Towards a new understanding of monsoon depressions

Transcription

1 Towards a new understanding of monsoon depressions William Boos Dept. of Geology & Geophysics May 2, 25 with contributions from John Hurley & Naftali Cohen Financial support:

2 What is a monsoon low pressure system? Vortex embedded in larger scale monsoon daily TRMM 3B42.v7 precip (colors, mm) ECMWF YOTC 85 hpa wind (vectors) Sept. 6, 28 (JTWC TS TWO) daily TRMM precip (3B42.v6), (mm) storm track 45 4 Diameter ~ 2 km, moderate winds, abundant precipitation Transit and even form over land latitude 2 25 Observed in northern and southern Indian Ocean, western Pacific m/s longitude 5

3 Indian monsoon depressions have well-known genesis and propagation characteristics 3 ERA-Interim June-Sept. (a) genesis (shading) & propagation (vectors) number per square degree 3 per summer latitude m/s longitude Boos et al. 24, QJRMS Figure 2. a) Color shading shows the number of genesis points per square degree (roughly 2, km 2 ) per sum

4 hpa hpa Cold-core cyclonic vortices Indian monsoon depression composite mean anomalies of potential temperature (colors, K) and zonal wind (contours, m/s) A A C A latitude (degrees) K K B Hurley & Boos 24, QJRMS B

5 Why are monsoon low pressure systems important? Produce about half the rainfall of much of monsoonal India, Australia Precursors for many tropical cyclones Create intense floods

6 Pakistan floods July/August 2! related to depression from Bay of Bengal! about 2 dead, $-3 billion loss (Aon Benfield disaster report) Satellite Photos of Indus River Before and After the Flood Event 25 km August 8, 29 Satellite image of the Indus River on August 8, 29 (Source: NASA) NASA MODIS images August 7, 2 Satellite image of the Indus River on August 7, 2 (Source: NASA)

7 Previous theory Growth: Like mid-latitude weather systems, monsoon depressions thought to obtain energy from equator-to-pole temperature gradient! Shukla 977 & 978, Goswami 98, Mak 983, Moorthi & Arakawa 985 Propagation: westward propagation thought to be controlled by low-level dynamics Rao and Rajamani 97, Sanders 984, Chen et al. 25

8 2 3 Not dry baroclinic instability pressure (hpa) pressure (hpa) composite mean Ertel s Potential Vorticity mid life (colors), potential temperature (contours) latitude 3

9 height (hpa) height (hpa) Downshear HUs tilt of PV column.5 in all phases of life DRWs cycle is inconsistent with moist baroclinic growth anomalous -8-4 PV (colors) relative longitude (deg) -2 - Tilt diagnostic relative longitude (deg) 2 - mean zonal wind MDs, q' (/3, -7 m 2 (vectors) K s - kg - TD (degrees) ) HUs, q' (/3, -7 m 2 K s - kg - ) 2.7 Tilt with/against (positive/negative) the shear, / ( 8 Pa s 3 ) 8 day before detection MDs HUs DRWs relative longitude (deg) -2-8 relative -4 longitude 4 (deg) 8 2 TD (degrees) MDs upshear tilt downshear tilt 2 MDs, q' (2/3, -7 m 2 K s - kg - Tilt with/against (positive/n HUs ) HUs, q' (2/3, -7 m 2 K s - kg - ) Cohen & Boos (in prep.) DRWs height (hpa) height (hpa) MDs HUs DRWs

10 Monsoon synoptic vortices and TCs have similar genesis statistics Poisson regression of observed genesis points on climatological mean variables, after Tippett et al. (2) for TCs µ =exp(b T x).4% monsoon%vorces%.3% TCs% coefficient(value(.2%.% %!.% 6%hPa%relave% humidity% 85%hPa%absolute% vorcity% approx.%cape% 85!2%hPa%wind% shear%!.2% (Ditchek et al., in prep.)

11 longitude are degree (roughly 2, km 2 ) Propagation: per summer season (June-Sept.), the after smoothing puzzle with a Gaussian sened to a 2 2 grid for clarity; vectors are shown only if the mean zonal or meridional propagation two-tailed t-test. b) June-Sept. climatological mean vertical shear vector, defined as the 2 hpa - pressure (hpa) relative vorticity (colors, -4 s - ) and climatological relvor zonal (colors) wind and (contours, uclim (contours, CI 2 m/s) cint 3) s the 2 ler spatial ere in the ors seems primarily the vortex ear vector might be ernatively, m motion stretching on of the low-level, these and f the lowlatitude Figure 3. The composite mean vertical component of relative vorticity (colors shading, vortex interval moves. upstream 4 s ) and climatological in mean mean eastward zonal wind flow (contours,... why? interval 2 m s, dashed negative and zero contour in bold). The climatological wind is a composite mean climatology relative to the storm center, as described in the Methods section. Boos et al. 24, QJRMS.5.5

12 Potential vorticity structure tells a different story pressure (hpa) PV (colors) (a) PV and (colors) climatological and low-pass mean u (contours) zonal wind (contours) PV (colors) (b) PV and (colors) total and zonal u (contours) wind (contours) 2.8 suggests propagation may be governed by nonlinear mid-tropospheric dynamics Boos et al. 24, QJRMS pressure (hpa) latitude latitude (degrees).6.4.2

13 Propagation by self-advection ( beta drift ) schematic of beta gyres in a tropical cyclone relative latitude b composite Indian monsoon depression: 5 hpa PV (colors) and azimuthally asymmetric 5 hpa streamfunction asymmetric (contours) ψ 2 m/s Boos et al. 24, QJRMS relative longitude.7 vectors show: total wind mean wind vortex propagation

14 Propagation by self-advection ( beta drift ) schematic of beta gyres in a tropical cyclone relative latitude b composite Indian monsoon depression: 5 hpa PV (colors) and azimuthally asymmetric 5 hpa streamfunction asymmetric (contours) ψ 2 m/s Boos et al. 24, QJRMS relative longitude beta gyres.7 vectors show: total wind mean wind vortex propagation

15 A big story: declining storm count trends in India Monsoon depression frequency (#/year) IMD smooth IMD # of storms first summers in > years with no depressions! year Cohen & Boos 24, GRL

16 latitude But that record is missing 2 some depressions latitude ERA-Interim sea level pressure (colors), surface winds (vectors) 8.5 m/s longitude Scatterometer surface wind (WindSat), 8.5 m/s surface wind speed (colors, SSM/I), TRMM 4 mm/day rain contour (magenta) longitude c) August 24, 2, UTC 3 d) August 24, 2, UTC latitude 5 2 latitude m/s longitude e) August 8, 22, UTC m/s longitude f) August 8, 22, UTC Cohen & Boos 24, GRL 25

17 Trend estimates trends trends the traditional datasets IMD (Fig. b) SIKKA (Fig. b) ERA-Interim Yale (Fig. c) MERAA NYUAD (Fig. c) ERA-Interim NYUAD (Fig. c) Averaged Reanalysis data (Fig. c) scatterometer KE (Figs. 4c and 4d) ERA-Interim Yale (Fig. 4c) IMD (Fig. 4c) Linear and nonlinear Poisson regression coefficeints Summer average KE (Fig. 4d) (scatterometer) year - Cohen & Boos 24, GRL

18 isentropic surface. 2 Summary Low pressure systems (e.g. monsoon depressions) are very important, yet are 2 6 poorly understood and little studied since 98s in number of Indian monsoon depressions Steep downward trend reported composite mean potential vorticity (colors), mid life (contours) potential temperature pressure (hpa) latitude 3.5 latitude pressure (hpa) genesis We have shown: 2.5 Indian monsoon depressions propagate westward by beta drift (not by QG lifting in 4.5 easterly shear) PV structure is inconsistent with baroclinic 2 instability/growth.5 There are problems with the only dataset 4that shows downward trend in.5depression counts 2.5 Figure 4. Composite mean EPV (colors) and potential temperature (contours at 35, 32, 33, and 35 K). (Probably delete top panel since don t want to focus on genesis here.) There is great need for community assessments of monsoon synoptic variability in observations and models (both climate & NWP) All of these characteristics of the EPV field confirm that barotropic or baroclinic instability, in the absence of longitude