, which was not apparent in infrared or microwave measurements from either geostationary or polar orbiting satellites. This spring event is investigated here through a numerical modeling study using the Weather and Research Forecasting (WRF) model, and examined in the context of the climatology of Cold Air Intrusions (CAIs) along the eastern flanks of the tropical Andes. The model results suggest significant precipitation enhancement from intense shallow convection at the CAI frontal boundary amplified by orographic lifting as it propagates northward latched to the slopes of Andes. Analysis of CAI mesoscale dynamics was conducted using four decades of European Center Medium-Range Weather Forecasts (ECMWF) reanalysis, Tropical Rainfall Measurement Mission (TRMM) data products, and rain-gauge observations with emphasis on characterizing year-round CAI frequency, CAI interactions with Andes topography, and their impact on orographic precipitation climatology. The data show a robust enhancement of the diurnal cycle of precipitation during CAI events in all seasons, and in particular increases in surface rainfall rate during early morning at intermediate elevations (∼ 1,500 m), that is the eastern Andes orographic maximum. This link between CAI frequency and rainfall suggests that they play an important role in maintaining the Andes to Amazon year-round terrestrial connectivity through runoff production and transport by the river networks.

Deforestation Impacts on Orographic Precipitation in the Tropical Andes

“…Upon the arrival of the CAI front, synoptic and regional scale dynamics are dominant, stable and cold conditions prevail behind the front, and thus EtPI are active ahead of the front only in the second and third CAI days. Note the spatial agreement between the standard deviation and the average daytime MCAPE differences consistent with intense convective activity at the intersection of the front with the topography, as pointed out by Eghdami and Barros (2019a) along the EADS, and ahead of the front at low elevations in the Amazon basin. Furthermore, the reduction in FIGURE 3 | WRF simulation (D01, 18 km) of the SLLJ case during February 06 00Z to 07 12 Z, 2003.…”

“…Detailed synoptic-scale analysis of the three events simulated here can be found in Sun and Barros (2015a,b) for the WLLJ and SLLJ cases, and in Eghdami and Barros (2019a) for the CAI case. For assessing the impact of land-cover change on atmospheric moist processes, the focus here is on lowlevel instability changes as measured by Maximum Convective Available Potential Energy (MCAPE) and precipitation changes along the EADS foothills.…”

“…In particular, the impact of extending land-cover changes due to anthropogenic activities from low (∼500 m) to high elevations near the treeline (∼3,500 m) is examined by converting TMFs to savanna in the tropical EADS (Figure 1) for three weather regimes that dominate regional precipitation climatology. Building on previous work, a Cold Air Intrusion (CAI) during October 3-6, 2013, associated with extreme rainfall in the central Andes (Eghdami and Barros, 2019a), is selected for the dry season. CAIs are recurrent synoptic-scale features in South-America, and Eghdami and Barros showed that during CAI events, high atmospheric moisture contents and strong shallow convection lead to localized extremes in orographic precipitation along the EADS foothills as the cold front propagates northward.…”

Parsing Synthetic Aperture Radar Measurements of Snow in Complex Terrain: Scaling Behaviour and Sensitivity to Snow Wetness and Landcover

“…Note that the speckle removal described in Section 3.1 to improve the signal-to-noise ratio (SNR) is expected to introduce a scaling break above and below the LRS filter scale. Given the small size (<20 km 2 ) of the homogenous areas of interest, instead of using 2D FT to calculate the power spectra, 1D FTs are conducted for rows and columns separately, and the average in the zonal (x) and meridional (y) directions is calculated for scaling analysis as following [56,57].…”

Parsing Synthetic Aperture Radar Measurements of Snow in Complex Terrain: Scaling Behaviour and Sensitivity to Snow Wetness and Landcover

“…Note that the speckle removal described in Section 3.1 to improve signal-to-noise ratio (SNR) is expected to introduce a scaling break above and below the LRS filter scale. in the zonal (x) and meridional (y) directions is calculated for scaling analysis as following [53,54]. If k x ( ) and k y � � are wavenumbers (wavelengths) corresponding to x and y directions, in the range of scales where the power spectrum |F′(k γ )| 2 exhibits power-law behaviour…”